Abstract:
A method for operating a drop generating device for selective formation of large-volume droplets and small-volume droplets, said method includes the steps of defining a small-drop waveform at least a pulse to form a small volume drop; defining a large-drop waveform that includes a set of pulses to form a large volume drop; creating a sequence of waveforms comprising waveforms of the small-drop and large-drop waveforms in response to image data the sequence of waveforms; and selectively inserting a perturbation pulse in the time interval between pulses of any two consecutive waveforms.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Reference is made to commonly assigned U.S. patent application Ser. No. ______ (Docket # 96204) filed concurrently herewith by Robert Link et al. al., entitled “Method For Operating Continuous Printers”, and commonly assigned U.S. patent application Ser. No. ______ (Docket # 96205) filed concurrently herewith by Robert Link et al., entitled “Drop Placement Method For Continuous Printers”, the disclosures of which are herein incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to the field of digitally controlled printing devices, such as continuous ink jet printers, having perturbations that break a liquid ink stream into large-volume droplets (print droplets) and small-volume droplets (deflected droplets) and having perturbations during the time period for creating the small-volume droplet that do are not sufficient to cause liquid breakage but are used selectively to calibrate the print droplets to corresponding pixels on the media. 
       BACKGROUND OF THE INVENTION 
       [0003]    Traditionally, digitally controlled color printing capability is accomplished by one of two technologies. Both require independent ink supplies for each of the colors of ink provided. Ink is fed through channels formed in the printhead. Each channel includes a nozzle from which droplets of ink are selectively extruded and deposited upon a medium. Typically, each technology requires separate ink delivery systems for each ink color used in printing. Ordinarily, the three primary subtractive colors, i.e. cyan, yellow and magenta, are used because these colors can produce, in general, up to several million shades or color combinations. 
         [0004]    The first technology, commonly referred to as “droplet on demand” ink jet printing, selectively provides ink droplets for impact upon a recording surface using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a flying ink droplet that crosses the space between the printhead and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink droplets, as is required to create the desired image. Typically, a slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle, and also forms a slightly concave meniscus at the nozzle helping to keep the nozzle clean. 
         [0005]    Conventional droplet on demand ink jet printers utilize a heat actuator or a piezoelectric actuator to produce the ink jet droplet at orifices of a printhead. With heat actuators, a heater, placed at a convenient location, heats the ink to cause a localized quantity of ink to phase change into a gaseous steam bubble that raises the internal ink pressure sufficiently for an ink droplet to be expelled. With piezoelectric actuators, a mechanical force causes an ink droplet to be expelled. 
         [0006]    The second technology, commonly referred to as “continuous stream” or simply “continuous” ink jet printing, uses a pressurized ink source that produces a continuous stream of ink droplets. Traditionally, the ink droplets are selectively electrically charged. Deflection electrodes direct those droplets that have been charged along a flight path different from the flight path of the droplets that have not been charged. Either the deflected or the non-deflected droplets can be used to print on receiver media while the other droplets go to an ink capturing mechanism (catcher, interceptor, gutter, etc.) to be recycled or disposed. U.S. Pat. No. 1,941,001, issued to Hansell, on Dec. 26, 1933, and U.S. Pat. No. 3,373,437 issued to Sweet et al., on Mar. 12, 1968, each disclose an array of continuous ink jet nozzles wherein ink droplets to be printed are selectively charged and deflected towards the recording medium. 
         [0007]    In another form of continuous ink jet printing, such as is described in U.S. Pat. No. 6,491,362 B1, issued to Jeanmaire, on Dec. 10, 2002, commonly assigned, included herein by reference, stimulation devices are associated with various nozzles of the printhead. These stimulation devices perturb the liquid streams emanating from the associated nozzle or nozzles in response to drop formation waveforms supplied to the stimulation devices by control means. The perturbations initiate the separation of a drop from the liquid stream. Different waveforms can be employed to create drops of a plurality of drop volumes. A controlled sequence of waveforms supplied to the stimulation device yields a sequence of drops, whose drop volumes are controlled by the waveforms used. A drop deflection means applies a force to the drops to cause the drop trajectories to separate based on the size of the drops. Some of the drop trajectories are allowed to strike the print media while others are intercepted by a catcher or gutter. 
         [0008]    Having understood some basics of a continuous inkjet printer, a brief description of synchronizing ejected print droplets to the print media is useful. In this regard, one or more printheads are positioned adjacent to a print media such that the printhead is able to deposit ink or other printing fluid on the print media as the print media is moved relative to the printhead. In many such printing systems, the relative velocity of the print media past the printhead (print speed) can vary widely, for example from 50 ft/min. to 1000 ft/min. The velocities are given by way of example and are not limiting to the claimed invention. While the print speed can vary widely, continuous inkjet printers typically have a base drop creation rate or frequency that is fixed, or at least can not be varied widely. In some cases the base drop creation frequency is defined by a printing system clock or by a natural characteristic of the drop generator such as its resonant frequency. As drops can be printed only when drops are created, the time between successive drops that are printed is limited to values that are an integer number of the base drop creation periods. When the print speed is low, the time between successive printed drops corresponds to the base drop creation period times a large integer, while for high print speeds the time between successive print drops corresponds to the base drop creation period times a small integer. 
