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Home | Alpha Telephone | Domain Names | Web Hosting | Get Traffic | xrEvidence | xrSoccer United States Patent
High capacity compressed document image storage for digital color printers A high capacity compressed document image storage apparatus for color printing systems. The compressed document storage system is highly suited for use with high speed color pagewidth print heads. However, the document storage apparatus can also be used with other printing systems, such as color electrophotographic and other systems. The compressed document storage system is composed of the following major components: 1) an image creation system which can operate on a band by band basis. 2) an input memory which stores at least one band of the uncompressed page image. Two bands are preferred to allow double buffering, so that the RIP and compression stages can proceed simultaneously. 3) a page image compression system which can operate on a band by band basis. A new page image compression scheme which achieves high compression ratio and preserves high image quality is disclosed. 4) a mass storage device for storing compressed page images. 5) a real-time page image expansion system which expands the compressed page images at the rate required by the print engine. 6) an output memory which stores at least one band of the expanded page image. The recommended minimum is the number of lines from the first line of the print engine to the last line of the print engine, plus two bands. 7) an interface which reads data from the output memory and provides it at the correct timing to the print engine. The page image compression scheme can achieve a typical compression ratio in excess of 80:1 when compared to a CC'MM'YK 800 dpi bitmap, and in excess of 420:1 when compared to an 800 dpi CMYK contone image. The high compression ratio allows the storage of more than 1,000 A4 800 dpi full color page images on a low cost 1 GByte disk drive. The compression scheme reduces the compressed data rate required for high speed (120 A4 ppm) printing to that which can be achieved using commonly available magnetic hard disk drives. An improvement to JPEG image compression when used with text or graphics overlays is also disclosed.
Assistant Examiner: Lamb; Twyler Attorney, Agent or Firm: CROSS REFERENCE TO RELATED APPLICATIONS Reference is made to my commonly assigned, co-pending U.S. Pat. Applications: Ser. No. 08/701,021 entitled CMOS PROCESS COMPATIBLE FABRICATION OF PRINT HEADS filed Aug. 21, 1996; Ser. No. 08/733,711 entitled CONSTRUCTION AND MANUFACTURING PROCESS FOR DROP ON DEMAND PRINT HEADS WITH NOZZLE HEATERS filed Oct. 17, 1996; Ser. No. 08/734,822 entitled A MODULAR PRINT HEAD ASSEMBLY filed Oct. 22, 1996; Ser. No. 08/736,537 entitled PRINT HEAD CONSTRUCTIONS FOR REDUCED ELECTROSTATIC INTERACTION BETWEEN PRINTED DROPLETS filed Oct. 24, 1996; Ser. No. 08/750,320 entitled NOZZLE DUPLICATION FOR FAULT TOLERANCE IN INTEGRATED PRINTING HEADS filed Nov. 26, 1996; Ser. No. 08/753,718 entitled NOZZLE PLACEMENT IN MONOLITHIC DROP-ON-DEMAND PRINT HEADS and Ser. No. 08/750,606 entitled A COLOR VIDEO PRINTER AND A PHOTO CD SYSTEM WITH INTEGRATED PRINTER both filed on Nov. 27, 1996; Ser. No. 08/750,438 entitled A LIQUID INK PRINTING APPARATUS AND SYSTEM, Ser. No. 08/750,599 entitled COINCIDENT DROP SELECTION, DROP SEPARATION PRINTING METHOD AND SYSTEM, Ser. No. 08/750,435 entitled MONOLITHIC PRINT HEAD STRUCTURE AND A MANUFACTURING PROCESS THEREFOR USING ANISTROPIC WET ETCHING, Ser. No. 08/750,436 entitled POWER SUPPLY CONNECTION FOR MONOLITHIC PRINT HEADS, Ser. No. 08/750,437 entitled MODULAR DIGITAL PRINTING, Ser. No. 08/750,439 entitled A HIGH SPEED DIGITAL FABRIC PRINTER, Ser. No. 08/750,763 entitled A COLOR PHOTOCOPIER USING A DROP ON DEMAND INK JET PRINTING SYSTEM, Ser. No. 08/765,756 entitled PHOTOGRAPH PROCESSING AND COPYING SYSTEMS, Ser. No. 08/750,646 entitled FAX MACHINE WITH CONCURRENT DROP SELECTION AND DROP SEPARATION INK JET PRINTING, Ser. No. 08/759,774 entitled FAULT TOLERANCE IN HIGH VOLUME PRINTING PRESSES, Ser. No. 08/750,429 entitled INTEGRATED DRIVE CIRCUITRY IN DROP ON DEMAND PRINT HEADS, Ser. No. 08/750,433 entitled HEATER POWER COMPENSATION FOR TEMPERATURE IN THERMAL PRINTING SYSTEMS, Ser. No. 08/750,640 entitled HEATER POWER COMPENSATION FOR THERMAL LAG IN THERMAL PRINTING SYSTEMS, Ser. No. 08/750,650 entitled DATA DISTRIBUTION IN MONOLITHIC PRINT HEADS, and Ser. No. 08/750,642 entitled PRESSURIZABLE LIQUID INK CARTRIDGE FOR COINCIDENT FORCES PRINTERS all filed Dec. 3, 1996; Ser. No. 08/750,647 entitled MONOLITHIC PRINTING HEADS AND MANUFACTURING PROCESSES THEREFOR, Ser. No. 08/750,604 entitled INTEGRATED FOUR COLOR PRINT HEADS, Ser. No. 08/750,605 entitled A SELF-ALIGNED CONSTRUCTION AND MANUFACTURING PROCESS FOR MONOLITHIC PRINT HEADS, Ser. No. 08/682,603 entitled A COLOR PLOTTER USING CONCURRENT DROP SELECTION AND DROP SEPARATION INK JET PRINTING TECHNOLOGY, Ser. No. 08/750,603 entitled A NOTEBOOK COMPUTER WITH INTEGRATED CONCURRENT DROP SELECTION AND DROP SEPARATION COLOR PRINTING SYSTEM, Ser. No. 08/765,130 entitled INTEGRATED FAULT TOLERANCE IN PRINTING MECHANISMS; Ser. No. 08/750,431 entitled BLOCK FAULT TOLERANCE IN INTEGRATED PRINTING HEADS, Ser. No. 08/750,607 entitled FOUR LEVEL INK SET FOR BI-LEVEL COLOR PRINTING, Ser. No. 08/750,430 entitled A NOZZLE CLEARING PROCEDURE FOR LIQUID INK PRINTING, Ser. No. 08/750,600 entitled METHOD AND APPARATUS FOR ACCURATE CONTROL OF TEMPERATURE PULSES IN PRINTING HEADS, Ser. No. 08/750,608 entitled A PORTABLE PRINTER USING A CONCURRENT DROP SELECTION AND DROP SEPARATION PRINTING SYSTEM, and Ser. No. 08/750,602 entitled IMPROVEMENTS IN IMAGE HALFTONING all filed Dec. 4, 1996; Ser. No. 08/765,127 entitled PRINTING METHOD AND APPARATUS EMPLOYING ELECTROSTATIC DROP SEPARATION, Ser. No. 08/750,643 entitled COLOR OFFICE PRINTER WITH A HIGH CAPACITY DIGITAL PAGE IMAGE STORE, and Ser. No. 08/765,035 entitled HEATER POWER COMPENSATION FOR PRINTING LOAD IN THERMAL PRINTING SYSTEMS all filed Dec. 5, 1996; Ser. No. 08/765,036 entitled APPARATUS FOR PRINTING MULTIPLE DROP SIZES AND FABRICATION THEREOF, Ser. No. 08/765,017 entitled HEATER STRUCTURE AND FABRICATION PROCESS FOR MONOLITHIC PRINT HEADS, Ser. No. 08/750,772 entitled DETECTION OF FAULTY ACTUATORS IN PRINTING HEADS, Ser. No. 08/765,037 entitled PAGE IMAGE AND FAULT TOLERANCE CONTROL APPARATUS FOR PRINTING SYSTEMS all filed Dec. 9, 1996; and Ser. No. 08/765,038 entitled CONSTRUCTIONS AND MANUFACTURING PROCESSES FOR THERMALLY ACTIVATED PRINT HEADS filed Dec. 10, 1996. I claim: 1. A high capacity compressed document image storage apparatus which is incorporated in a printing system having a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles; (c) pressure means for subjecting ink in said body of ink to a pressure of at least 2% above ambient pressure, at least during drop selection and separation; (d) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and drop separating means for causing ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles; said image storage apparatus comprising: (1) an image creation system which can operate on a band by band basis; (2) an input memory which stores at least one band of the uncompressed page image, but less than 50% of said uncompressed page image; (3) a page image compression system which can operate on a band by band basis; (4) a mass storage device able to store a plurality of compressed page images; (5) a page image expansion system which expands compressed page image data to form an expanded page image at a rate which is within 20% of the page printing rate when measured over the duration of the page printing time; (6) an output memory which stores at least one band of said expanded page image, but less than 50% of said expanded page image; and (7) an apparatus which provides page image data from said output memory to a printing device wherein the time taken to provide a band of image data to the printing device does not vary by more than .+-.50% of the mean time to provide a band of image data to the printing device. 2. A high capacity compressed document image storage apparatus which is incorporated in a printing system having a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles; (c) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separating means for causing ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles, said drop selecting means being capable of producing said difference in meniscus position in the absence of said drop separation means; said image storage apparatus comprising: (1) an image creation system which can operate on a band by band basis; (2) an input memory which stores at least one band of the uncompressed page image, but less than 50% of said uncompressed page image; (3) a page image compression system which can operate on a band by band basis; (4) a mass storage device able to store a plurality of compressed page images; (5) a page image expansion system which expands compressed page image data to form an expanded page image at a rate which is within 20% of the page printing rate when measured over the duration of the page printing time; (6) an output memory which stores at least one band of said expanded page image, but less than 50% of said expanded page image; and (7) an apparatus which provides page image data from said output memory to a printing device wherein the time taken to provide a band of image data to the printing device does not vary by more than .+-.50% of the mean time to provide a band of image data to the printing device. 3. A high capacity compressed document image storage apparatus which is incorporated in a printing system having a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles, said ink exhibiting a surface tension decrease of at least 10 mN/m over a 30.degree. C. temperature range; (c) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separating means for causing ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles; said image storage apparatus comprising: (1) an image creation system which can operate on a band by band basis; (2) an input memory which stores at least one band of the uncompressed page image, but less than 50% of said uncompressed page image; (3) a page image compression system which can operate on a band by band basis; (4) a mass storage device able to store a plurality of compressed page images; (5) a page image expansion system which expands compressed page image data to form an expanded page image at a rate which is within 20% of the page printing rate when measured over the duration of the page printing time; (6) an output memory which stores at least one band of said expanded page image, but less than 50% of said expanded page image; and (7) an apparatus which provides page image data from said output memory to a printing device wherein the time taken to provide a band of image data to the printing device does not vary by more than .+-.50% of the mean time to provide a band of image data to the printing device. 4. A high capacity compressed document image storage apparatus including: (1) an image creation system which can operate on a band by band basis; (2) an input memory which stores at least one band of the uncompressed page image, but less than 50% of said uncompressed page image; (3) a page image compression system which can operate on a band by band basis in which page images are compressed by an average of more than 400:1 compared to a 32 bit contone representation of said page images; (4) a mass storage device able to store a plurality of compressed page images; (5) a page image expansion system which expands compressed page image data to form an expanded page image at a rate which is within 20% of the page printing rate when measured over the duration of the page printing time; (6) an output memory which stores at least one band of said expanded page image, but less than 50% of said expanded page image; and (7) an apparatus which provides page image data from said output memory to a printing device wherein the time taken to provide a band of image data to the printing device does not vary by more than .+-.50% of the mean time to provide a band of image data to the printing device. 5. A high capacity compressed document image storage apparatus including: (1) an image creation system which can operate on a band by band basis; (2) an input memory which stores at least one band of the uncompressed page image, but less than 50% of said uncompressed page image; (3) a page image compression system which can operate on a band by band basis; (4) a mass storage device able to store a plurality of compressed page images in which more than 1,000 page images can be stored per GByte of said mass storage; (5) a page image expansion system which expands compressed page image data to form an expanded page image at a rate which is within 20% of the page printing rate when measured over the duration of the page printing time; (6) an output memory which stores at least one band of said expanded page image, but less than 50% of said expanded page image; and (7) an apparatus which provides page image data from said output memory to a printing device wherein the time taken to provide a band of image data to the printing device does not vary by more than .+-.50% of the mean time to provide a band of image data to the printing device. 