Exceeding the surface settling limit in acoustic ink printing

In an acoustic ink printing system, the time between droplet ejection in predetermined cases is increased where full optical density is not required. This is accomplished by choosing an order in which drops are ejected to be strictly alternate whenever possible. In a system which has a 10 maximum ink droplet per pixel only requires, for example, five drops for a particular situation, twice as much time is available for settling of an ink surface than would be available for sequential bursts of droplets. The longer ink surface settling time allows for high ink droplet directionality control. When more than five drops per area are needed, less time exists between droplet ejection, increasing droplet misdirectionality. This drop misdirectionality will occur within shadow or dark regions where it has been shown in many cases to be helpful in providing coverage of the substrate.

BACKGROUND OF THE INVENTION
 This invention relates to acoustic ink printing, and more particularly to a
 method and apparatus that allows an acoustic ink printer to operate at
 operational speeds greater than previously achievable and, which extends
 the ink types which may be used with the acoustic ink printer, while at
 the same time ensuring appropriate ink drop ejection directionality to
 achieve desired output printing.
 It has been shown that acoustic ink printers which have printheads
 comprising acoustically illuminated spherical or Fresnel focusing lenses
 can print precisely positioned picture elements (pixels) at resolutions
 that are sufficient for high-quality printing of complex images.
 Although acoustic lens-type droplet emitters currently are favored, there
 are other types of droplet emitters which may be utilized for acoustic ink
 printing, including (1) piezoelectric shell transducers, such as described
 in Lovelady et al., U.S. Pat. No. 4,308,547, and (2) interdigitated
 transducers (IDTs), such as described in commonly assigned U.S. Pat. No.
 4,697,195. Furthermore, acoustic ink printing technology is compatible
 with various printhead configurations; including (1) single emitter
 embodiments for raster scan printing, (2) matrix configured arrays for
 matrix printing, and (3) several different types of page and width arrays,
 ranging from (i) single row sparse arrays for hybrid forms of
 parallel/serial printing, and (ii) multiple row staggered arrays with
 individual emitters for each of the pixel positions or addresses within a
 page width address field (i.e., single emitter/pixel/line) for ordinary
 line printing.
 For performing acoustic ink printing with any of the aforementioned droplet
 emitters, each of the emitters launches a converging acoustic beam into a
 pool of ink, with the angular convergence of the beam being selected so
 that it comes to focus at or near the free surface (i.e., the liquid/air
 interface) of the pool. Moreover, controls are provided for modulating the
 radiation pressure which each beam exerts against the free surface of the
 ink. That permits the radiation pressure from each beam to make brief,
 controlled excursions to a sufficiently high pressure level to overcome
 the restraining force of surface tension, whereby individual droplets of
 ink are emitted from the free surface of the ink on command, with
 sufficient velocity to deposit them on a nearby recording medium.
 An attraction of acoustic ink printing is the ability to control droplet
 size based on the frequency of the signal provided, rather than relying on
 the size of the nozzle emitting the droplet. For example, an acoustic ink
 printer may emit droplets which are a magnitude or more smaller than the
 acoustic ink printhead openings. On the other hand, conventional ink jet
 printing requires a minimization of the nozzle itself to obtain smaller
 droplets.
 Ideally, in an acoustic ink printer, the acoustic wave propagates in a
 direction perpendicular to the air-ink surface. The acoustic wave causes a
 droplet to be ejected in a direction which is parallel to the direction of
 the acoustic wave propagates. Thus, ideally the droplet is ejected in a
 direction perpendicular to the air-ink interface. To achieve high-quality
 printing, it has been considered necessary that the direction of droplet
 ejection must be the same for all ejectors across a printhead. Very slight
 misdirections cause droplets to land on a substrate, e.g., paper, at a
 location distant from their intended locations.