         [0009]    In many types of continuous inkjet, a print drop can not be created at the base drop creation rate. For example in some printing systems that electro-statically deflect the non-print drops so that they strike the catcher, successive print drops must be separated by two or more catch drops. Similarly, by way of example, some print systems that separate print and catch drops by a means of a flow of gas across the drop trajectory, the print drops are formed from the ink that passes through the nozzle during not just one base drop creation period but rather in a plurality, typically three, of the base drop creation periods. 
         [0010]    As a result, there are certain print speeds at which the pixel locations on the print media move past the printhead at a rate, called the pixel rate, which exactly matches a frequency at which printable drops can be created. At such speeds the print drop creation rate becomes synchronized with the pixel rate. At these speeds, the time between successive pixel locations on the pint media passing the printhead is equal to an integer N times the base drop creation period; where N must be 2 or 3 or more, depending on the drop deflection mechanism. For example, if the base-drop creation frequency is 360 kHz, N=3, and the print resolution is 600 drops per inch, this occurs at 200 in/sec print speed.  FIG. 4A  illustrates this. There are three fundamental drops  100  created at the base-drop creation frequency for each pixel spacing  102  so the drop formation is synchronized with the pixel rate.  FIG. 4B  illustrates a sequence of drops  104  printed in a print media in one such printer in which the print drops  104  have three times the volume of the non-print drops  100 . Since the print drop formation is synchronized with the rate at which pixel moves past the printhead, the print drops are evenly spaced on the print media, landing at a consistent location within the respective pixels locations. 
         [0011]    In addition to the 200 in/sec speed at which the pixel rate equaled the base drop creation frequency divided by N=3, other print speeds at which the pixel rate equals the base drop creation rate divided by other larger integer values allow the pixel rate to be synchronized with the print drop creation rate. For the same base drop frequency and print resolution as in the example above and using N=4, a print speed of 150 in/sec is required to match the pixel rate exactly with the print drop creation rate.  FIG. 5A  illustrates such a case, four fundamental drops  100  are created for each pixel spacing  102 . The base drop creation rate is again synchronized with the pixel rate.  FIG. 5B  illustrates a sequence of print and catch drops where one print drop  104  is created for every four of the base drops  100  and where the print drop has a volume equal to three times the volume of the non-print base drops. A repeated pattern of one print drop  104  and one catch drop  100  are produced for each pixel location  102 . Again the print drops are uniformly spaced and land at a consistent place within each pixel interval. 
         [0012]    The printing system, however, needs to be able to print not just at those print speeds at which the pixel rate equals the print drop creation rate, but also at all intermediate speeds. For example, it must be able to print not just at 150 in/sec (where N=4) and 200 in/sec (where N=3), but also at print speeds between these two values. At such intermediate print speeds, the time between successive print drops is not fixed. The time between successive print drops is three times the base drop creation period for some of the drops, while other print drop pairs are separated by four times the base drop creation rate.  FIG. 6A  illustrates a sequence of the base drops  100  created at such a speed. During pixel intervals  1 ,  3 ,  4 ,  6 ,  7 ,  9 , and  10 , three drops  100  were created, while in pixel intervals  2 ,  5 , and  8  four drops were created. When creating print drops to print in each of the pixels, it is necessary to account for this variation in number of base drops that could be created in the pixel time interval. Therefore as shown in  FIG. 6B , during pixel intervals  1 ,  3 ,  4 ,  6 ,  7 ,  9 , and  10 , a single print drop  104  was created, while in pixel intervals  2 ,  5 , and  8  one print drop  106  and one catch drop  100  were created. While this ensures that the print drops land within the proper pixel locations, the spacing between print drops is not consistent. This is seen more clearly in  FIG. 6C , where the catch drops and the pixel interval markings have been removed to more clearly show how the print drops  106  would look on the print media. Some of the drops are separated from the preceding drop more than the other drops are from the drop that precedes them. These drop spacing intervals where the spacing is different (typically larger) than most of the drop spacing intervals are called synchronization bands or synch incidents  108 . 
         [0013]    As the print drops are not created at consistent time intervals, their spacing as they drop through the air is also not consistent. As a result the print drops do not encounter the same amount of air drag as they drop from the drop generator to the print media. The print drops  105  preceded by a synch incident  108  encounter more air drag than the other drops. The impact of these drops gets shifted as a result of the increased air drag, to produce a larger apparent synch band as seen in  FIG. 6D . Depending on the print conditions, the synch bands may be readily observed by to a person looking at the print. A means to overcome the visibility of synch bands is desired. The present invention addresses this needed improvement. 