6. A high capacity compressed document image storage apparatus including: (1) an image creation system which can operate on a band by band basis; (2) an input memory which stores at least one band of the uncompressed page image, but less than 50% of said uncompressed page image; (3) a page image compression system which can operate on a band by band basis wherein cells of the page image which contain more than two colors are compressed using at least JPEG compression; (4) a mass storage device able to store a plurality of compressed page images; (5) a page image expansion system which expands compressed page image data to form an expanded page image at a rate which is within 20% of the page printing rate when measured over the duration of the page printing time; (6) an output memory which stores at least one band of said expanded page image, but less than 50% of said expanded page image; and (7) an apparatus which provides page image data from said output memory to a printing device wherein the time taken to provide a band of image data to the printing device does not vary by more than .+-.50% of the mean time to provide a band of image data to the printing device. 7. An image storage apparatus as claimed in claim 6 wherein cells of the page image which contain only black and white pixels are not compressed using JPEG compression. 8. A high capacity compressed document image storage apparatus including: (1) an image creation system which can operate on a band by band basis; (2) an input memory which stores at least one band of the uncompressed page image, but less than 50% of said uncompressed page image: (3) a page image compression system which can operate on a band by band basis; (4) a mass storage device able to store a plurality of compressed page images; (5) a page image expansion system which expands compressed page image data at an average rate of more than 30 million pixels per second to form an expanded page image at a rate which is within 20% of the page printing rate when measured over the duration of the page printing time; (6) an output memory which stores at least one band of said expanded page image, but less than 50% of said expanded page image; and (7) an apparatus which provides page image data from said output memory to a printing device wherein the time taken to provide a band of image data to the printing device does not vary by more than .+-.50% of the mean time to provide a band of image data to the printing device. 9. A high capacity compressed document image storage apparatus including: (1) an image creation system which can operate on a band by band basis; (2) an input memory which stores at least one band of the uncompressed page image, but less than 50% of said uncompressed page image; (3) a page image compression system which can operate on a band by band basis; (4) a mass storage device able to store a plurality of compressed page images; (5) a page image expansion system which can expand a plurality of said compressed page images simultaneously, each at an average rate of more than 30 million pixels per second to form an expanded page image at a rate which is within 20% of the page printing rate when measured over the duration of the page printing time; (6) an output memory which stores at least one band of said expanded page image, but less than 50% of said expanded page image; and (7) an apparatus which provides page image data from said output memory to a printing device wherein the time taken to provide a band of image data to the printing device does not vary by more than .+-.50% of the mean time to provide a band of image data to the printing device. 10. A method of page image compression said method including the following steps: (a) dividing said page image into an array of cells of pixels; (b) encoding said cells containing only white pixels to a specific code; (c) encoding said cells containing only black and white pixels to a different specific code, and supplementing said code with information encoding the pattern of black and white pixels in said cell; and (d) encoding said cells containing a plurality of colors to a mutually different code, and supplementing said code with information encoding the colors of the pixels in said cell. 11. A compression system as claimed in claim 10 said compression system being incorporated in a printing system which includes a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles; (c) a pressurizing device adapted to subject ink in said body of ink to a pressure of at least 2% above ambient pressure, at least during drop selection and separation to form a meniscus with an air/ink interface; (d) drop selection apparatus operable upon the air/ink interface to select predetermined nozzles and to generate a difference in meniscus position between ink in selected and non-selected nozzles; and (e) drop separation apparatus adapted to cause ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles. 12. A compression system as claimed in claim 10 said compression system being incorporated in a printing system which includes a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles, said body of ink forming a meniscus with an air/ink interface at each nozzle; (c) drop selection apparatus operable upon the air/ink interface to select predetermined nozzles and to generate a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separation apparatus adapted to cause ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles, said drop selection apparatus being capable of producing said difference in meniscus position in the absence of said drop separation apparatus. 13. A compression system as claimed in claim 10 said compression system being incorporated in a printing system which includes a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles, said body of ink forming a meniscus with an air/ink interface at each nozzle and said ink exhibiting a surface tension decrease of at least 10 mN/m over a 30.degree. C. temperature range; (c) drop selection apparatus operable upon the air/ink interface to select predetermined nozzles and to generate a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separation apparatus adapted to cause ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles. 14. A compression system as claimed in claim 10 wherein said cells of pixels are rectangular and the number of pixels in said cells is an integral multiple of eight in one dimension, and a further integral multiple of eight in the perpendicular dimension, wherein said multiples of eight may be identical or may be different. 15. A compression system as claimed in claim 14 wherein the number of pixels in said cells is 24 in both dimensions. 16. A compression system as claimed in claim 10 wherein said specific code is a two bit code. 17. A compression system as claimed in claim 10 wherein said cells containing only black and white pixels are further divided into arrays of subcells. 18. A compression system as claimed in claim 10 wherein: (a) Subcells containing only white pixels are encoded to a specific subcode; (b) Subcells containing only black pixels are encoded to a further specific subcode; (c) Subcells containing a pattern which matches one of a predetermined set of patterns are encoded to a further specific subcode, plus a further subcode representing the pattern which is matched; and (d) Subcells containing a pattern which is neither all black, all white, nor match any of said predetermined set of patterns are encoded to a further specific subcode and supplemented with data which directly encodes said pattern. 19. A compression system as claimed in claim 10 wherein an exact match between the pattern of pixels in said subcell and one of a predetermined set of patterns is not required for said subcell to be encoded using said subcode representing said pattern which is matched. 20. A compression system as claimed in claim 10 wherein said cells containing only color pixels are subsampled to form an 8.times.8 JPEG cell. 21. A compression system as claimed in claim 20 wherein said JPEG cells are JPEG compressed. 22. A compression system as claimed in claim 20 wherein said cells are low pass filtered before subsampling. 23. A printing system incorporating a page image compression system wherein said printing system includes a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles; (c) a pressurizing device adapted to subject ink in said body of ink to a pressure of at least 2% above ambient pressure, at least during drop selection and separation to form a meniscus with an air/ink interface; (d) drop selection apparatus operable upon the air/ink interface to select predetermined nozzles and to generate a difference in meniscus position between ink in selected and non-selected nozzles; and (e) drop separation apparatus adapted to cause ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles. 24. A printing system incorporating a page image compression system wherein said printing system includes a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles, said body of ink forming a meniscus with an air/ink interface at each nozzle; (c) drop selection apparatus operable upon the air/ink interface to select predetermined nozzles and to generate a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separation apparatus adapted to cause ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles, said drop selection apparatus being capable of producing said difference in meniscus position in the absence of said drop separation apparatus. 25. A printing system incorporating a page image compression system wherein said printing system includes a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles, said body of ink forming a meniscus with an air/ink interface at each nozzle and said ink exhibiting a surface tension decrease of at least 10 mN/m over a 30.degree. C. temperature range; (c) drop selection apparatus operable upon the air/ink interface to select predetermined nozzles and to generate a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separation apparatus adapted to cause ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles. 26. An expansion apparatus for expanding compressed page image data in real-time as the page is being printed, said apparatus including: (a) a mass storage unit for storage of the compressed page images; (b) a JPEG decoder which expands the JPEG encoded portions of the compressed page image into contone image data; (c) a contone band memory which temporarily holds at least one band of said contone image data; (d) an expander circuit which expands compressed black and white text and graphics bitmaps and combines said bitmaps with said contone image data read from said contone band memory. 27. An expansion apparatus as claimed in claim 26 which is incorporated in a printing system having a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles; (c) a pressurizing device adapted to subject ink in said body of ink to a pressure of at least 2% above ambient pressure, at least during drop selection and separation to form a meniscus with an air/ink interface; (d) drop selection apparatus operable upon the air/ink interface to select predetermined nozzles and to generate a difference in meniscus position between ink in selected and non-selected nozzles; and (e) drop separation apparatus adapted to cause ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles. 28. An expansion apparatus as claimed in claim 26 which is incorporated in a printing system having a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles, said body of ink forming a meniscus with an air/ink interface at each nozzle; (c) drop selection apparatus operable upon the air/ink interface to select predetermined nozzles and to generate a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separation apparatus adapted to cause ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles, said drop selection apparatus being capable of producing said difference in meniscus position in the absence of said drop separation apparatus. 29. An expansion apparatus as claimed in claim 26 which is incorporated in a printing system having a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles, said body of ink forming a meniscus with an air/ink interface at each nozzle and said ink exhibiting a surface tension decrease of at least 10 mN/m over a 30.degree. C. temperature range; (c) drop selection apparatus operable upon the air/ink interface to select predetermined nozzles and to generate a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separation apparatus adapted to cause ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles. 30. An expansion apparatus as claimed in claim 26 which also contains a head control circuit which provides data to the print head when required by the print head. 31. An expansion apparatus as claimed in claim 26 which also includes a halftoner circuit which calculates bi-level image data from the image data created by said expander circuit. 32. An expansion apparatus as claimed in claim 31 which also includes a halftone image memory which stores a plurality of lines of said bi-level image data. 33. An expansion apparatus as claimed in claim 32 where the number of lines of said bi-level image data stored in said halftone image memory is less than the number of lines in the printed page. 34. An expansion apparatus as claimed in claim 31 in which said expander circuit and said halftoner circuit are contained on the same integrated circuit. 35. An expansion apparatus as claimed in claim 26 wherein the image resolution of said contone image data is less than the image resolution of said bitmap image data. 36. An expansion apparatus as claimed in claim 35 wherein said contone image data is pixel replicated after JPEG expansion so that the image resolution of said pixel replicated contone image data is the same as the image resolution of said bitmap image data. 37. A method of compressing an image said method including the steps of: (a) dividing the image into a plurality of cells; (b) detecting which of the pixels in each cell are of a specific color; (c) compressing information specifying the location of said pixels; (d) replacing said specific color in said pixels with a different color; and (e) compressing said cells by JPEG compression. 