 Typically, a 1 mm gap separates the air-ink interface from the substrate. A
 droplet ejected one degree off from the ideal ejection direction is
 displaced 17.5 .mu.m from its intended location on the substrate. For a
 1200 spi (spots per inch) printer, this displacement constitutes 80% of
 one pixel. Thus, in existing systems it has been a high priority to ensure
 that the direction of ejection of the droplets must be controlled very
 closely to achieve high-quality printing.
 A common cause of misdirectionality is that waves generated from a previous
 droplet ejection have not settled sufficiently before the next droplet is
 ejected.
 Thus, for conventional acoustic ink printing systems, a design constraint
 is the time between droplet ejection must be sufficient so as to ensure
 settling of the surface acoustic waves so that the next ejected droplet
 maintains good directionality as it moves toward the substrate. In this
 regard, time required for acoustic waves to settle is a fundamental limit
 on the print speed of an acoustic ink printer.
 Ink settling time decreases with increased ink surface tension. Thus,
 aqueous inks in acoustic ink printing tend to be high-surface tension
 inks.
 Substantial effort has been directed to improving the directionality of the
 ink droplets ejected from an acoustic ink ejector, and to designs which
 decrease the ink settling time, in order to increase printing speed.
 Examples of efforts in these areas are described in many commonly assigned
 U.S. patents including: U.S. Pat. No. 4,697,195 entitled Nozzleless Liquid
 Droplet Ejectors; U.S. Pat. No. 4,748,453 Entitled Spot Deposition for
 Liquid Ink Printing; U.S. Pat. No. 4,748,461 Entitled Capillary Wave
 Controllers for Nozzleless Droplet Ejectors; U.S. Pat. No. 4,719,480
 entitled Spatial Stabilization of Standing Capillary Surface Waves; U.S.
 Pat. No. 4,719,476 entitled Spatially Addressing Capillary Wave Droplet
 Ejectors and the Like; U.S. Pat. No. 5,919,354 entitled Method and
 Apparatus for Suppressing Capillary Waves in an Ink-jet Printer; U.S. Pat.
 No. 5,229,793 entitled Liquid Surface Control with an Applied Pressure
 Signal in Acoustic Ink Printing; U.S. Pat. No. 5,216,451 entitled Surface
 Ripple Wave Diffusion in Apertured Free Ink Surface Level Controllers for
 Acoustic Ink Printers; U.S. Pat. No. 5,450,107 entitled Surface Ripple
 Wave Suppression by Anti-reflection in Apertured Free Ink Surface Level
 Controllers for Acoustic Ink Printers; U.S. Pat. No. 5,629,724 entitled
 Stabilization of the Free Surface of Liquid; U.S. Pat. No. 5,808,636
 entitled Reduction of Droplet Misdirectionality in Acoustic Ink Printing;
 U.S. Pat. No. 5,870,112 entitled Dot Scheduling for Liquid Ink Printers,
 all hereby incorporated by reference.
 Various ones of the above references specifically note the importance of
 directionality in acoustic ink printing as well as the importance of
 surface waves in achieving desired directionality.
 However, the ink ejection process in these documents, as well as the
 conventional state of the art, is to provide a sequential burst of ink
 droplets when printing to a substrate or to generate a checkerboard type
 print output.
 Checkerboard printing is a two pass process, wherein each pass prints a
 portion of the pixels in a dot pattern known as a "checkerboard" pattern.
 In this type of two pass printing, a first pass of the printhead carriage
 prints a swath of information in which odd numbered pixels of odd numbered
 rows or scanlines and even numbered pixels of even numbered rows or
 scanlines of a bitmap are printed. In a second pass of the carriage
 printhead, the complementary pattern consisting of even numbered pixels in
 odd numbered rows and odd numbered pixels in even numbered rows is
 printed. By printing in two passes, the ink printed in the first pass has
 time to dry partially before the ink from the second pattern is deposited.
 The cited material does not however, recognize the potential benefits of
 relaxing ink ejection constraints when in a dark/shadow image area, and
 thus does not apply this understanding through the use of specialized
 filler patterns which adjust ink droplet ejection.