       SUMMARY OF THE INVENTION 
       [0014]    A method for operating a drop generating device for selective formation of large-volume droplets and small-volume droplets, said method comprising the steps of defining a small-drop waveform at least a pulse to form a small volume drop; defining a large-drop waveform that includes a set of pulses to form a large volume drop; creating a sequence of waveforms comprising waveforms of the small-drop and large-drop waveforms in response to image data the sequence of waveforms; and selectively inserting a perturbation pulse in the time interval between pulses of any two consecutive waveforms. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  shows a simplified block schematic diagram of an example embodiment of a printer system made in accordance with the present invention; 
           [0016]      FIG. 2  is a schematic view of an example embodiment of a continuous printhead made in accordance with the present invention; 
           [0017]      FIG. 3  is a schematic view of a simplified gas flow deflection mechanism of the present invention; 
           [0018]      FIG. 4A  is a plot of base-drop creation frequency (N=3) per corresponding pixel in which the drop frequency and pixel rate synchronize; 
           [0019]      FIG. 4B  is a plot of large-volume droplets having three times the base drop frequency per corresponding pixel in which the drop frequency and pixel rate synchronize; 
           [0020]      FIG. 5A  is a plot of base-drop creation frequency (N=4) per corresponding pixel in which the drop frequency and pixel rate synchronize; 
           [0021]      FIG. 5B  is a plot of large-volume droplets having three times the base drop frequency and a catch drop per corresponding pixel in which the drop frequency and pixel rate synchronize; 
           [0022]      FIG. 6A  is plot of drop rate frequency (N=3) per corresponding pixel in which the drop rate and pixel rate are not synchronized; 
           [0023]      FIG. 6B  is a plot of large-volume droplets having three times the base drop frequency and a catch drop per corresponding pixel in which the drop frequency and pixel rate are not synchronized; 
           [0024]      FIG. 6C  is a plot of  FIG. 6B  with the catch drops removed; 
           [0025]      FIG. 6D  is a plot of  FIG. 6C  illustrating the air drag produced by the drop pattern of  FIG. 6C ; 
           [0026]      FIG. 7  is a plot showing a prior art sequence of waveforms for the creation of a sequence of drops from a nozzle. 
           [0027]      FIG. 8  is a plot showing a sequence of waveforms according to one embodiment of the invention for the creation of a sequence of drops from a nozzle. 
           [0028]      FIG. 9  is a plot showing a sequence of waveforms according to another embodiment of the invention for the creation of a sequence of drops from a nozzle. 
           [0029]      FIG. 10  is a plot showing a sequence of waveform according to an embodiment of the invention to compensate for first print drop air drag. 
           [0030]      FIG. 11  shows a portion of a single pixel wide line printed with the creation time for the print drops from the odd numbered jets phase shifted relative to the print drops from the even number jets 
           [0031]      FIGS. 12   a  and  b  show prior art waveforms for the odd and even numbered jets, respectively, used for printing the single pixel wide line of  FIG. 11 . 
           [0032]      FIG. 12   c - h  show waveforms for the odd and even numbered jets for printing a single pixel line according various embodiments of the invention. 
           [0033]      FIG. 13  show shows a portion of a sloping line that may be enhanced according to an embodiment of the invention 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0034]    The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. In the following description and drawings, identical reference numerals have been used, where possible, to designate identical elements. 
         [0035]    The example embodiments of the present invention are illustrated schematically and not to scale for the sake of clarity. One of the ordinary skills in the art will be able to readily determine the specific size and interconnections of the elements of the example embodiments of the present invention. 
         [0036]    As described herein, the example embodiments of the present invention provide a printhead or printhead components typically used in inkjet printing systems. However, many other applications are emerging which use inkjet printheads to emit liquids (other than inks) that need to be finely metered and deposited with high spatial precision. As such, as described herein, the terms “liquid” and “ink” refer to any material that can be ejected by the printhead or printhead components described below. 
         [0037]    Referring to  FIG. 1 , a continuous ink jet printer system  20  includes an image source  22  such as a scanner or computer which provides raster image data, outline image data in the form of a page description language, or other forms of digital image data. This image data is converted to half-toned bitmap image data by an image processing unit  24  which also stores the image data in memory. A plurality of drop forming mechanism control circuits  26  read data from the image memory and applies time-varying electrical pulses to a drop forming mechanism(s)  28  that are associated with one or more nozzles of a printhead  30 . These pulses are applied at an appropriate time, and to the appropriate nozzle, so that drops formed from a continuous ink jet stream will form spots on a recording medium  32  in the appropriate position designated by the data in the image memory. 