38. A method of compressing an image as claimed in claim 37, said method being used in a printing system having a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles; (c) a pressurizing device adapted to subject ink in said body of ink to a pressure of at least 2% above ambient pressure, at least during drop selection and separation to form a meniscus with an air/ink interface; (d) drop selection apparatus operable upon the air/ink interface to select predetermined nozzles and to generate a difference in meniscus position between ink in selected and non-selected nozzles; and (e) drop separation apparatus adapted to cause ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles. 39. A method of compressing an image as claimed in claim 37, said method being used in a printing system having a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles, said body of ink forming a meniscus with an air/ink interface at each nozzle; (c) drop selection apparatus operable upon the air/ink interface to select predetermined nozzles and to generate a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separation apparatus adapted to cause ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles, said drop selection apparatus being capable of producing said difference in meniscus position in the absence of said drop separation apparatus. 40. A method of compressing an image as claimed in claim 37, said method being used in a printing system having a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles, said body of ink forming a meniscus with an air/ink interface at each nozzle and said ink exhibiting a surface tension decrease of at least 10 mN/m over a 30.degree. C. temperature range; (c) drop selection apparatus operable upon the air/ink interface to select predetermined nozzles and to generate a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separation apparatus adapted to cause ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles. 41. A method of compressing an image as claimed in claim 37 wherein said specific color is black. 42. A method of compressing an image as claimed in claim 37 wherein said specific color is determined by a color specification which is included in the compressed data. 43. A method of compressing an image as claimed in claim 37 wherein said specific color in said pixels is replaced by a color which is the average color of all of the pixels in said cell which are not the specific color. 44. A method of compressing an image as claimed in claim 37 wherein said specific color in said pixels is replaced by a color which is the weighted average of all of the pixels which are not the specific color and which are within a specific radius of the pixel in which the color is being replaced. 45. A method of compressing an image as claimed in claim 37 wherein said specific color in said pixels is replaced by a color which is the first color found which is not said specific color when following a search path from the pixel in which the color is being replaced. 46. A method of compressing an image as claimed in claim 37 wherein said cells of pixels are rectangular and the number of pixels in said cells is an integral multiple of eight in one dimension, and a further integral multiple of eight in the perpendicular dimension, wherein said multiples of eight may be identical or may be different. 47. A method of compressing an image as claimed in claim 46 wherein the number of pixels in said cells is 24 in both dimensions. 48. A method of compressing an image as claimed in claim 37 wherein the pixels in said cells are subsampled to form 8.times.8 JPEG cells prior to JPEG encoding. 49. A method of compressing an image as claimed in claim 37 wherein the colors of pixels in 64 subsets of each of said cells are mathematically combined to form each pixel of 8.times.8 JPEG cells. 50. A compressed image expansion apparatus comprising: (a) a JPEG decoder which converts first compressed image data to contone pixel data at a first resolution; (b) a resolution conversion device which converts said first contone pixel data to a second contone pixel data of a second resolution, said second resolution being different from said first resolution; (c) a compressed bitmap decoder which converts second compressed image data to bitmap image data at said second resolution; (d) creation means to create a third contone pixel data at said second resolution, based upon the pattern of true and false entries in said bitmap image data; and (e) a combination means which combines said second contone pixel data with said third contone pixel data to produce a fourth contone pixel data. 51. A compressed image expansion apparatus as claimed in claim 50, said expansion apparatus being used to expand image data for a printing system having a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles; (c) a pressurizing device adapted to subject ink in said body of ink to a pressure of at least 2% above ambient pressure, at least during drop selection and separation to form a meniscus with an air/ink interface; (d) drop selection apparatus operable upon the air/ink interface to select predetermined nozzles and to generate a difference in meniscus position between ink in selected and non-selected nozzles; and (e) drop separation apparatus adapted to cause ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles. 52. A compressed image expansion apparatus as claimed in claim 50, said expansion apparatus being used to expand image data for a printing system having a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles said body of ink forming a meniscus with an air/ink interface at each nozzle; (c) drop selection apparatus operable upon the air/ink interface to select predetermined nozzles and to generate a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separation apparatus adapted to cause ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles, said drop selection apparatus being capable of producing said difference in meniscus position in the absence of said drop separation apparatus. 53. A compressed image expansion apparatus as claimed in claim 50, said expansion apparatus being used to expand image data for a printing system having a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles, said body of ink forming a meniscus with an air/ink interface at each nozzle and said ink exhibiting a surface tension decrease of at least 10 mN/m over a 30.degree. C. temperature range; (c) drop selection apparatus operable upon the air/ink interface to select predetermined nozzles and to generate a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separation apparatus adapted to cause ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles. 54. A compressed image expansion apparatus as claimed in claim 50 wherein said second resolution is an integral multiple of said first resolution in at least one dimension. 55. A compressed image expansion apparatus as claimed in claim 50 wherein said second resolution is the same integral multiple of said first resolution in both dimensions. 56. A compressed image expansion apparatus as claimed in claim 50 wherein the conversion method employed by said resolution conversion device to convert said first contone pixel data to said second resolution is pixel replication. 57. A compressed image expansion apparatus as claimed in claim 50 wherein the conversion method employed by said resolution conversion device to convert said first contone pixel data to said second resolution is bi-linear interpolation. 58. A compressed image expansion apparatus as claimed in claim 50 wherein the conversion method employed by said resolution conversion device to convert said first contone pixel data to said second resolution is bi-cubic interpolation. 59. A compressed image expansion apparatus as claimed in claim 50 wherein said compressed bitmap decoder includes a memory which contains a set of bitmap patterns which are substituted into said bitmap image data at said second resolution when codes specifying a member of said set of bitmap patterns is encountered in said second compressed image data. 60. A compressed image expansion apparatus as claimed in claim 50 which also includes a halftoning means which halftones said fourth contone pixel data to produce bi-level pixel data. 61. A compressed image expansion apparatus as claimed in claim 60 where said halftoning means operates by dithering. 62. A compressed image expansion apparatus as claimed in claim 60 where said halftoning means operates by error diffusion. 63. A compressed image expansion apparatus as claimed in claim 60 where said halftoning means operates by dispersed dot ordered dithering. 64. A compressed image expansion apparatus as claimed in claim 60 where said halftoning means is included on the same integrated circuit as said resolution conversion device and said compressed bitmap decoder. FIELD OF THE INVENTION The present invention is in the field of computer controlled printing devices. In particular, the field is data compression and expansion systems for high resolution color printing BACKGROUND OF THE INVENTION Many different types of digitally controlled printing systems have been invented, and many types are currently in production. These printing systems use a variety of actuation mechanisms, a variety of marking materials, and a variety of recording media. Examples of digital printing systems in current use include: laser electrophotographic printers; LED electrophotographic printers; dot matrix impact printers; thermal paper printers; film recorders; thermal wax printers; dye diffusion thermal transfer printers; and ink jet printers. However, at present, such electronic printing systems have not significantly replaced mechanical printing presses, even though this conventional method requires very expensive setup and is seldom commercially viable unless a few thousand copies of a particular page are to be printed. Thus, there is a need for improved digitally controlled printing systems, for example, being able to produce high quality color images at a high-speed and low cost, using standard paper. Inkjet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because, e.g., of its non-impact, low-noise characteristics, its use of plain paper and its avoidance of toner transfers and fixing. Many types of ink jet printing mechanisms have been invented. These can be categorized as either continuous ink jet (CIJ) or drop on demand (DOD) ink jet. Continuous ink jet printing dates back to at least 1929: Hansell, U.S. Pat. No. 1,941,001. Sweet et al U.S. Pat. No. 3,373,437, 1967, discloses an array of continuous ink jet nozzles where ink drops to be printed are selectively charged and deflected towards the recording medium. This technique is known as binary deflection CIJ, and is used by several manufacturers, including Elmjet and Scitex. Hertz et al U.S. Pat. No. 3,416,153, 1966, discloses a method of achieving variable optical density of printed spots in CIJ printing using the electrostatic dispersion of a charged drop stream to modulate the number of droplets which pass through a small aperture. This technique is used in ink jet printers manufactured by Iris Graphics. Kyser et al U.S. Pat. No. 3,946,398, 1970, discloses a DOD ink jet printer which applies a high voltage to a piezoelectric crystal, causing the crystal to bend, applying pressure on an ink reservoir and jetting drops on demand. Many types of piezoelectric drop on demand printers have subsequently been invented, which utilize piezoelectric crystals in bend mode, push mode, shear mode, and squeeze mode. Piezoelectric DOD printers have achieved commercial success using hot melt inks (for example, Tektronix and Dataproducts printers), and at image resolutions up to 720 dpi for home and office printers (Seiko Epson). Piezoelectric DOD printers have an advantage in being able to use a wide range of inks. However, piezoelectric printing mechanisms usually require complex high voltage drive circuitry and bulky piezoelectric crystal arrays, which are disadvantageous in regard to manufacturability and performance. Endo et al GB Pat. No. 2,007,162, 1979, discloses an electrothermal DOD ink jet printer which applies a power pulse to an electrothermal transducer (heater) which is in thermal contact with ink in a nozzle. The heater rapidly heats water based ink to a high temperature, whereupon a small quantity of ink rapidly evaporates, forming a bubble. The formation of these bubbles results in a pressure wave which cause drops of ink to be ejected from small apertures along the edge of the heater substrate. This technology is known as Bubblejet.TM. (trademark of Canon K.K. of Japan), and is used in a wide range of printing systems from Canon, Xerox, and other manufacturers. Vaught et al U.S. Pat. No. 4,490,728, 1982, discloses an electrothermal drop ejection system which also operates by bubble formation. In this system, drops are ejected in a direction normal to the plane of the heater substrate, through nozzles formed in an aperture plate positioned above the heater. This system is known as Thermal Ink Jet, and is manufactured by Hewlett-Packard. In this document, the term Thermal Ink Jet is used to refer to both the Hewlett-Packard system and systems commonly known as Bubblejet.TM.. Thermal Ink Jet printing typically requires approximately 20 .mu.J over a period of approximately 2 .mu.s to eject each drop. The 10 Watt active power consumption of each heater is disadvantageous in itself and also