 While other printing arts such as those using half-toning concepts do
 include the concept of staggered or varying print sequences (i.e., as in
 the generation of half-tone cells,) such use is directed towards achieving
 a desired tone scaling. In other words, in half-toning it is desirable to
 provide smooth transition variations during printing and that is where the
 half-toning print sequences are directed. However, the concepts of the
 present invention are specifically directed to directionality and are not
 concerned with such tone scaling concepts.
 The present invention departs from conventional acoustic ink printer
 designs which have constraints on firing frequency due to the need to
 allow an ink surface to settle sufficiently before a next ejection. The
 invention also takes advantage of the inventor's understanding that
 constraints against misdirectionality within dark or shadow areas of an
 image may be relaxed in a beneficial manner. It is noted the constraints
 of existing systems result in an inherent limitation on the speed with
 which a device may print. For example, existing systems based on aqueous
 inks, are known to have an upper level operating frequency of 48 kHz.
 In consideration of the above, it has been deemed desirable to develop an
 apparatus and method directed to maintaining high directionality control
 of droplet ejections during the printing of image areas with predetermined
 first optical density requirements, while at the same time relaxing
 certain constraints which will increase misdirectionality of droplet
 ejection when printing image areas which have an optical density greater
 than the first optical density. Such constraints are directed to the time
 between droplet ejection required for the settling of an ink surface.
 SUMMARY OF THE INVENTION
 It is an object of the invention to provide an apparatus and method which
 relaxes the design constraints of conventional systems to ensure
 sufficient settling of the surface acoustic waves for each droplet
 ejection, to maintain directionality control of all ejected drops as they
 are propelled toward a substrate.
 The present invention provides an increase in the time between drop
 ejection in cases where full density printing is not required. The
 relaxation is accomplished by choosing an order of droplet ejection which
 permits, where physically possible, strictly alternate drop ejection in
 lower print density areas thereby maintaining desired control of droplet
 directionality, while control of drop ejection in higher print density
 areas permits droplet misdirectionality. The implemented droplet control
 allows the overall operating speed of the acoustic ink printer to be
 increased and/or ink having properties with lower surface tensions than
 previously determined allowable by design constraints to be used.
 In accordance with a more limited aspect of the present invention, the
 order of droplet ejection takes place in accordance with a filler pattern
 supplied from a controller to individual acoustic ink ejectors, in order
 to maintain directionality during printing of low optical density areas
 while providing beneficial misdirectionality in the high-density dark or
 shadow regions of an image.
 A first benefit of the present invention is an ability to operate the
 acoustic ink printing system at an operating speed higher than previously
 considered appropriate.
 With attention to another aspect of the present invention, inks which were
 previously believed to be non-compatible with design constraints of the
 acoustic ink printer may be now implemented.
 Still further advantages of the present invention will become apparent to
 those of ordinary skill in the art upon reading and understanding the
 following detailed description of the preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring now to the drawings which are provided for illustrating the
 preferred embodiments of the invention only and not for purposes of
 limiting same, FIG. 1 provides a view of an exemplary acoustic ink
 printing ejector 10 to which the present invention is directed. Of course,
 other configurations may also have the present invention applied thereto.
 Additionally, while a single ejector is illustrated, an acoustic ink
 printhead will consist of a number of the ejectors arranged in an array
 configuration, and the present invention is intended to work with such an
 array.
 As shown, ejector 10 includes a glass layer 12 having an electrode 14
 disposed thereon. A piezoelectric layer 16, preferably formed of zinc
 oxide, is positioned on the electrode layer 14 and an electrode 18 is
 disposed on the piezoelectric layer 16. Electrode layer 14 and electrode
 18 are connected through a surface wiring pattern representatively shown
 by lines 20 and 22 to a radio frequency (rf) power source 24 which
 generates power that is transferred to the electrodes 14 and 18. On a side
 opposite the electrode layer 14, a lens 26, such as a concentric Fresnel
 lens or other appropriate lens, is formed. Spaced from the lens 26 is a
 liquid level control plate (also called an orifice plate) 28, having an
 orifice 30 formed therein. Ink 32 is retained between the orifice plate 28
 and the glass layer 12. The orifice 30 is aligned with the lens 26 to
 facilitate emission of a droplet 34 from ink surface 36 to a substrate 38.