         [0038]    Recording medium  32  is moved relative to printhead  30  by a recording medium transport system  34 , which is electronically controlled by a recording medium transport control system  36 , and which in turn is controlled by a micro-controller  38 . The recording medium transport system shown in  FIG. 1  is a schematic only, and many different mechanical configurations are possible. For example, a transfer roller could be used as recording medium transport system  34  to facilitate transfer of the ink drops to recording medium  32 . Such transfer roller technology is well known in the art. In the case of page width printheads, it is most convenient to move recording medium  32  past a stationary printhead. However, in the case of scanning print systems, it is usually most convenient to move the printhead along one axis (the sub-scanning direction) and the recording medium along an orthogonal axis (the main scanning direction) in a relative raster motion. 
         [0039]    Ink is contained in an ink reservoir  40  under pressure. In the non-printing state, continuous ink jet drop streams are unable to reach recording medium  32  due to an ink catcher  42  that blocks the stream and which may allow a portion of the ink to be recycled by an ink recycling unit  44 . The ink recycling unit reconditions the ink and feeds it back to reservoir  40 . Such ink recycling units are well known in the art. The ink pressure suitable for optimal operation will depend on a number of factors, including geometry and thermal properties of the nozzles and thermal properties of the ink. A constant ink pressure can be achieved by applying pressure to ink reservoir  40  under the control of ink pressure regulator  46 . 
         [0040]    The ink is distributed to printhead  30  through an ink channel  47 . The ink preferably flows through slots or holes etched through a silicon substrate of printhead  30  to its front surface, where a plurality of nozzles and drop forming mechanisms, for example, heaters, are situated. When printhead  30  is fabricated from silicon, drop forming mechanism control circuits  26  can be integrated with the printhead. Printhead  30  also includes a deflection mechanism (not shown in  FIG. 1 ) which is described in more detail below with reference to  FIGS. 2 and 3 . 
         [0041]    Referring to  FIG. 2 , a schematic view of a continuous liquid printhead  30  is shown. A jetting module  48  of printhead  30  includes an array or a plurality of nozzles  50  formed in a nozzle plate  49 . In  FIG. 2 , nozzle plate  49  is affixed to jetting module  48 . However, if preferred, nozzle plate  49  can be integrally formed with jetting module  48 . 
         [0042]    Liquid, for example, ink, is emitted under pressure through each nozzle  50  of the array to form filaments of liquid  52 . In  FIG. 2 , the array or plurality of nozzles extends into and out of the figure and preferably the nozzle array is a linear array of nozzles. 
         [0043]    Jetting module  48  is operable to form liquid drops having a first size and liquid drops having a second size through each nozzle. To accomplish this, jetting module  48  includes a drop stimulation or drop forming device or transducer  28 , for example, a heater, piezoelectric transducer, EHD transducer, or a MEMS actuator, that, when selectively activated, perturbs each filament of liquid  52 , for example, ink, to induce portions of each filament to break off from the filament and coalesce to form drops  54 ,  56 . 
         [0044]    In  FIG. 2 , drop forming device  28  is a heater  51  located in a nozzle plate  49  on one or both sides of nozzle  50 . This type of drop formation is known and has been described in, for example, U.S. Pat. No. 6,457,807 B1, issued to Hawkins et al., on Oct. 1, 2002; U.S. Pat. No. 6,491,362 B1, issued to Jeanmaire, on Dec. 10, 2002; U.S. Pat. No. 6,505,921 B2, issued to Chwalek et al., on Jan. 14, 2003; U.S. Pat. No. 6,554,410 B2, issued to Jeanmaire et al., on Apr. 29, 2003; U.S. Pat. No. 6,575,566 B1, issued to Jeanmaire et al., on Jun. 10, 2003; U.S. Pat. No. 6,588,888 B2, issued to Jeanmaire et al., on Jul. 8, 2003; U.S. Pat. No. 6,793,328 B2, issued to Jeanmaire, on Sep. 21, 2004; U.S. Pat. No. 6,827,429 B2, issued to Jeanmaire et al., on Dec. 7, 2004; and U.S. Pat. No. 6,851,796 B2, issued to Jeanmaire et al., on Feb. 8, 2005. 
         [0045]    Typically, one drop forming device  28  is associated with each nozzle  50  of the nozzle array. However, a drop forming device  28  can be associated with groups of nozzles  50  or all of nozzles  50  of the nozzle array. 
         [0046]    When printhead  30  is in operation, drops  54 ,  56  are typically created in a plurality of sizes, for example, in the form of large drops  56 , a first size, and small drops  54 , a second size. The ratio of the mass of the large drops  56  to the mass of the small drops  54  is typically approximately an integer between 2 and 10. A drop stream  58  including drops  54 ,  56  follows a drop path or trajectory  57 . 