necessitates special inks, complicates the driver electronics and precipitates deterioration of heater elements. Other ink jet printing systems have also been described in technical literature, but are not currently used on a commercial basis. For example, U.S. Pat. No. 4,275,290 discloses a system wherein the coincident address of predetermined print head nozzles with heat pulses and hydrostatic pressure, allows ink to flow freely to spacer-separated paper, passing beneath the print head. U.S. Pat. Nos. 4,737,803; 4,737,803 and 4,748,458 disclose ink jet recording systems wherein the coincident address of ink in print head nozzles with heat pulses and an electrostatically attractive field cause ejection of ink drops to a print sheet. Each of the above-described inkjet printing systems has advantages and disadvantages. However, there remains a widely recognized need for an improved ink jet printing approach, providing advantages for example, as to cost, speed, quality, reliability, power usage, simplicity of construction and operation, durability and consumables. SUMMARY OF THE INVENTION My concurrently filed applications, entitled "Liquid Ink Printing Apparatus and System" and "Coincident Drop-Selection, Drop-Separation Printing Method and System" describe new methods and apparatus that afford significant improvements toward overcoming the prior art problems discussed above. Those inventions offer important advantages, e.g., in regard to drop size and placement accuracy, as to printing speeds attainable, as to power usage, as to durability and operative thermal stresses encountered and as to other printer performance characteristics, as well as in regard to manufacturability and the characteristics of useful inks. One important purpose of the present invention is to further enhance the structures and methods described in those applications and thereby contribute to the advancement of printing technology. The invention provides a high capacity compressed document image storage apparatus including: (a) an image creation system which can operate on a band by band basis; (b) an input memory which stores at least one band of the uncompressed page image, but less than 50% of said uncompressed page image; (c) a page image compression system which can operate on a band by band basis; (d) a mass storage device able to store a plurality of compressed page images; (e) a page image expansion system which expands compressed page image data to form an expanded page image at a rate which is within 20% of the page printing rate when measured over the duration of the page printing time; (f) an output memory which stores at least one band of said expanded page image, but less than 50% of said expanded page image; and (g) an apparatus which provides page image data from said output memory to a printing device wherein the time taken to provide a band of image data to the printing device does not vary by more than .+-.50% of the mean time to provide a band of image data to the printing device. The invention also provides a method of page image compression including the following steps: (a) dividing said page image into an array of cells of pixels; (b) encoding said cells containing only white pixels to a specific code; (c) encoding said cells containing only black and white pixels to a different specific code, and supplementing said code with information encoding the pattern of black and white pixels in said cell; and (d) encoding said cells containing a plurality of colors to a mutually different code, and supplementing said code with information encoding the colors of the pixels in said cell. The invention also provides a printing system incorporating a page image compression system, where the printing system includes a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles; (c) pressure means for subjecting ink in said body of ink to a pressure of at least 2% above ambient pressure, at least during drop selection and separation; (d) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (e) drop separating means for causing ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles. The invention also provides a printing system incorporating a page image compression system, where the printing system includes a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles; (c) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separating means for causing ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles, said drop selecting means being capable of producing said difference in meniscus position in the absence of said drop separation means. The invention also provides a printing system incorporating a page image compression system, where the printing system includes a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles, said ink exhibiting a surface tension decrease of at least 10 mN/m over a 30.degree. C. temperature range; (c) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separating means for causing ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles. The invention also provides an expansion apparatus for expanding compressed page image data in real-time as the page is being printed, said apparatus including: (a) a mass storage unit for storage of the compressed page images; (b) a JPEG decoder which expands the JPEG encoded portions of the compressed page image into contone image data; (c) a contone band memory which temporarily holds at least one band of said contone image data; (d) an expander circuit which expands compressed black and white text and graphics bitmaps and combines said bitmaps with said contone image data read from said contone band memory. The invention also provides a method of compressing an image which includes the steps of: (a) dividing the image into a plurality of cells; (b) detecting which of the pixels in each cell are of a specific color; (c) compressing information specifying the location of said pixels; (d) replacing said specific color in said pixels with a different color; and (e) compressing said cells by JPEG compression. The invention also provides a compressed image expansion apparatus comprising: (a) a JPEG decoder which converts first compressed image data to contone pixel data at a first resolution; (b) a resolution conversion device which converts said first contone pixel data to a second contone pixel data of a second resolution, said second resolution being different from said first resolution; (c) a compressed bitmap decoder which converts second compressed image data to bitmap image data at said second resolution; (d) creation means to create a third contone pixel data at said second resolution, based upon the pattern of true and false entries in said bitmap image data; and (e) a combination means which combines said second contone pixel data with said third contone pixel data to produce a fourth contone pixel data. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1(a) shows a simplified block schematic diagram of one exemplary printing apparatus according to the present invention. FIG. 1(b) shows a cross section of one variety of nozzle tip in accordance with the invention. FIGS. 2(a) to 2(f) show fluid dynamic simulations of drop selection. FIG. 3(a) shows a finite element fluid dynamic simulation of a nozzle in operation according to an embodiment of the invention. FIG. 3(b) shows successive meniscus positions during drop selection and separation. FIG. 3(c) shows the temperatures at various points during a drop selection cycle. FIG. 3(d) shows measured surface tension versus temperature curves for various ink additives. FIG. 3(e) shows the power pulses which are applied to the nozzle heater to generate the temperature curves of FIG. 3(c) FIG. 4 shows a block schematic diagram of print head drive circuitry for practice of the invention. FIG. 5 shows projected manufacturing yields for an A4 page width color print head embodying features of the invention, with and without fault tolerance. FIG. 6 is a block schematic diagram of a typical current office or home printing system. FIG. 7 is a block schematic diagram of a typical current commercial printing system. FIG. 8 is a block schematic diagram of a typical office printing system using page image compression for electronic collation and print on demand digital page storage. FIG. 9 is a diagram representing the data encoding scheme of the page image compression system. FIGS. 10(a) and 10(b) are an example set of patterns to which subcells may be matched for encoding. FIG. 11 is a block schematic diagram of a real-time page image expansion system for high speed color printing. FIG. 12 is a block schematic diagram of an ASIC to implement real-time expansion of compressed page images. FIG. 13 is a block schematic diagram of the expansion section of the ASIC shown in FIG. 12. FIG. 14 is a block schematic diagram of the halftoning section of the ASIC shown in FIG. 12. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The present invention is a high capacity compressed document image storage apparatus for color printing systems. The compressed document storage system is highly suited for use with high speed color pagewidth print heads. However, the document storage apparatus can also be used with other printing systems, such as color electrophotographic and other systems. The compressed document storage system uses a new compression scheme which is described herein. This page image compression scheme can achieve a typical compression ratio in excess of 80:1 when compared to a CC'MM'YK 800 dpi bitmap, and in excess of 420:1 when compared to an 800 dpi CMYK contone image. This compression is achieved while maintaining a high image quality which is largely indistinguishable from the original image with the naked eye. The high compression ratio allows the storage of more than 1,000 A4 800 dpi full color page images on a low cost 1 GByte disk drive. The compression scheme reduces the compressed data rate required for high speed (120 A4 ppm) printing to that which can be achieved using commonly available magnetic hard disk drives. The compressed document storage system can be used for electronic collation. Electronic collation is relevant when multiple copies of a multiple page document are printed. A system without electronic collation will print all copies of the first page, followed by all copies of the second page, and so on. This requires either manual collation, which is inconvenient and time consuming for the user, or mechanical collation, which is bulky, expensive, and normally limited to a maximum number of copies (typically 10 or 20 copies). The compressed document storage system can also be used for print-on-demand applications, where compressed digital copies of various documents are stored locally to the printer, and can be printed when required. The compressed document storage system is composed of the following major components: 1) an image creation system which preferably can operate on a band by band basis. This will usually be a Adobe Postscript' interpreter and Raster Image Processor (RIP). 2) an input memory which stores at least one band of the uncompressed page image. Two bands are preferred to allow double buffering, so that the RIP and compression stages can proceed simultaneously. 3) a page image compression system which can operate on a band by band basis. A new page image compression scheme which achieves high compression ratio and preserves high image quality is described herein. 4) a mass storage device for storing compressed page images. A 1 GByte HDD can be used to store 1,000 or more pages. 5) a real-time page image expansion system which expands the compressed page images at the rate required by the print engine. 6) an output memory which stores at least one band of the expanded page image. The recommended minimum is the number of lines from the first line of the print engine to the last line of the print engine, plus two bands. 