 Ink surface 36 is, of course, exposed by the orifice 30.
 The lens 26, the electrode layer 14, the piezoelectric layer 16 and the
 electrode 28 are formed in the glass layer 12 through photolithographic
 techniques. The orifice plate 28 is subsequently positioned to be spaced
 from the glass layer 12. The ink 32 is fed into the space between the
 orifice plate 28 and the glass layer 12 from an ink supply (not shown but
 such supply is well known in the art).
 A controller 40 generates control signals 42 which are selectively supplied
 to rf power source 24. Upon receipt of an appropriate control signal 42,
 ink droplet ejection is initiated, causing droplet 34 to be ejected.
 As previously noted, due to the requirement of allowing the ink surface to
 settle prior to ejection of each droplet, to avoid droplet
 misdirectionality, the speed of the existing systems are constrained, and
 various types of inks having lower surface tension having longer settling
 times. For example, an acoustic ink printing system which has a maximum of
 ten drops to an area, i.e. a pixel area, is known to operate at
 approximately 48 kHz for use with a high-surface tension ink. It was
 observed by the inventors that in existing systems the controller would
 send bursts of control signals to rf power source 24, to thereby cause a
 sequence of ink droplets to be ejected each immediately following the
 other.
 For example, if up to ten droplets can be printed in an area, i.e. in a
 pixel, and for a particular image the selected pixel requires only six
 droplets, then the device would generate six droplets sequentially.
 Similarly, if two droplets were required, then two droplets would be
 sequentially generated, etc. This design requires the device operating
 frequency (the time between sequential drops) to be sufficiently slow to
 allow the ink surface to resettle prior to the next time adjacent droplet
 ejection, in order to maintain drop directionality for each ejected
 droplet.
 The present proposal is to design acoustic ink printers to print up to ten
 drops per pixel for each color. The "normal mode" for such printing is to
 divide these drops into two groups. During a first pass, the printhead
 will print up to five drops, and on a second pass up to the next five
 drops are ejected.
 One embodiment of the present invention is to reorder the drops to be
 printed from a sequential burst into a substantially alternating pattern.
 A basic filler pattern would have a first group of filler pattern data
 (e.g. 1, 3, 5, 7, 9) and filler pattern data (e.g. 2, 4, 6, 8, 10) in a
 second group. In a situation where the level of drops to be included
 within a pixel are ten, the preceding pattern would simply eject five
 drops on the first pass (i.e. corresponding to the filler data 1, 3, 5, 7
 and 9) and five drops on the second pass corresponding to the supplied
 filler pattern data (i.e. 2, 4, 6, 8, 10). Thus if a full ten-droplet
 printing were required, a sequential type emission would exist. However,
 when the level of droplets to be included within a pixel are less than
 ten, benefits of the present invention come into play. For example, if the
 number of droplets to be placed in a pixel are five, then on a first pass,
 droplets would be ejected corresponding to filler pattern data 1, 3, and
 5, and also on the first pass no droplets would be ejected when filler
 data pattern received is 7 and 9. On the second pass, droplets would be
 ejected when filler pattern data of 2 and 4 is received, whereas no
 droplets would be ejected when filler pattern data 6, 8 or 10 is received.
 While the above is discussed in connection with a single pixel, it is to be
 appreciated that multiple pixels are to be printed on a line. The
 preceding discussed pattern will provide a checkerboard pattern from pixel
 to pixel in order to prevent all "1" drops from printing at the same pass.
 Thus, in consideration of multiple pixel printing and where the input
 level to each pixel is five, the printing pattern would be XXX00 (i.e.