         [0047]    Printhead  30  also includes a gas flow deflection mechanism  60  that directs a flow of gas  62 , for example, air, past a portion of the drop trajectory  57 . This portion of the drop trajectory is called the deflection zone  64 . As the flow of gas  62  interacts with drops  54 ,  56  in deflection zone  64  it alters the drop trajectories. As the drop trajectories pass out of the deflection zone  64  they are traveling at an angle, called a deflection angle, relative to the un-deflected drop trajectory  57 . 
         [0048]    Small drops  54  are more affected by the flow of gas than are large drops  56  so that the small drop trajectory  66  diverges from the large drop trajectory  68 . That is, the deflection angle for small drops  54  is larger than for large drops  56 . The flow of gas  62  provides sufficient drop deflection and therefore sufficient divergence of the small and large drop trajectories so that catcher  42  (shown in  FIG. 3 ) can be positioned to intercept the small drop trajectory  66  so that drops following this trajectory are collected by catcher  42  while drops following the other trajectory bypass the catcher and impinge a recording medium  32  (shown in  FIG. 3 ). 
         [0049]    When catcher  42  is positioned to intercept small drop trajectory  66 , large drops  56  are deflected sufficiently to avoid contact with catcher  42  and strike the print media. When catcher  42  is positioned to intercept small drop trajectory  66 , large drops  56  are the drops that print, and this is referred to as large drop print mode. 
         [0050]    Jetting module  48  includes an array or a plurality of nozzles  50 . Liquid, for example, ink, supplied through channel  47 , is emitted under pressure through each nozzle  50  of the array to form filaments of liquid  52 . In  FIG. 2 , the array or plurality of nozzles  50  extends into and out of the figure. 
         [0051]    Drop stimulation or drop forming device  28  (shown in  FIGS. 1 and 2 ) associated with jetting module  48  is selectively actuated to perturb the filament of liquid  52  to induce portions of the filament to break off from the filament to form drops. In this way, drops are selectively created in the form of large drops and small drops that travel toward a recording medium  32 . 
         [0052]    Referring to  FIGS. 2 and 3 , positive pressure gas flow structure  61  of gas flow deflection mechanism  60  is located on a first side of drop trajectory  57 . Positive pressure gas flow structure  61  includes first gas flow duct  72  that includes a lower wall  74  and an upper wall  76 . Gas flow duct  72  directs gas flow  62  supplied from a positive pressure source  92  at downward angle θ of approximately a 45° relative to liquid filament  52  toward drop deflection zone  64  (also shown in  FIG. 2 ). An optional seal(s)  80  provides an air seal between jetting module  48  and upper wall  76  of gas flow duct  72 . 
         [0053]    Upper wall  76  of gas flow duct  72  does not need to extend to drop deflection zone  64  (as shown in  FIG. 3 ). In  FIG. 3 , upper wall  76  ends at a wall  96  of jetting module  48 . Wall  96  of jetting module  48  serves as a portion of upper wall  76  ending at drop deflection zone  64 . 
         [0054]    Negative pressure gas flow structure  63  of gas flow deflection mechanism  60  is located on a second side of drop trajectory  57 . Negative pressure gas flow structure includes a second gas flow duct  78  located between catcher  42  and an upper wall  82  that exhausts gas flow from deflection zone  64 . Second duct  78  is connected to a negative pressure source  94  that is used to help remove gas flowing through second duct  78 . An optional seal(s)  80  provides an air seal between jetting module  48  and upper wall  82 . 
         [0055]    As shown in  FIG. 3 , gas flow deflection mechanism  60  includes positive pressure source  92  and negative pressure source  94 . However, depending on the specific application contemplated, gas flow deflection mechanism  60  can include only one of positive pressure source  92  and negative pressure source  94 . 
         [0056]    Gas supplied by first gas flow duct  72  is directed into the drop deflection zone  64 , where it causes large drops  56  to follow large drop trajectory  68  and small drops  54  to follow small drop trajectory  66 . As shown in  FIG. 3 , small drop trajectory  66  is intercepted by a front face  90  of catcher  42 . Small drops  54  contact face  90  and flow down face  90  and into a liquid return duct  86  located or formed between catcher  42  and a plate  88 . Collected liquid is either recycled and returned to ink reservoir  40  (shown in  FIG. 1 ) for reuse or discarded. Large drops  56  bypass catcher  42  and travel on to recording medium  32 . Alternatively, catcher  42  can be positioned to intercept large drop trajectory  68 . Large drops  56  contact catcher  42  and flow into a liquid return duct located or formed in catcher  42 . Collected liquid is either recycled for reuse or discarded. Small drops  54  bypass catcher  42  and travel on to recording medium  32 . 
         [0057]    Referring to  FIG. 2 , alternatively, deflection can be accomplished by applying heat asymmetrically to filament of liquid  52  using an asymmetric heater  51 . When used in this capacity, asymmetric heater  51  typically operates as the drop forming mechanism in addition to the deflection mechanism. This type of drop formation and deflection is known having been described in, for example, U.S. Pat. No. 6,079,821, issued to Chwalek et al., on Jun. 27, 2000. 