7) an interface which reads data from the output memory and provides it at the correct timing to the print engine. The compression scheme can operate on a sequence of page bands, allowing the use of band by band rasterization and avoiding the requirement for a full page memory of uncompressed information. The compression scheme is also suited for real-time expansion for high speed color printing systems. A real-time expansion system capable of expanding 120 pages per minute is disclosed. The clock to the print head 50 is generated from the system clock 408 by the Head clock generator 407, and buffered by the buffer 406. To facilitate testing of the Head control ASIC, JTAG test circuits 499 may be included. In one general aspect, the invention constitutes a drop-on-demand printing mechanism wherein the means of selecting drops to be printed produces a difference in position between selected drops and drops which are not selected, but which is insufficient to cause the ink drops to overcome the ink surface tension and separate from the body of ink, and wherein an alternative means is provided to cause separation of the selected drops from the body of ink. The separation of drop selection means from drop separation means significantly reduces the energy required to select which ink drops are to be printed. Only the drop selection means must be driven by individual signals to each nozzle. The drop separation means can be a field or condition applied simultaneously to all nozzles. The drop selection means may be chosen from, but is not limited to, the following list: 1) Electrothermal reduction of surface tension of pressurized ink 2) Electrothermal bubble generation, with insufficient bubble volume to cause drop ejection 3) Piezoelectric, with insufficient volume change to cause drop ejection 4) Electrostatic attraction with one electrode per nozzle The drop separation means may be chosen from, but is not limited to, the following list: 1) Proximity (recording medium in close proximity to print head) 2) Proximity with oscillating ink pressure 3) Electrostatic attraction 4) Magnetic attraction The table "DOD printing technology targets" shows some desirable characteristics of drop on demand printing technology. The table also lists some methods by which some embodiments described herein, or in other of my related applications, provide improvements over the prior art. ______________________________________ DOD printing technology targets Target Method of achieving improvement over prior art ______________________________________ High speed Practical, low cost, pagewidth printing heads with operation more than 10,000 nozzles. Monolithic A4 pagewidth print heads can be manufactured using standard 300 mm (12") silicon wafers High image High resolution (800 dpi is sufficient for most quality applications), six color process to reduce image noise Full color Halftoned process color at 800 dpi using stochastic operation screening Ink flexibility Low operating ink temperature and no requirement for bubble formation Low power Low power operation results from drop selection requirements measn not being required to fully eject drop Low cost Monohlithic print head without aperture plate, high manufacturing yield, small number of electrical connections, use of modified existing CMOS manufacturing facilities High manufac- Integrated fault tolerance in printing head turing yield High reliability Integrated fault tolerance in printing head. Elimination of cavitation and kogation. Reduction of thermal shock. Small number of Shift registers, control logic, and drive circuitry can be electrical integrated on a monolithic print head using standard connections CMOS processes Use of existing CMOS compatibility. This can be achieved because VLSI manufac- the heater drive power is less is than 1% of Thermal turing facilities Ink Jet heater drive power Electronic A new page compression system which can achieve collation 100:1 compression with insignificant image degradation, resulting in a compressed data rate low enough to allow real-time printing of any combination of thousands of pages stored on a low cost magnetic disk drive. ______________________________________ In thermal ink jet (TIJ) and piezoelectric ink jet systems, a drop velocity of approximately 10 meters per second is preferred to ensure that the selected ink drops overcome ink surface tension, separate from the body of the ink, and strike the recording medium. These systems have a very low efficiency of conversion of electrical energy into drop kinetic energy. The efficiency of TIJ systems is approximately 0.02%). This means that the drive circuits for TIJ print heads must switch high currents. The drive circuits for piezoelectric ink jet heads must either switch high voltages, or drive highly capacitive loads. The total power consumption of pagewidth TIJ printheads is also very high. An 800 dpi A4 full color pagewidth TIJ print head printing a four color black image in one second would consume approximately 6 kW of electrical power, most of which is converted to waste heat. The difficulties of removal of this amount of heat precludes the production of low cost, high speed, high resolution compact pagewidth TIJ systems. One important feature of embodiments of the invention is a means of significantly reducing the energy required to select which ink drops are to be printed. This is achieved by separating the means for selecting ink drops from the means for ensuring that selected drops separate from the body of ink and form dots on the recording medium. Only the drop selection means must be driven by individual signals to each nozzle. The drop separation means can be a field or condition applied simultaneously to all nozzles. The table "Drop selection means" shows some of the possible means for selecting drops in accordance with the invention. The drop selection means is only required to create sufficient change in the position of selected drops that the drop separation means can discriminate between selected and unselected drops. __________________________________________________________________________ Drop selection means Method Advantage Limitation __________________________________________________________________________ 1. Electrothermal Low temperature Requires ink pressure reduction of surface increase and low drop regulating mechanism. Ink tension of selection energy. Can be surface tension must reduce pressurized ink used with many ink substantially as temperature types. Simple fabrication. increases CMOS drive circuits can be fabricated on same substrate 2. Electrothermal Medium drop selection Requires ink pressure reduction of ink energy, suitable for hot oscillation mechanism. Ink viscosity, combined melt and oil based inks. must have a large decrease with oscillating ink Simple fabrication. in viscosity as temperature pressure CMOS drive circuits can increases be fabricated on same substrate 3. Electrothermal Well known technology, High drop selection energy, bubble generation, sinrple fabrication, requires water based ink, with insufficient bipolar drive circuits can problems with kogation, bubble volume to be fabricated on same cavitation, thermal stress cause drop ejection substrate 4. Piezoelectric, with Many types of ink base High manufacturing cost, insufficient volume can be used incompatible with change to cause drop integrated circuit processes, ejection high drive voltage, mechanical complexity, bulky 5. Electrostatic Simple electrode Nozzk pitch must be attraction with one fabrication relatively large. Crosstalk electrode per nozzle between adjacent electric fields. Requires high voltage drive circuits __________________________________________________________________________ Other drop selection means may also be used. The preferred drop selection means for water based inks is method 1: "Electrothermal reduction of surface tension of pressurized ink". This drop selection means provides many advantages over other systems, including; low power operation (approximately 1% of TIJ), compatibility with CMOS VLSI chip fabrication, low voltage operation (approx. 10 V), high nozzle density, low temperature operation, and wide range of suitable ink formulations. The ink must exhibit a reduction in surface tension with increasing temperature. The preferred drop selection means for hot melt or oil based inks is method 2: "Electrothermal reduction of ink viscosity, combined with oscillating ink pressure". This drop selection means is particularly suited for use with inks which exhibit a large reduction of viscosity with increasing temperature, but only a small reduction in surface tension. This occurs particularly with non-polar ink carriers with relatively high molecular weight. This is especially applicable to hot melt and oil based inks. The table "Drop separation means" shows some of the possible methods for separating selected drops from the body of ink, and ensuring that the selected drops form dots on the printing medium. The drop separation means discriminates between selected drops and unselected drops to ensure that unselected drops do not form dots on the printing medium. __________________________________________________________________________ Drop separation means Method Advantage Limitation __________________________________________________________________________ 1. Electrostatic Can print on rough Requires high voltage attraction surfaces, simple power supply implementation 2. AC electric field Higher field strength is Requires high voltage AC possible than electrostatic, power supply synchronized operating margins can be to drop ejection phase. increased, ink pressure Multiple drop phase reduced, and dust operation is difficult accumulation is reduced 3. Proximity Very small spot sizes can Requires print medium to be (print head in close be achieved. Very low very close to print head proximity to, but power dissipation. High surface, not suitable for not touching, drop position accuracy rough print media, usually recording medium) requires transfer roller or belt 4. Transfer Very small spot sizes can Not compact due to size of Proximity (print be achieved, very low transfer roller or transfer head is in close power dissipation, high belt. proximity to a accuracy, can print on transfer roller or rough paper belt 5. Proximity with Useful for hot melt inks Requires print medium to be oscillating ink using viscosity reduction very close to print head pressure. drop selection method, surface, not suitable for reduces possibility of rough print media. Requires nozzle clogging, can use ink pressure oscillation pigments instead of dyes apparatus 6. Magnetic Can print on rough Requires uniform high attraction surfaces. Low power if magnetic field strength, permanent magnets are requires magnetic ink used __________________________________________________________________________ Other drop separation means may also be used. The preferred drop separation means depends upon the intended use. For most applications, method 1: "Electrostatic attraction", or method 2: "AC electric field" are most appropriate. For applications where smooth coated paper or film is used, and very high speed is not essential, method 3: "Proximity" may be appropriate. For high speed, high quality systems, method 4: "Transfer proximity" can be used. Method 6: "Magnetic attraction" is appropriate for portable printing systems where the print medium is too rough for proximity printing, and the high voltages required for electrostatic drop separation are undesirable. There is no clear 'best' drop separation means which is applicable to all circumstances. Further details of various types of printing systems according to the present invention are described in the following Australian patent specifications filed on Apr. 12, 1995, the disclosure of which are hereby incorporated by reference: 'A Liquid ink Fault Tolerant (LIFT) printing mechanism' (Filing no.: PN2308); 'Electrothermal drop selection in LIFT printing' (Filing no.: PN2309); 'Drop separation in LIFT printing by print media proximity' (Filing no.: PN2310); 'Drop size adjustment in Proximity LIFT printing by varying head to media distance' (Filing no.: PN2311); 'Augmenting Proximity LIFT printing with acoustic ink waves' (Filing no.: PN2312); 'Electrostatic drop separation in LIFT printing' (Filing no.: PN2313); 'Multiple simultaneous drop sizes in Proximity LIFT printing' (Filing no.: PN2321); 'Self cooling operation in thermally activated print heads' (Filing no.: PN2322); and 'Thermal Viscosity Reduction LIFT printing' (Filing no.: PN2323). A simplified schematic diagram of one preferred printing system according to the invention appears in FIG. 1(a). An image source 52 may be raster image data from a scanner or computer, or outline image data in the form of a page description language (PDL), or other forms of digital image representation. This image data is converted to a pixel-mapped page image by the image processing system 53. This may be a raster image processor (RIP) in the case of PDL image data, or may be pixel image manipulation in the case of raster image data. Continuous tone data produced by the image processing unit 53 is halftoned. Halftoning is performed by the Digital Halftoning unit 54. Halftoned bitmap image data is stored in the image memory 72. Depending upon the printer and system configuration, the image memory 72 may be a full page memory, or a band memory. Heater control circuits 71 read data from the image memory 72 and apply time-varying electrical pulses to the nozzle heaters (103 in FIG. 1(b)) that are part of the print head 50. These pulses are applied at an appropriate time, and to the appropriate nozzle, so that selected drops will form spots on the recording medium 51 in the appropriate position designated by the data in the image memory 72. The recording medium 51 is moved relative to the head 50 by a paper transport system 65, which is electronically controlled by a paper transport control system 66, which in turn is controlled by a microcontroller 315. The paper transport system shown in FIG. 1(a) is schematic only, and many different mechanical configurations are possible. In the case of pagewidth print heads, it is most convenient to move the recording medium 51 past a stationary head 50. However, in the case of scanning print systems, it is usually most convenient to move the head 50 along one axis (the sub-scanning direction) and the recording medium 51 along the orthogonal axis (the main scanning direction), in a relative raster motion. The microcontroller 315 may also control the ink pressure regulator 63 and the heater control circuits 71. For printing using surface tension reduction, ink is contained in an ink reservoir 64 under pressure. In the quiescent state (with no ink drop ejected), the ink pressure is insufficient to overcome the ink surface tension and eject a drop. A constant ink pressure can be achieved by applying pressure to the ink reservoir 64 under the control of an ink pressure regulator 63. Alternatively, for larger printing systems, the ink pressure can be very accurately generated and controlled by situating the top surface of the ink in the reservoir 64 an appropriate distance above the head 50. This ink level can be regulated by a simple float valve (not shown). For printing using viscosity reduction, ink is contained in an ink reservoir 64 under pressure, and the ink pressure is caused to oscillate. The means of producing this oscillation may be a piezoelectric actuator mounted in the ink channels (not shown). When properly arranged with the drop separation means, selected drops proceed to form spots on the recording medium 51, while unselected drops remain part of the body of ink. The ink is distributed to the back surface of the head 50 by an ink channel device 75. The ink preferably flows through slots and/or holes etched through the silicon substrate of the head 50 to the front surface, where the nozzles and actuators are situated. In the case of thermal selection, the nozzle actuators are electrothermal heaters. In some types of printers according to the invention, an external field 74 is required to ensure that the selected drop separates from the body of the ink and moves towards the recording medium 51. A convenient external field 74 is a constant electric field, as the ink is easily made to be electrically conductive. In this case, the paper guide or platen 67 can be made of electrically conductive material and used as one electrode generating the electric field. The other electrode can be the head 50 itself. Another embodiment uses proximity of the print medium as a means of discriminating between selected drops and unselected drops. For small drop sizes gravitational force on the ink drop is very small; approximately 10.sup.