 1,3,5,7,9) on the first pass, and XX000 (i.e. 2,4,6,8,10) on the return
 pass for the first pixel (where the X stands for printing a drop, and 0
 stands for not printing). In consideration of the checkerboarding concept
 of printing discussed, the next pixel would be printed with a first pass
 of XX000 (i.e. 2,4,6,8,10) and a second pass of XXX00 (i.e. 1,3,5,7,9).
 It is to be noted that once the pixel requires three or greater drops, two
 of the drops will always be printed in adjacent cycles in one of the
 passes. Thus, under the foregoing scenario, the acoustic ink printer would
 still be tied to the intrinsic drop rate of the printer that will allow
 for a settling of the ink surface.
 The following embodiments are directed to providing a filler pattern which
 allows the acoustic ink printer to go beyond what is considered the
 intrinsic drop rate of the device while at the same time maintaining the
 drop directionality, which result in high quality prints.
 An important concept of the present invention is that when a printer is
 printing in an area which is high density, i.e. in the shadow or black
 area of an image, then it is acceptable to have some misdirectionality
 from the ejector, which allows the ejector to scatter ink droplets in
 areas not otherwise appropriate. Particularly, since the shadow/black
 areas are going to be black or dark in any case, there is no overall
 decrease in print quality, and in fact, there may be an increase in print
 quality by allowing a relaxation of droplet directionality constraints.
 Implementing this concept makes it possible to operate an acoustic ink
 printer at a higher speed than what was previously an accepted optimal
 speed. When not in dark or shadow regions, the filler patterns provide
 time between the ejected droplets, delivering them across the whole drop
 cycle such that they are not ejected at times immediately adjacent to each
 other. Providing a time period between the ejected droplets allows for the
 ink surface to stabilize to a degree which results in the desired
 directionality for these mid-range (i.e. non-black/shadow) areas.
 One example of a filler pattern which may be used in conjunction with the
 present invention is 1,7,3,9,5 for a first group and 6,2,8,4,10 for a
 second group, as shown in FIG. 2A. Under this scenario, the number of
 drops which are to be ejected from droplet ejector 10 to pixel 50 is five.
 In particular, there will be five droplets ejected onto pixel 50. Using
 the above-noted filler pattern, prior to any operation, no droplets have
 been ejected (this state is illustrated by "00000") 52. During a first
 pass of a printhead, the ejector 10 ejects droplets when filler data
 (1,7,3,9,5) 52 causes a control signal to activate the acoustic ink
 ejector. For example, since only five droplets are to be ejected, during
 the first pass (which uses filler pattern 1,7,3,9,5) an ink droplet will
 be ejected corresponding to filler pattern data 1, 3 and 5. Since data 7
 and 9 are above the input print level (i.e. five), no droplets are ejected
 corresponding to this data (X0X0X) 54. As can be seen, during the first
 pass, the time between the first ejection of a droplet (i.e. from the time
 the ejector received filler pattern data "1"), until the ejector 10 ejects
 a second droplet due to filler pattern data "3", is doubled from a system
 which ejects drops in an adjacent manner. When the acoustic ink ejector 10
 received the filler pattern data "7", it was noted to not be within the
 input level and therefore, no droplet was ejected.
 During a second pass, the filler pattern, 6,2,8,4,10, 56, results in
 droplets being ejected corresponding to filler pattern data "2" and "4"
 resulting in a pattern OXOXO, 58. By operation of the first and second
 passes, all five droplets (XXXXX) 60 are appropriately ejected. However,
 since time was inserted between each of the ejections, i.e. there is no
 sequential ejection, and the surface of the ink was able to settle thereby
 allowing proper directionality of the ink droplets.