         [0058]    As shown in  FIG. 3 , catcher  42  is a type of catcher commonly referred to as a “Coanda” catcher. However, the “knife edge” catcher shown in  FIG. 1  and the “Coanda” catcher shown in  FIG. 3  are interchangeable and work equally well. Alternatively, catcher  42  can be of any suitable design including, but not limited to, a porous face catcher, a delimited edge catcher, or combinations of any of those described above. 
         [0059]    Referring to  FIG. 7 , there is shown a sequence of waveforms for the creation of a sequence of drops from a nozzle. The sequence includes a small drop waveform  200  for creation of a fundamental drop  100 , and a large drop waveforms  206  for the creation of drop  106  having larger volume than the fundamental drop. In this illustration, the waveform  206  has three times the period as the waveform  200 , and the drops  106  created by the waveforms  206  have three times the volume of the fundamental drop  100 . The waveforms may comprise a single pulse such as waveform  200 , or they may comprise a plurality of pulses such as waveform  206  includes. According to the teaching of US Patent Application 2008/0284827, included herein by reference, waveform  200 , having a time period equal to the base drop formation period, creates first set of perturbations on the diameter of the liquid stream, having a spatial period x on the liquid stream, which causes the liquid stream to form into small-volume droplets. The final pulse of waveform  206 , the waveform having a time period of M times the time period of waveform  200 , produces a second set of perturbations on the liquid stream, the second set of perturbations having a spatial period M times x that causes a large-volume droplet to form, in which the large-volume droplet is M times the volume of the small-volume droplets. The earlier pulses in the large drop waveform create produce a third set of perturbations on the diameter of the liquid stream during the large drop waveform period. The spatial period between the perturbations of the third set of perturbations is sufficiently short so that the segment of the liquid stream that forms the large-volume droplet is not broken up thereby. 
         [0060]    As explained in the background, printing at speeds at which the drop formation rate is not synchronized with the pixel rate requires periodic synch bands, a non-uniform spacing of print drops to keep the print drops aligned to with the appropriate pixel locations. These synch bands or synch incidents are most noticeable when printing at speeds close the maximum speed of the printer. For the example described in the background, synch bands would be most noticeable at speeds approaching 200 in/sec; 200 in/sec being the print speed at which three fundamental drops are created for each pixel interval. At speeds approaching the 200 in/sec speed, three fundamental drops were created in some of the pixels intervals while four fundamental drops were created for other pixel intervals as illustrated in  FIG. 6A . When printing drops in each pixel location, this requires inclusion of a catch drop between the some of the print drops as shown in  FIG. 6B . But that leads to the non-uniform spacing of printed drops on the print media illustrated in  FIG. 6C  or  6 D. 
         [0061]    The visibility of the synch bands can be reduced significantly, according the present invention, by altering the velocity of the print drops on one or both sides of the catch drop at the synch incident. By slowing down the print drop  106   c  that precedes the catch drop  100  that forms the synch band gap  110 , the impact point of the print drop  106   c  can be shifted slightly into the gap  110   a  to reduce the visibility of the gap. In the same way, speeding up the print drop  106   d , the drop that follows catch drop  100 , the impact point of print drop  106   d  can be shifted slightly into the gap  110 , reducing the visibility of the synch band. 
         [0062]    One method for altering the velocity of a drop is to alter the energy of the activation pulse that created the drop. For example, increasing the duty cycle of the pulse can increase the velocity of the drop, and decreasing the duty cycle of the pulse can decrease the velocity of the drop. While altering the duty cycle is effective at altering the drop velocity, it has been seen to also affect the drop velocity of the both the drop that proceeds and the drop that follows the target drop for the velocity adjustment. Under some conditions, it also can alter the drop formation characteristics, leading to increased satellite drop formation or altering the drop breakoff distance or the time to properly coalesce into a well formed drop. 
         [0063]    An alternate means for altering the velocity of a drop is by inserting a narrow activation pulse either before or after the pulses employed to create the drop.  FIG. 8  shows a sequence of waveforms like that shown in  FIG. 7 , but a velocity modifying pulse  208  is inserted between the last pulse of waveform  206   c  and the pulse of waveform  200 . The inserted velocity modifying pulse  208  has the effect of slowing down the drop  106   c  produced by the waveform  206   c  and speeding up the drop  100  produced by the waveform  200 . By slowing down print drop  106   c , its impact position on the print media is shifted as indicated by arrow  109  to partially fill in the synch band  110 . The speeding up of drop  100  has no effect on the printed image as drop  100  is deflected to the catcher and does not strike the paper. 