-4 of the surface tension forces, so gravity can be ignored in most cases. This allows the print head 50 and recording medium 51 to be oriented in any direction in relation to the local gravitational field. This is an important requirement for portable printers. FIG. 1(b) is a detail enlargement of a cross section of a single microscopic nozzle tip embodiment of the invention, fabricated using a modified CMOS process. The nozzle is etched in a substrate 101, which may be silicon, glass, metal, or any other suitable material. If substrates which are not semiconductor materials are used, a semiconducting material (such as amorphous silicon) may be deposited on the substrate, and integrated drive transistors and data distribution circuitry may be formed in the surface semiconducting layer. Single crystal silicon (SCS) substrates have several advantages, including: 1) High performance drive transistors and other circuitry can be fabricated in SCS; 2) Print heads can be fabricated in existing facilities (fabs) using standard VLSI processing equipment; 3) SCS has high mechanical strength and rigidity; and 4) SCS has a high thermal conductivity. In this example, the nozzle is of cylindrical form, with the heater 103 forming an annulus. The nozzle tip 104 is formed from silicon dioxide layers 102 deposited during the fabrication of the CMOS drive circuitry. The nozzle tip is passivated with silicon nitride. The protruding nozzle tip controls the contact point of the pressurized ink 100 on the print head surface. The print head surface is also hydrophobized to prevent accidental spread of ink across the front of the print head. Many other configurations of nozzles are possible, and nozzle embodiments of the invention may vary in shape, dimensions, and materials used. Monolithic nozzles etched from the substrate upon which the heater and drive electronics are formed have the advantage of not requiring an orifice plate. The elimination of the orifice plate has significant cost savings in manufacture and assembly. Recent methods for eliminating orifice plates include the use of 'vortex' actuators such as those described in Domoto et al U.S. Pat. No. 4,580,158, 1986, assigned to Xerox, and Miller et al U.S. Pat. No. 5,371,527, 1994 assigned to Hewlett-Packard. These, however are complex to actuate, and difficult to fabricate. The preferred method for elimination of orifice plates for print heads of the invention is incorporation of the orifice into the actuator substrate. This type of nozzle may be used for print heads using various techniques for drop separation. Operation with Electrostatic Drop Separation As a first example, operation using thermal reduction of surface tension and electrostatic drop separation is shown in FIG. 2. FIG. 2 shows the results of energy transport and fluid dynamic simulations performed using FIDAP, a commercial fluid dynamic simulation software package available from Fluid Dynamics Inc., of Illinois, USA. This simulation is of a thermal drop selection nozzle embodiment with a diameter of 8 .mu.m, at an ambient temperature of 30.degree. C. The total energy applied to the heater is 276 nJ, applied as 69 pulses of 4 nJ each. The ink pressure is 10 kPa above ambient air pressure, and the ink viscosity at 30.degree. C. is 1.84 cPs. The ink is water based, and includes a sol of 0.1% palmitic acid to achieve an enhanced decrease in surface tension with increasing temperature. A cross section of the nozzle tip from the central axis of the nozzle to a radial distance of 40 .mu.m is shown. Heat flow in the various materials of the nozzle, including silicon, silicon nitride, amorphous silicon dioxide, crystalline silicon dioxide, and water based ink are simulated using the respective densities, heat capacities, and thermal conductivities of the materials. The time step of the simulation is 0.1 .mu.s. FIG. 2(a) shows a quiescent state, just before the heater is actuated. An equilibrium is created whereby no ink escapes the nozzle in the quiescent state by ensuring that the ink pressure plus external electrostatic field is insufficient to overcome the surface tension of the ink at the ambient temperature. In the quiescent state, the meniscus of the ink does not protrude significantly from the print head surface, so the electrostatic field is not significantly concentrated at the meniscus. FIG. 2(b) shows thermal contours at 5.degree. C. intervals 5 .mu.s after the start of the heater energizing pulse. When the heater is energized, the ink in contact with the nozzle tip is rapidly heated. The reduction in surface tension causes the heated portion of the meniscus to rapidly expand relative to the cool ink meniscus. This drives a convective flow which rapidly transports this heat over part of the free surface of the ink at the nozzle tip. It is necessary for the heat to be distributed over the ink surface, and not just where the ink is in contact with the heater. This is because viscous drag against the solid heater prevents the ink directly in contact with the heater from moving. FIG. 2(c) shows thermal contours at 5.degree. C. intervals 10 .mu.s after the start of the heater energizing pulse. The increase in temperature causes a decrease in surface tension, disturbing the equilibrium of forces. As the entire meniscus has been heated, the ink begins to flow. FIG. 2(d) shows thermal contours at 5.degree. C. intervals 20 .mu.s after the start of the heater energizing pulse. The ink pressure has caused the ink to flow to a new meniscus position, which protrudes from the print head. The electrostatic field becomes concentrated by the protruding conductive ink drop. FIG. 2(e) shows thermal contours at 5.degree. C. intervals 30 .mu.s after the start of the heater energizing pulse, which is also 6 .mu.s after the end of the heater pulse, as the heater pulse duration is 24 .mu.s. The nozzle tip has rapidly cooled due to conduction through the oxide layers, and conduction into the flowing ink. The nozzle tip is effectively 'water cooled' by the ink. Electrostatic attraction causes the ink drop to begin to accelerate towards the recording medium. Were the heater pulse significantly shorter (less than 16 .mu.s in this case) the ink would not accelerate towards the print medium, but would instead return to the nozzle. FIG. 2(f) shows thermal contours at 5.degree. C. intervals 26 .mu.s after the end of the heater pulse. The temperature at the nozzle tip is now less than 5.degree. C. above ambient temperature. This causes an increase in surface tension around the nozzle tip. When the rate at which the ink is drawn from the nozzle exceeds the viscously limited rate of ink flow through the nozzle, the ink in the region of the nozzle tip 'necks', and the selected drop separates from the body of ink. The selected drop then travels to the recording medium under the influence of the external electrostatic field. The meniscus of the ink at the nozzle tip then returns to its quiescent position, ready for the next heat pulse to select the next ink drop. One ink drop is selected, separated and forms a spot on the recording medium for each heat pulse. As the heat pulses are electrically controlled, drop on demand ink jet operation can be achieved. FIG. 3(a) shows successive meniscus positions during the drop selection cycle at 5 .mu.s intervals, starting at the beginning of the heater energizing pulse. FIG. 3(b) is a graph of meniscus position versus time, showing the movement of the point at the centre of the meniscus. The heater pulse starts 10 .mu.s into the simulation. FIG. 3(c) shows the resultant curve of temperature with respect to time at various points in the nozzle. The vertical axis of the graph is temperature, in units of 100.degree. C. The horizontal axis of the graph is time, in units of 10 .mu.s. The temperature curve shown in FIG. 3(b) was calculated by FIDAP, using 0.1 .mu.s time steps. The local ambient temperature is 30 degrees C. Temperature histories at three points are shown: A--Nozzle tip: This shows the temperature history at the circle of contact between the passivation layer, the ink, and air. B--Meniscus midpoint: This is at a circle on the ink meniscus midway between the nozzle tip and the centre of the meniscus. C--Chip surface: This is at a point on the print head surface 20 .mu.m from the centre of the nozzle. The temperature only rises a few degrees. This indicates that active circuitry can be located very close to the nozzles without experiencing performance or lifetime degradation due to elevated temperatures. FIG. 3(e) shows the power applied to the heater. Optimum operation requires a sharp rise in temperature at the start of the heater pulse, a maintenance of the temperature a little below the boiling point of the ink for the duration of the pulse, and a rapid fall in temperature at the end of the pulse. To achieve this, the average energy applied to the heater is varied over the duration of the pulse. In this case, the variation is achieved by pulse frequency modulation of 0.1 .mu.s sub-pulses, each with an energy of 4 nJ. The peak power applied to the heater is 40 mW, and the average power over the duration of the heater pulse is 11.5 mW. The sub-pulse frequency in this case is 5 Mhz. This can readily be varied without significantly affecting the operation of the print head. A higher sub-pulse frequency allows finer control over the power applied to the heater. A sub-pulse frequency of 13.5 Mhz is suitable, as this frequency is also suitable for minimizing the effect of radio frequency interference (RFI). Inks With a Negative Temperature Coefficient of Surface Tension The requirement for the surface tension of the ink to decrease with increasing temperature is not a major restriction, as most pure liquids and many mixtures have this property. Exact equations relating surface tension to temperature for arbitrary liquids are not available. However, the following empirical equation derived by Ramsay and Shields is satisfactory for many liquids: ##EQU1## Where .gamma..sub.T is the surface tension at temperature T, k is a constant, T.sub.c is the critical temperature of the liquid, M is the molar mass of the liquid, x is the degree of association of the liquid, and .rho. is the density of the liquid. This equation indicates that the surface tension of most liquids falls to zero as the temperature reaches the critical temperature of the liquid. For most liquids, the critical temperature is substantially above the boiling point at atmospheric pressure, so to achieve an ink with a large change in surface tension with a small change in temperature around a practical ejection temperature, the admixture of surfactants is recommended. The choice of surfactant is important. For example, water based ink for thermal ink jet printers often contains isopropyl alcohol (2-propanol) to reduce the surface tension and promote rapid drying. Isopropyl alcohol has a boiling point of 82.4.degree. C., lower than that of water. As the temperature rises, the alcohol evaporates faster than the water, decreasing the alcohol concentration and causing an increase in surface tension. A surfactant such as 1-Hexanol (b.p. 158.degree. C.) can be used to reverse this effect, and achieve a surface tension which decreases slightly with temperature. However, a relatively large decrease in surface tension with temperature is desirable to maximize operating latitude. A surface tension decrease of 20 mN/m over a 30.degree. C. temperature range is preferred to achieve large operating margins, while as little as 10 mN/m can be used to achieve operation of the print head according to the present invention. Inks With Large -.DELTA..gamma..sub.T Several methods may be used to achieve a large negative change in surface tension with increasing temperature. Two such methods are: 1) The ink may contain a low concentration sol of a surfactant which is solid at ambient temperatures, but melts at a threshold temperature. Particle sizes less than 1,000 .ANG. are desirable. Suitable surfactant melting points for a water based ink are between 50.degree. C. and 90.degree. C., and preferably between 60.degree. C. and 80.degree. C. 2) The ink may contain an oilwater microemulsion with a phase inversion temperature (PIT) which is above the maximum ambient temperature, but below the boiling point of the ink. For stability, the PIT of the microemulsion is preferably 20.degree. C. or more above the maximum non-operating