 Turning to another aspect of the present invention, and FIG. 2B, pixels A,
 B and C are shown in a state prior to operation (Pixel A--00000; Pixel
 B--00000; Pixel C--00000) 62. Using the previously discussed filler
 pattern 64, following a first pass not only are there no adjacent ink
 droplets ejected within pixel A, (i.e. the pattern of Pixel A is--X0X0X;
 the pattern of Pixel B is--0X0X0; and the pattern of Pixel C is--X0X0X)
 66, but there also are no adjacent ink droplet ejections at the borders
 between the pixels. For example, an ink droplet ejection occurs in pixel A
 in response to filler pattern data "5" 68, and the next time period there
 is no ejection of an ink droplet in pixel B, since the next filler pattern
 data is "6" 70. The same is true between pixel B at space 72 and Pixel C
 at space 74. For the second pass, the remaining filler pattern data is
 applied 76. Specifically, Pixel A now has applied to it the filling
 pattern 6,2,8,4,10 (second pass, Pixel A is--0X0X0), pixel B has the
 filling pattern 1,7,3,9,5 (second pass, Pixel B is--X0X0X), and pixel C
 has the filling pattern 6,2,8,4,10 (second pass, Pixel C is--0X0X0) 78.
 The remaining ink droplets necessary for the image are ejected 80.
 Using the droplet maximum and the described filler patterns, it is possible
 to provide up to five drops with no two drops being printed on adjacent
 cycles. So after a drop is fired, the present invention provides for twice
 the settling time as opposed to systems which perform adjacent or
 sequential droplet ejection. By increasing the time period between droplet
 ejections in non-black/shadow areas, it is possible to increase the
 overall operational speed of the acoustic ink printer. As previously
 noted, the highest optimal speed acoustic ink printers have been
 approximately 48 kHz or less. Under this new design, the inventors have
 determined that it is possible to deliver ink with an equivalent level of
 print quality using 40 kHz or greater, with a speed up to 55 kHz
 operation, which is approximately a 15% increase in speed. This provides
 for the overall printing system to increase the throughput of page
 printing.
 It is to be appreciated that when printing in lighter areas such as those
 with an input level of five droplets per pixel, i.e. half the ten pixels
 to which the system can print, the printing is actually printing at
 approximately 1/2 of 55 khz which is 27.5 kHz. As more ink drops are
 required per pixel, the rate goes up. By the time the system is asked to
 print six drops, the printing is in an area that is at the dark end of the
 spectrum. It is approximately 75% of the maximum optical density for most
 ink and media combinations.
 Printing using a filler pattern such as described, the black/shadow
 patterns are likely to be somewhat darker than in existing systems because
 the drops are spread more evenly across the paper. Thus, droplet "6" is
 occurring in the image black/shadows (not in the light or mid-tone areas).
 Misdirectionality errors are likely to be much less noticeable in these
 black/shadow regions. In fact, some misdirectionality is actually helpful
 to fill out the image and provide darker, more saturated colors by
 ensuring greater coverage of the paper.
 While the increase in misdirectionality is helpful in the middle or solid
 dark objects, it can degrade image quality at the edges. Thus it can be
 helpful to avoid firing drops on adjacent time cycles at the edges of
 objects. This may require adjustments in the pattern of drops. For
 example, for an object at full density, at the edges perpendicular to the
 process direction, patterns XOXXX and XXXOX might be used for left and
 right edges respectively. For edges parallel to the process direction
 patterns OXOXO and XOXOX are preferred.
 FIGS. 2A and 2B are directed to a single acoustic ink ejector, such as
 depicted in FIG. 1. So what is being discussed about pixels are pixels
 created along a line. What has therefore been described is directed to a
 single ejector as a device, and that there is a desire to minimize the
 repetition of that device in pixel ejection. In particular, there is a
 desire to generate a droplet ejection sequence to obtain, if possible,
 non-adjacent droplet ejections.
 This concept is discussed in connection with FIGS. 3 and 4. FIG. 3 depicts
 an example of a 10-drop simulation with underfilling spots and 2 micron
 misdirectionality (1 sigma). In particular, FIG. 3 illustrates the output
 of an acoustic ink printer operating at a speed no greater than 48 kHz
 such that misdirectionality is minimized. FIG. 4 shows the results of the
 same printing characteristics but with 5 micron misdirectionality. It can
 be seen the image in FIG. 4 has a coverage that is greater with less
 visible line structures than FIG. 3.