         [0064]      FIG. 9  shows a sequence of waveforms in which a velocity modifying pulse  209  is inserted between the pulse of waveform  200  and the first pulse of waveform  206   d . Inserted pulse  209  has the effect of slowing down drop  100 , and speeding up drop  106   d . The speeding up of drop  106   d  shifts its impact position on the print media, as indicated by arrow  111  to partially fill in gap  110 . The slowing down of drop  100  has no effect on print as drop  100  is directed to the catcher. The slight velocity changes of the catch drops by the inserted velocity modifying pulses have no detrimental effect on the ability to catch such drops. 
         [0065]    Insertion of the velocity modifying pulse does not produce any shift of the waveforms that follow it. The inserted pulse is not an inserted waveform that delays all the following waveforms, but rather a pulse that is inserted into the time interval between the last pulse of the one waveform and the first pulse of the next waveform. In  FIG. 8 , the pulse is inserted after the last pulse  356  of waveform  206   c  into the time interval of waveform  200  prior to pulse  300  of waveform  200 . In  FIG. 9 , the velocity modifying pulse  209  is inserted after the pulse  300  of waveform  200  into the time interval of waveform  206   d  that precedes the first pulse  316  of waveform  206   d . As the time interval into which the velocity modifying pulse is inserted is sufficiently short, the velocity modifying pulse doesn&#39;t cause an additional drop to break off from the continuous stream flowing from the nozzle. 
         [0066]    The velocity modifying pulses have been described relative to their use for reducing the visibility of synch incidents. They may also be employed for other applications in which it is advantageous to modify the drop&#39;s velocity relative to the velocity of other drops. For example, when printing the stroke of a character with several consecutive print drops, the first print drop typically encounters more air drag than the following print drops. This can cause the first print drop to impact the print media closer to the second print drop than intended. By inserting a velocity modifying pulse in the time interval just prior to the first pulse of the waveform that creates the first print drop, its velocity can be increased to compensate at least partially for the increased air drag that it encounters. This is illustrated in  FIG. 10 . Drop  106   a  is the first of several print drops  106   b - 106   d  created after several catch drops  100   a - 100   c . These catch drops are created by waveforms  200   a - 200   c , and the print drops are created by waveforms  206   a - 206   d . The first print drop  206  encounters more air drag than the subsequent print drops, causing the impact location on the print media to be shifted to the right as denoted by arrow  112 . To compensate for the increased air drag on drop  106   a  encounters, a velocity modifying pulse  209  is inserted after the pulse of waveform  200   c  and prior to the first pulse  316  of waveform  206   a . The inserted velocity modifying pulse increases the initial velocity of the first print drop  106   a  relative to that of the other print drops, causing its impact position on the print media to be shifted to the left as denoted by arrow  113 , at least partially compensating for the air drag on the first print drop. 
         [0067]    In US Published Application 20080231669, which is herein incorporated by reference, and in application Ser. No. 12/613,683 filed Nov. 6, 2009, which is herein incorporated by reference, it disclosed that print quality can be improved y intentionally phase shifting the creation of print drops from the odd number jets relative to the creation of print drops from the even number jets. While the phase shift is effective in improving the overall print quality, it can introduce a small stagger in the impact positions of the drops from the odd and even numbered jets. This stagger has been found to depend on the spacing between the printhead and the print media and on the drop to drop spacing. Under certain conditions, the stagger of the dots on the print media can be opposite of what one would expect based on which jets have the drop creation phase delayed behind the other.  FIG. 11  shows a portion of a single pixel wide line printed on the print media with the print drops from the odd numbered jets phase shifted relative to the print drops from the even number jets. The direction of the print media motion relative to the printhead is indicated by arrow  249 . The phase shift produces about a stagger  248  between print location of the dots printed by the even and odd jets,  250 , and  252  respectively, with the printed dots from the odd jets  252  appearing to lag behind the drops from the even drops  250 . In this context, the even jets while be called leading jets and the odd jets will be called lagging jets, because the dots printed by the even jets appear to lag behind the dots printed by the odd jets. The terms leading and lagging jets are not intended to indicate which set of jets has its drop creation phase delayed relative to the other. As suggested by the arrows  254 , the print location stagger can be reduced by adjusting the velocity of the drops from the even jets, the leading jets. Alternatively, the print location stagger can be reduced by adjusting the velocity of the drops from the odd numbered jets, the lagging jets; denoted by arrows  256 . In yet another embodiment, the print location stagger can be reduced by both adjusting the velocity of the drops from the leading jets and adjusting the velocity of the drops from the lagging jets, suggested by both arrows  254  and  256 . 
         [0068]      FIGS. 12   a  and  b  show the waveforms and the drops created by the waveforms for creating a single pixel line for the odd numbered jets and the even numbered jets respectively.  FIG. 12   a  shows a series of catch drops  100 , a print drop  252 , and several more catch drops  100  from an odd numbered jet. These drops are created by a series of waveforms  200  for creating catch drops, a waveform  188  for creating the print drop, and several more waveforms  200 .  FIG. 12   b  shows a similar sequence of catch drops  100  with one print drop  250  created by a sequence of catch drop waveforms  200 , print drop waveform  190 , and additional catch drop waveforms  200 . The waveforms in  FIGS. 12   a  and  b  do not include velocity modifying pulses. 