temperature encountered by the ink. A PIT of approximately 80.degree. C. is suitable. Inks with Surfactant Sols Inks can be prepared as a sol of small particles of a surfactant which melts in the desired operating temperature range. Examples of such surfactants include carboxylic acids with between 14 and 30 carbon atoms, such as: ______________________________________ Name Formula m.p. Synonym ______________________________________ Tetradecanoic acid CH.sub.3 (CH.sub.2).sub.12 COOH 58.degree. C. Myristic acid Hexadecanoic acid CH.sub.3 (CH.sub.2).sub.14 COOH 63.degree. C. Palmitic acid Octadecanoic acid CH.sub.3 (CH.sub.2).sub.15 COOH 71.degree. C. Stearic acid Eicosanoic acid CH.sub.3 (CH.sub.2).sub.16 COOH 77.degree. C. Arachidic acid Docosanoic acid CH.sub.3 (CH.sub.2).sub.20 COOH 80.degree. C. Behenic acid ______________________________________ As the melting point of sols with a small particle size is usually slightly less than of the bulk material, it is preferable to choose a carboxylic acid with a melting point slightly above the desired drop selection temperature. A good example is Arachidic acid. These carboxylic acids are available in high purity and at low cost. The amount of surfactant required is very small, so the cost of adding them to the ink is insignificant. A mixture of carboxylic acids with slightly varying chain lengths can be used to spread the melting points over a range of temperatures. Such mixtures will typically cost less than the pure acid. It is not necessary to restrict the choice of surfactant to simple unbranched carboxylic acids. Surfactants with branched chains or phenyl groups, or other hydrophobic moieties can be used. It is also not necessary to use a carboxylic acid. Many highly polar moieties are suitable for the hydrophilic end of the surfactant. It is desirable that the polar end be ionizable in water, so that the surface of the surfactant particles can be charged to aid dispersion and prevent flocculation. In the case of carboxylic acids, this can be achieved by adding an alkali such as sodium hydroxide or potassium hydroxide. Preparation of Inks with Surfactant Sols The surfactant sol can be prepared separately at high concentration, and added to the ink in the required concentration. An example process for creating the surfactant sol is as follows: 1) Add the carboxylic acid to purified water in an oxygen free atmosphere. 2) Heat the mixture to above the melting point of the carboxylic acid. The water can be brought to a boil. 3) Ultrasonicate the mixture, until the typical size of the carboxylic acid droplets is between 100 .ANG. and 1,000 .ANG.. 4) Allow the mixture to cool. 5) Decant the larger particles from the top of the mixture. 6) Add an alkali such as NaOH to ionize the carboxylic acid molecules on the surface of the particles. A pH of approximately 8 is suitable. This step is not absolutely necessary, but helps stabilize the sol. 7) Centrifuge the sol. As the density of the carboxylic acid is lower than water, smaller particles will accumulate at the outside of the centrifuge, and larger particles in the centre. 8) Filter the sol using a microporous filter to eliminate any particles above 5000 .ANG.. 9) Add the surfactant sol to the ink preparation. The sol is required only in very dilute concentration. The ink preparation will also contain either dye(s) or pigment(s), bactericidal agents, agents to enhance the electrical conductivity of the ink if electrostatic drop separation is used, humectants, and other agents as required. Anti-foaming agents will generally not be required, as there is no bubble formation during the drop ejection process. Cationic Surfactant Sols Inks made with anionic surfactant sols are generally unsuitable for use with cationic dyes or pigments. This is because the cationic dye or pigment may precipitate or flocculate with the anionic surfactant. To allow the use of cationic dyes and pigments, a cationic surfactant sol is required. The family of alkylamines is suitable for this purpose. Various suitable alkylamines are shown in the following table: ______________________________________ Name Formula Synonym ______________________________________ Hexadecylamine CH.sub.3 (CH.sub.2).sub.14 CH.sub.2 NH.sub.2 Palmityl amine Octadecylamine CH.sub.3 (CH.sub.2).sub.16 CH.sub.2 NH.sub.2 Stearyl amine Eicosylamine CH.sub.3 (CH.sub.2).sub.18 CH.sub.2 NH.sub.2 Arachidyl amine Docosylamine CH.sub.3 (CH.sub.2).sub.20 CH.sub.2 NH.sub.2 Behenyl amine ______________________________________ The method of preparation of cationic surfactant sols is essentially similar to that of anionic surfactant sols, except that an acid instead of an alkali is used to adjust the pH balance and increase the charge on the surfactant particles. A pH of 6 using HCl is suitable. Microemulsion Based Inks An alternative means of achieving a large reduction in surface tension as some temperature threshold is to base the ink on a microemulsion. A microemulsion is chosen with a phase inversion temperature (PIT) around the desired ejection threshold temperature. Below the PIT, the microemulsion is oil in water (O/W), and above the PIT the microemulsion is water in oil (W/O). At low temperatures, the surfactant forming the microemulsion prefers a high curvature surface around oil, and at temperatures significantly above the PIT, the surfactant prefers a high curvature surface around water. At temperatures close to the PIT, the microemulsion forms a continuous 'sponge' of topologically connected water and oil. There are two mechanisms whereby this reduces the surface tension. Around the PIT, the surfactant prefers surfaces with very low curvature. As a result, surfactant molecules migrate to the ink/air interface, which has a curvature which is much less than the curvature of the oil emulsion. This lowers the surface tension of the water. Above the phase inversion temperature, the microemulsion changes from O/W to W/O, and therefore the ink/air interface changes from water/air to oil/air. The oil/air interface has a lower surface tension. There is a wide range of possibilities for the preparation of microemulsion based inks. For fast drop ejection, it is preferable to chose a low viscosity oil. In many instances, water is a suitable polar solvent. However, in some cases different polar solvents may be required. In these cases, polar solvents with a high surface tension should be chosen, so that a large decrease in surface tension is achievable. The surfactant can be chosen to result in a phase inversion temperature in the desired range. For example, surfactants of the group poly(oxyethylene)alkylphenyl ether (ethoxylated alkyl phenols, general formula: C.sub.n H.sub.2n+1 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub.m OH) can be used. The hydrophilicity of the surfactant can be increased by increasing m, and the hydrophobicity can be increased by increasing n. Values of m of approximately 10, and n of approximately 8 are suitable. Low cost commercial preparations are the result of a polymerization of various molar ratios of ethylene oxide and alkyl phenols, and the exact number of oxyethylene groups varies around the chosen mean. These commercial preparations are adequate, and highly pure surfactants with a specific number of oxyethylene groups are not required. The formula for this surfactant is C.sub.8 H.sub.17 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub.n OH (average n=10). Synonyms include Octoxynol-10, PEG-10 octyl phenyl ether and POE (10) octyl phenyl ether The HLB is 13.6, the melting point is 7.degree. C., and the cloud point is 65.degree. C. Commercial preparations of this surfactant are available under various brand names. Suppliers and brand names are listed in the following table: ______________________________________ Trade name Supplier ______________________________________ Akyporox OP100 Chem-Y GmbH Alkasurf OP-10 Rhone-Poulenc Surfactants and Specialties Dehydrophen POP 10 Pulcra SA Hyonic OP-10 Henkel Corp. Iconol OP-10 BASF Corp. Igepal O Rhone-Poulenc France Macol OP-10 PPG Industries Malorphen 810 Huls AG Nikkol OP-10 Nikko Chem. Co. Ltd. Renex 750 ICI Americas Inc. Rexol 45/10 Hart Chemical Ltd. Synperonic OP10 ICI PLC Teric X10 ICI Australia ______________________________________ These are available in large volumes at low cost (less than one dollar per pound in quantity), and so contribute less than 10 cents per liter to prepared microemulsion ink with a 5% surfactant concentration. Other suitable ethoxylated alkyl phenols include those listed in the following table: ______________________________________ Trivial name Formula HLB Cloud point ______________________________________ Nonoxynol-9 C.sub.9 H.sub.19 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub..about .9 OH 13 54.degree. C. Nonoxynol-10 C.sub.9 H.sub.19 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub..about .10 OH 13.2 62.degree. C. Nonoxynol-11 C.sub.9 H.sub.19 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub..about .11 OH 13.8 72.degree. C. Nonoxynol-12 C.sub.9 H.sub.19 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub..about .12 OH 14.5 81.degree. C. Octoxynol-9 C.sub.8 H.sub.17 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub..about .9 OH 12.1 61.degree. C. Octoxynol-10 C.sub.8 H.sub.17 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub..about .10 OH 13.6 65.degree. C. Octoxynol-12 C.sub.8 H.sub.17 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub..about .12 OH 14.6 88.degree. C. Dodoxynol-10 C.sub.12 H.sub.25 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub..abou t.10 OH 12.6 42.degree. C. Dodoxynol-11 C.sub.12 H.sub.25 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub..abou t.11 OH 13.5 56.degree. C. Dodoxynol-14 C.sub.12 H.sub.25 C.sub.4 H.sub.6 (CH.sub.2 CH.sub.2 O).sub..abou t.14 OH 14.5 87.degree. C. ______________________________________ Microemulsion based inks have advantages other than surface tension control: 1) Microemulsions are thermodynamically stable, and will not separate. Therefore, the storage time can be very long. This is especially significant for office and portable printers, which may be used sporadically. 2) The microemulsion will form spontaneously with a particular drop size, and does not require extensive stirring, centrifuging, or filtering to ensure a particular range of emulsified oil drop sizes. 3) The amount of oil contained in the ink can be quite high, so dyes which are soluble in oil or soluble in water, or both, can be used. It is also possible to use a mixture of dyes, one soluble in water, and the other soluble in oil, to obtain specific colors. 4) Oil miscible pigments are prevented from flocculating, as they are trapped in the oil microdroplets. 5) The use of a microemulsion can reduce the mixing of different dye colors on the surface of the print medium. 6) The viscosity of microemulsions is very low. 7) The requirement for humectants can be reduced or eliminated. Dyes and Pigments in Microemulsion Based Inks Oil in water mixtures can have high oil contents--as high as 40%--and still form O/W microemulsions. This allows a high dye or pigment loading. Mixtures of dyes and pigments can be used. An example of a microemulsion based ink mixture with both dye and pigment is as follows: 1) 70% water 2) 5% water soluble dye 3) 5% surfactant 4) 10% oil 5) 10% oil miscible pigment The following table shows the nine basic combinations of colorants in the oil and water phases of the microemulsion that may be used. ______________________________________ Combination Colorant in water phase Colorant in oil phase ______________________________________ 1 none oil miscible pigment 2 none oil soluble dye 3 water soluble dye none 4 water soluble dye oil miscible pigment 5 water soluble dye oil soluble dye 6 pigment dispersed in water none 7 pigment dispersed in water oil miscible pigment 8 pigment dispersed in water oil soluble dye 9 none none ______________________________________ The ninth combination, with no colorants, is useful for printing transparent coatings, UV ink, and selective gloss highlights. As many dyes are amphiphilic, large quantities of dyes can also be solubilized in the oil-water boundary layer as this layer has a very large surface area. It is also possible to have multiple dyes or pigments in each phase, and to have a mixture of dyes and pigments in each phase. When using multiple dyes or pigments the absorption spectrum of the resultant ink will be the weighted average of the absorption spectra of the different colorants used. This presents two problems: 1) The absorption spectrum will tend to become broader, as the absorption peaks of both colorants are averaged. This has a tendency to 'muddy' the colors. To obtain brilliant color, careful choice of dyes and pigments based on their absorption spectra, not just their human-perceptible color, needs to be made. 2) The color of the ink may be different on different substrates. If a dye and a pigment are used in combination, the color of the dye will tend to have a smaller contribution to the printed ink color on more absorptive papers, as the dye will be absorbed into the paper, while the pigment will tend to 'sit on top' of the paper. This may be used as an advantage in some circumstances. Surfactants with a Krafft Point in the Drop Selection Temperature Range For ionic surfactants there is a temperature (the Krafft point) below which the solubility is quite low, and the solution contains essentially no micelles. Above the Krafft temperature micelle formation becomes possible and there is a rapid increase in solubility of the surfactant. If the critical micelle concentration (CMC) exceeds the solubility of a surfactant at a particular temperature, then the minimum surface tension will be achieved at the point of maximum solubility, rather than at the CMC. Surfactants are usually much less effective below the Krafft point. This factor can be used to achieve an increased reduction in surface tension with increasing temperature. At ambient temperatures, only a portion of the surfactant is in solution. When the nozzle heater is turned on, the temperature rises, and more of the surfactant goes into solution, decreasing the surface tension. A surfactant should be chosen with a Krafft point which is near the top of the range of temperatures to which the ink is raised. This gives a maximum margin between the concentration of surfactant in solution at ambient temperatures, and the concentration of surfactant in solution at the drop selection temperature. The concentration of surfactant should be approximately equal to the CMC at the Krafft point. In this manner, the surface tension is reduced to the maximum amount at elevated temperatures, and is reduced to a minimum amount at ambient temperatures. The following table shows some commercially available surfactants with Krafft points in the desired range. ______________________________________ Formula Krafft point ______________________________________ C.sub.16 H.sub.33 SO.sub.3.sup.