 In FIG. 3, by maintaining the directionality levels within the dark area,
 the ink is being applied to the paper on a substantially straight line,
 allowing an observer user to see line patterns 82. However, with increased
 misdirectionality which would occur in the present invention (and shown in
 FIG. 4), an observer perceives a darker paper due to the lack of the line
 patterns, and slightly better coverage on the paper. In other words the
 droplets are scattered in less than optimal line placement, which
 eliminates the noticeable line patterns.
 Turning attention to FIG. 5, shown is a block diagram of a printhead 90
 having multiple ejectors 92a-92n which are fired in accordance with
 actuation of rf power source 94. A controller 96 provides control signal
 98 to the rf power source which is configured through either a plurality
 of individual rf power sources or multiplexing designs for a single rf
 source, to actuate ink ejectors 92a-92n. With attention to the filler
 pattern, such patterns may be stored in a look-up table 100 within
 controller 96 or external thereto. Use of lookup table 100 provides a fast
 manner of obtaining the filler pattern data information which is used by
 controller 96 to generate control signals 98 for rf power source assembly
 94. Thus, while the discussion of FIGS. 1, 2A and 2B have been
 substantially in connection with a single ink droplet ejector, the present
 invention is applicable to an entire printhead where each individual ink
 jet ejector 92a-92n is operated in accordance with individualized filler
 pattern data associated therewith.
 It has been noted that the present invention will allow for an increase in
 the operational speed of acoustic ink printers. However, the present
 invention is also beneficial for acoustic ink printheads which have
 already been designed. When a given printhead has been designed, the
 design essentially freezes the firing or operational speed of the
 printhead. Therefore, while the concepts of the present invention are
 especially beneficial for increasing the speed of future designs, there
 are also benefits for existing conventionally designed systems. By
 relaxing the directionality constraints when in a shadow or dark area, the
 ink types which may be used with existing systems may be broadened.
 Particularly, inks with lower surface tension may be used in existing
 systems when the concepts of providing unique filler patterns are
 implemented. Use of lower surface tension inks can allow for a faster
 drying (though the inks would still be slow dry in an absolute sense) and
 potentially relax the requirements on the drying system.
 Thus, the present invention provides a manner of increasing the speed at
 which acoustic ink printers operate while at the same time maintaining
 directionality within non-dense color areas, and beneficially using
 misdirectionality which will occur due to the high operational speeds when
 in shadow or black areas. In addition to allowing a printer to operate at
 faster speeds, in devices where the speed is already fixed, the use of
 unique filling patterns can lead to an expanded use of different ink types
 that may be incorporated within the acoustic ink printing system.
 It is to also be noted that while some examples of fill patterns are
 described, there are numerous calculations available to generate
 sophisticated fill patterns which avoid sequential ink droplet ejection.
 It is to also be noted that while the present invention was described in
 conjunction with a maximum of ten droplets per pixel, systems having a
 larger or smaller number may also incorporate the concepts of the present
 invention.
 There are numerous ways that images can be processed to insure that drops
 are not fired on adjacent time cycles, for example,
 1. Halftoning at high addressability may be used; or
 2. Feedback controlled dithering (such as error diffusion) such that the
 threshold is increased on subsequent time cycles.
 However, an important aspect of the present invention is that adjacent
 firing of drops is avoided. While specific implementations have been shown
 to avoid adjacent firings, it is, again, understood other processes may
 exist and these should be considered within the scope of the described
 broader concept.
 The invention has been described with reference to the preferred
 embodiments. Obviously, modifications and alterations will occur to others
 upon a reading and understanding of this specification. It is intended to
 include all such modifications and alterations insofar as they come within
 the scope of the appended claims or the equivalents thereof.