         [0069]      FIG. 12   c  shows a sequence of waveforms according to the invention where a velocity modifying pulse  258  is included to increase the velocity of the print drop  252  from the lagging jets as indicated by arrow  256 , but no velocity modifying pulses are employed to modify the velocity of the print drop  250  for the leading jets in  FIG. 12   d .  FIG. 12   e  shows a sequence of waveforms according to the invention where no velocity modifying pulses are employed to modify the velocity of the print drop  252  from the lagging jets, but a velocity modifying pulse  260  is inserted after the pulses of waveform  190 , which created the print drop, to reduce the velocity of the print drop  250  from the leading jets in  FIG. 12   f .  FIGS. 12   g  and  h  show waveforms where a velocity modify pulse  262  is used to reduce the velocity of the print drop  250  from the leading jets and a velocity modifying pulse  264  is used to increase the velocity of the print drop  252  of the lagging jets. The use of velocity modifying pulses to modify the velocity of print drops from both the even and the odd jets, allows a larger drop placement adjustment to be made or allows lower energy velocity modifying pulses to be used than are required if the velocity modifying pulses are applied to only one of the even or the odd numbered jets. 
         [0070]    The embodiments of  FIG. 12   c - h  illustrate that the velocity modifying pulses can be applied differently for the odd and even number jets when there is a phase shift in the drop creation time for odd and even numbered jets for the printing of single pixel wide lines. In a similar manner, velocity modifying pulses can be applied differently for the odd and even number jets on the leading and trailing edges of strokes and even at synch incidents. For example, velocity modifying pulse may be inserted just prior to the waveforms form for creating print drops from only the even numbered jets at the leading edge of a stroke and may be inserted immediately after the waveforms for creating print drops from only the odd numbered jets at the trailing edge of a stroke. 
         [0071]    When printing sloping lines or strokes of characters, it is necessary to stair step the edges of the lines or strokes as shown in  FIG. 13 . In some instances, the steps can be detected by an observer, resulting in a reduction in perceived print quality. In an embodiment of the invention pre-pulses and/or post pulses can be employed to reduce the visibility of such steps. Inserting a velocity modifying pulse after the pulse or pulses of the waveform that created drop  264 , a post-pulse, can reduce the velocity of drop  264  causing the print location of drop  264  to be shifted to the right as indicated by arrow  266 . Inserting a velocity modifying pulse before the pulse or pulses of the waveform that created drop  268 , a pre-pulse, can increase the velocity of drop  268  causing the print location of drop  268  to be shifted to the left as indicated by arrow  270 . By means of either or both of a velocity modifying pre-pulse and a velocity modifying post-pulse as described above, the step can be rounded to reduce its visibility. In a similar manner, velocity modifying pre-pulses and post pulses can be employed for drops  272  and  274  to alter their velocities to cause their impact positions to be slightly shifted as indicated by arrows  276  and  278  respectively, to reduce the visibility of the step on the trailing edge of the line or stroke. 
         [0072]    The amount by which the impact location of a print drop is shifted by a velocity modifying pulse is proportional to velocity shift of the print drop produced by the pulse. The velocity shift produced the velocity modifying pulses is related to the energy of the pulse. Increasing the pulse energy, by either increasing the pulse amplitude or pulse width, increases the amount of velocity shift produced. Adjustment of the pulse energy therefore serves as a means to adjust the impact position shift produced by velocity modifying pulses. The impact point shift produced by the velocity modifying pulses also depends on the spacing between the nozzle plate and the print media. As a result the preferred pulse energy for optimizing some aspect of the print can depend of the spacing between the nozzle plate and the print media. In some embodiments, the printing system can include a test pattern or other test to determine the optimum pulse energies for the velocity modifying pulses. 
         [0073]    In yet another embodiment the width of character stroke can be modulated by means of velocity modifying pulses to pull forward or push back the drops at make the edges or the trailing edges of the strokes. Then can be used to enhance the readability of bar codes for example by refining the width ratios of wide and narrow strokes. 
         [0074]    For some of these embodiments the velocity modifying pulses would be applied based on characteristics of the print data including, but not limited to, speeding up the first print drop in a series of print drops, smoothing out a step and refining the width of character strokes. In other embodiments, the need for a velocity modifying pulse is based on characteristics present at the printing, such as the odd-even or the synch band correction. Still further, in some embodiments, determining the need for a velocity modifying pulse includes determining the need based on at least the sequence of the following drops. For example, the following drop may include that the following drop is a catch drop. Alternatively, determining the need for a velocity modifying pulse includes determining the need based on the sequence of drops from an adjacent jet. 
         [0075]    The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.