- Na.sup.+ 57.degree. C. C.sub.18 H.sub.37 SO.sub.3.sup.- Na.sup.+ 70.degree. C. C.sub.16 H.sub.33 SO.sub.4.sup.- Na.sup.+ 45.degree. C. Na.sup.+- O.sub.4 S(CH.sub.2).sub.16 SO.sub.4.sup.- Na.sup.+ 44.9.degree. C. K.sup.+- O.sub.4 S(CH.sub.2).sub.16 SO.sub.4.sup.- K.sup.+ 55.degree. C. C.sub.16 H.sub.33 CH(CH.sub.3)C.sub.4 H.sub.6 SO.sub.3.sup.- Na.sup.+ 60.8.degree. C. ______________________________________ Surfactants with a Cloud Point in the Drop Selection Temperature Range Non-ionic surfactants using polyoxyethylene (POE) chains can be used to create an ink where the surface tension falls with increasing temperature. At low temperatures, the POE chain is hydrophilic, and maintains the surfactant in solution. As the temperature increases, the structured water around the POE section of the molecule is disrupted, and the POE section becomes hydrophobic. The surfactant is increasingly rejected by the water at higher temperatures, resulting in increasing concentration of surfactant at the air/ink interface, thereby lowering surface tension. The temperature at which the POE section of a nonionic surfactant becomes hydrophilic is related to the cloud point of that surfactant. POE chains by themselves are not particularly suitable, as the cloud point is generally above 100.degree. C. Polyoxypropylene (POP) can be combined with POE in POE/POP block copolymers to lower the cloud point of POE chains without introducing a strong hydrophobicity at low temperatures. Two main configurations of symmetrical POE/POP block copolymers are available. These are: 1) Surfactants with POE segments at the ends of the molecules, and a POP segment in the centre, such as the poloxamer class of surfactants (generically CAS 9003-11-6) 2) Surfactants with POP segments at the ends of the molecules, and a POE segment in the centre, such as the meroxapol class of surfactants (generically also CAS 9003-11-6) Some commercially available varieties of poloxamer and meroxapol with a high surface tension at room temperature, combined with a cloud point above 40.degree. C. and below 100.degree. C. are shown in the following table: __________________________________________________________________________ Surface BASF Tension Cloud Trivial name Trade name Formula (mN/m) point __________________________________________________________________________ Meroxapol 105 Pluronic 10R5 HO(CHCH.sub.3 CH.sub.2 O).sub..about.7 (CH.sub.2 CH.sub.2 O).sub..about.22 (CHCH.sub.3 CH.sub.2 O).sub..about.7 50.9 69.degree. C. Meroxapol 108 Pluronic 10R8 HO(CHCH.sub.3 CH.sub.2 O).sub..about.7 (CH.sub.2 CH.sub.2 O).sub..about.91 (CHCH.sub.3 CH.sub.2 O).sub..about.7 54.1 99.degree. C. Meroxapol 178 Pluronic 17R8 HO(CHCH.sub.3 CH.sub.2 O).sub..about.12 (CH.sub.2 CH.sub.2 O).sub..about.136 (CHCH.sub.3 CH.sub.2 O).sub..about.12 47.3 81.degree. C. Meroxapol 258 Pluronic 25R8 HO(CHCH.sub.3 CH.sub.2 O).sub..about.18 (CH.sub.2 CH.sub.2 O).sub..about.163 (CHCH.sub.3 CH.sub.2 O).sub..about.18 46.1 80.degree. C. Poloxamer 105 Pluronic L35 HO(CH.sub.2 CH.sub.2 O).sub..about.11 (CHCH.sub.3 CH.sub.2 O).sub..about.16 (CH.sub.2 CH.sub.2 O).sub..about.11 48.8 77.degree. C. Poloxamer 124 Pluronic LM HO(CH.sub.2 CH.sub.2 O).sub..about.11 (CHCH.sub.3 CH.sub.2 O).sub..about.21 (CH.sub.2 CH.sub.2 O).sub..about.11 45.3 65.degree. C. __________________________________________________________________________ Other varieties of poloxamer and meroxapol can readily be synthesized using well known techniques. Desirable characteristics are a room temperature surface tension which is as high as possible, and a cloud point between 40.degree. C. and 100.degree. C., and preferably between 60.degree. C. and 80.degree. C. Meroxapol [HO(CHCH.sub.3 CH.sub.2 O).sub.x (CH.sub.2 CH.sub.2 O).sub.y (CHCH.sub.3 CH.sub.2 O).sub.z OH] varieties where the average x and z are approximately 4, and the average y is approximately 15 may be suitable. If salts are used to increase the electrical conductivity of the ink, then the effect of this salt on the cloud point of the surfactant should be considered. The cloud point of POE surfactants is increased by ions that disrupt water structure (such as I.sup.-), as this makes more water molecules available to form hydrogen bonds with the POE oxygen lone pairs. The cloud point of POE surfactants is decreased by ions that form water structure (such as Cl.sup.-, OH.sup.-), as fewer water molecules are available to form hydrogen bonds. Bromide ions have relatively little effect. The ink composition can be 'tuned' for a desired temperature range by altering the lengths of POE and POP chains in a block copolymer surfactant, and by changing the choice of salts (e.g Cl.sup.- to Br.sup.- to I.sup.-) that are added to increase electrical conductivity. NaCl is likely to be the best choice of salts to increase ink conductivity, due to low cost and non-toxicity. NaCl slightly lowers the cloud point of nonionic surfactants. Hot Melt Inks The ink need not be in a liquid state at room temperature. Solid 'hot melt' inks can be used by heating the printing head and ink reservoir above the melting point of the ink. The hot melt ink must be formulated so that the surface tension of the molten ink decreases with temperature. A decrease of approximately 2 mN/m will be typical of many such preparations using waxes and other substances. However, a reduction in surface tension of approximately 20 mN/m is desirable in order to achieve good operating margins when relying on a reduction in surface tension rather than a reduction in viscosity. The temperature difference between quiescent temperature and drop selection temperature may be greater for a hot melt ink than for a water based ink, as water based inks are constrained by the boiling point of the water. The ink must be liquid at the quiescent temperature. The quiescent temperature should be higher than the highest ambient temperature likely to be encountered by the printed page. The quiescent temperature should also be as low as practical, to reduce the power needed to heat the print head, and to provide a maximum margin between the quiescent and the drop ejection temperatures. A quiescent temperature between 60.degree. C. and 90.degree. C. is generally suitable, though other temperatures may be used. A drop ejection temperature of between 160.degree. C. and 200.degree. C. is generally suitable. There are several methods of achieving an enhanced reduction in surface tension with increasing temperature. 1) A dispersion of microfine particles of a surfactant with a melting point substantially above the quiescent temperature, but substantially below the drop ejection temperature, can be added to the hot melt ink while in the liquid phase. 2) A polar/non-polar microemulsion with a PIT which is preferably at least 20.degree. C. above the melting points of both the polar and non-polar compounds. To achieve a large reduction in surface tension with temperature, it is desirable that the hot melt ink carrier have a relatively large surface tension (above 30 mN/m) when at the quiescent temperature. This generally excludes alkanes such as waxes. Suitable materials will generally have a strong intermolecular attraction, which may be achieved by multiple hydrogen bonds, for example, polyols, such as Hexanetetrol, which has a melting point of 88.degree. C. Surface Tension Reduction of Various Solutions FIG. 3(d) shows the measured effect of temperature on the surface tension of various aqueous preparations containing the following additives: 1) 0.1% sol of Stearic Acid 2) 0.1% sol of Palmitic acid 3) 0.1% solution of Pluronic 10R5 (trade mark of BASF) 4) 0.1% solution of Pluronic L35 (trade mark of BASF) 5) 0.1% solution of Pluronic L44 (trade mark of BASF) Inks suitable for printing systems of the present invention are described in the following Australian patent specifications, the disclosure of which are hereby incorporated by reference: 'Ink composition based on a microemulsion' (Filing no.: PN5223, filed on Sep. 6, 1995); 'Ink composition containing surfactant sol' (Filing no.: PN5224, filed on Sep. 6, 1995); 'Ink composition for DOD printers with Krafft point near the drop selection temperature sol' (Filing no.: PN6240, filed on Oct. 30, 1995); and 'Dye and pigment in a microemulsion based ink' (Filing no.: PN6241, filed on Oct. 30, 1995). Operation Using Reduction of Viscosity As a second example, operation of an embodiment using thermal reduction of viscosity and proximity drop separation, in combination with hot melt ink, is as follows. Prior to operation of the printer, solid ink is melted in the reservoir 64. The reservoir, ink passage to the print head, ink channels 75, and print head 50 are maintained at a temperature at which the ink 100 is liquid, but exhibits a relatively high viscosity (for example, approximately 100 cP). The Ink 100 is retained in the nozzle by the surface tension of the ink. The ink 100 is formulated so that the viscosity of the ink reduces with increasing temperature. The ink pressure oscillates at a frequency which is an integral multiple of the drop ejection frequency from the nozzle. The ink pressure oscillation causes oscillations of the ink meniscus at the nozzle tips, but this oscillation is small due to the high ink viscosity. At the normal operating temperature, these oscillations are of insufficient amplitude to result in drop separation. When the heater 103 is energized, the ink forming the selected drop is heated, causing a reduction in viscosity to a value which is preferably less than 5 cP. The reduced viscosity results in the ink meniscus moving further during the high pressure part of the ink pressure cycle. The recording medium 51 is arranged sufficiently close to the print head 50 so that the selected drops contact the recording medium 51, but sufficiently far away that the unselected drops do not contact the recording medium 51. Upon contact with the recording medium 51, part of the selected drop freezes, and attaches to the recording medium. As the ink pressure falls, ink begins to move back into the nozzle. The body of ink separates from the ink which is frozen onto the recording medium. The meniscus of the ink 100 at the nozzle tip then returns to low amplitude oscillation. The viscosity of the ink increases to its quiescent level as remaining heat is dissipated to the bulk ink and print head. One ink drop is selected, separated and forms a spot on the recording medium 51 for each heat pulse. As the heat pulses are electrically controlled, drop on demand ink jet operation can be achieved. Manufacturing of Print Heads Manufacturing processes for monolithic print heads in accordance with the present invention are described in the following Australian patent specifications filed on Apr. 12, 1995, the disclosure of which are hereby incorporated by reference: 'A monolithic LIFT printing head' (Filing no.: PN2301); 'A manufacturing process for monolithic LIFT printing heads' (Filing no.: PN2302); 'A self-aligned heater design for LIFT print heads' (Filing no.: PN2303); 'Integrated four color LIFT print heads' (Filing no.: PN2304); 'Power requirement reduction in monolithic LIFT printing heads' (Filing no.: PN2305); 'A manufacturing process for monolithic LIFT print heads using anisotropic wet etching' (Filing no.: PN2306); 'Nozzle placement in monolithic drop-on-demand print heads' (Filing no.: PN2307); 'Heater structure for monolithic LIFT print heads' (Filing no.: PN2346); 'Power supply connection for monolithic LIFT print heads' (Filing no.: PN2347); 'External connections for Proximity LIFT print heads' (Filing no.: PN2348); and 'A self-aligned manufacturing process for monolithic LIFT print heads' (Filing no.: PN2349); and 'CMOS process compatible fabrication of LIFT print heads' (Filing no.: PN5222, Sep. 6, 1995). 'A manufacturing process for LIFT print heads with nozzle rim heaters' (Filing no.: PN6238, Oct. 30, 1995); 'A modular LIFT print head' (Filing no.: PN6237, Oct. 30, 1995); 'Method of increasing packing density of printing nozzles' (Filing no.: PN6236, Oct. 30, 1995); and 'Nozzle dispersion for reduced electrostatic interaction between simultaneously printed droplets' (Filing no.: PN6239, Oct. 30, 1995). Control of Print Heads Means of providing page image data and controlling heater temperature in print heads of the present invention is described in the following Australian patent specifications filed on Apr. 12, 1995, the |