FIELD OF THE INVENTION 
     The present invention relates to continuous inkjet printing in general and in particular to producing ink drops with a reduced set of waveforms. 
     BACKGROUND OF THE INVENTION 
     Traditionally, digitally controlled color inkjet printing 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 drops 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. 
     The first technology, commonly referred to as “drop on demand” inkjet printing, selectively provides ink drops 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 drop 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 drops, as 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. 
     Conventional drop on demand inkjet printers utilize a heat actuator or a piezoelectric actuator to produce the ink drop 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 drop to be expelled. With piezoelectric actuators, a mechanical force causes an ink drop to be expelled. 
     The second technology, commonly referred to as “continuous stream” or simply “continuous” inkjet printing, uses a pressurized ink source that produces a continuous stream of ink drops. Traditionally, the ink drops are selectively electrically charged. Deflection electrodes direct those drops that have been charged along a flight path different from the flight path of the drops that have not been charged. Either the deflected or the non-deflected drops can be used to print on receiver media while the other drops go to an ink capturing mechanism (catcher, interceptor, gutter, etc.) to be recycled or disposed. U.S. Pat. No. 1,941,001 (Hansell) and U.S. Pat. No. 3,373,437 (Sweet et al.) each disclose an array of continuous inkjet nozzles wherein ink drops to be printed are selectively charged and deflected towards the recording medium. 
     In another form of continuous inkjet printing, such as is described in commonly-assigned U.S. Pat. No. 6,491,362 (Jeanmaire), 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. 
     In this form of continuous inkjet printing, typically a printhead includes a large number of nozzles formed on a nozzle plate, with each nozzle having an associated stimulation device that is also formed on the nozzle plate. Since each stimulation device is typically activated by an independently controlled sequence of waveforms, a large number of electrical connections must be made between the stimulation devices on the nozzle plate and the drop formation mechanism control circuit that provides the sequences of waveforms. Typically the drop forming mechanism control circuitry is also formed on the nozzle plate to reduce the number of electrical connections that must be made to the nozzle plate. The drop forming mechanism control circuitry formed on the nozzle plate is typically formed using a CMOS process. The drop forming mechanism control circuit receives a set of waveforms and waveform selection control information from an image synchronization controller, which is typically located on a circuit board. 
     In this printing system, typically two volumes of drops are used, a small drop having a small drop volume and a large drop whose volume is approximately N times the small drop volume, where N is an integer. Small drops are formed by small drop waveforms having a period, called the small drop period. Large drops are formed by large drop waveforms having a large drop period equal to N times the small drop period. The small drop frequency, the inverse of the small drop period, serves as the base or fundamental frequency for drop formation. The base, or fundamental, drop creation rate or frequency is typically fixed, or at least cannot 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 described in commonly assigned U.S. Pat. No. 7,828,420 (Fagerquist et al), the large drop waveform can include a number of activation pulses within the large drop period to improve the formation or coalescence time of the large drop, uniformity of drop velocity, and the drop-to-drop spacing. As discussed therein, the large drop waveform can influence the uniformity of drop velocity and drop-to-drop spacing for small drops formed after the large drop formed by the large drop waveform. While the large drop waveform can be designed to improve the drop velocity uniformity of subsequent small drops, it is useful to provide more than one small drop waveform: one small drop waveform for use when the preceding drop is a large drop and another small drop waveform for use when the preceding drop is a small drop. Similarly, it is desirable to provide more than one large drop waveform: one large drop waveform for use when the preceding drop is a large drop and another large drop waveform for use when the preceding drop is a small drop. As the small drop period serves as the basic time period for drop formation, it is useful to define the large drop waveforms as defined sequences of large drop sub-waveforms, where each large drop sub-waveform has a period equal to the small drop period. 
     As the base drop frequency is fixed, or at least cannot be varied widely, and since there are a plurality of small drop waveforms and large drop sub-waveforms, the traditional method of controlling the sequence of drops formed by each nozzle in the printhead has involved the image synchronization controller providing all of the small drop waveforms and large drop sub-waveforms along with waveform selection control signals to the drop forming mechanism control circuit during each base drop period. Providing all of the waveforms and waveform selection control signals from the image synchronization controller to the drop forming mechanism control circuit during each base drop period requires many interconnects between the image synchronization controller and the drop forming mechanism control circuit. For example, in one implementation, there are eight unique waveforms for a 512-nozzle segment of the nozzle plate. The control circuitry associated with each nozzle requires a 3-bit waveform selection control signal to select one of the eight waveforms. This results in a total of 1536 select bits to be sent to the nozzle plate segment during each base drop period. The printhead operates with a base drop frequency of 480 kHz, resulting in a required bandwidth of approximately 750 megabits/second for the select signals. To keep the data rate low enough for the CMOS process used to fabricate the nozzle plate, the interconnect between the image synchronization controller and the nozzle plate segment that carries the waveform selection signals must be at least 8 bits wide. When combined with clock, latch, and enable signals necessary to operate the nozzle plate segment, this results in a total of 19 interconnects to control the nozzle plate segment. It is desirable to minimize the number of interconnects to the nozzle plate to reduce manufacturing costs and improve reliability. 
     It is also desirable to minimize the drop forming mechanism control circuitry on the nozzle plate to improve manufacturing yield and increase the number of nozzle plates that can be produced from one silicon wafer, thereby reducing the manufacturing cost. 
     SUMMARY OF THE INVENTION 
     Briefly, according to one aspect of the present invention a method of producing ink drops in a printing apparatus sends print-nonprint data from a controller to at least one inkjet nozzle. The print-nonprint data includes data on a current ink drop and data on at least one previous ink drop. A set of waveforms is provided to the at least one nozzle and a waveform based on the print-nonprint data is selected. The selected waveform is supplied to an ink droplet formation device associated with the at least one nozzle and an ink drop is produced from the at least one nozzle. 
     According to a feature of the present invention, the number of waveforms in the set of waveforms supplied by the controller to the nozzle plate is reduced without limiting the ability of the drop forming device to produce different types of drops. This reduction in the number of supplied waveforms reduces the number of interconnects to the printhead, reducing manufacturing cost and improving reliability. 
     According to another feature of the present invention, the number of waveform selection signals supplied by the controller to the nozzle plate and the frequency with which the selection signals are supplied are reduced. This reduction in the amount of supplied waveform selection data further reduces the number of interconnects to the nozzle plate. 
     According to yet another feature of the present invention, the amount of control circuitry to load and latch the waveform selection signals, distribute the waveforms to the drop forming devices and select the appropriate waveform for each drop forming device is reduced. If the control circuitry is implemented on the silicon substrate of the printhead, the reduction in control circuitry may improve nozzle plate manufacturing yield as well as increase the number of nozzle plates that can be produced from a silicon wafer. 
     The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a simplified block schematic diagram of an example embodiment of a printer system made in accordance with the present invention; 
         FIG. 2  is a schematic view of an example embodiment of a continuous printhead made in accordance with the present invention; 
         FIG. 3  is a schematic view of a simplified gas flow deflection mechanism of the present invention; 
         FIG. 4  is a plot of waveform and drop sequences for print and non-print pixels when there are three base drop periods per pixel. 
         FIG. 5  is a table of pixel waveform sequences when there are three base drop periods per pixel. 
         FIG. 6  is a plot of waveform and drop sequences for print and non-print pixels when there are four base drop periods per pixel. 
         FIG. 7  is a table of pixel waveform sequences when there are four base drop periods per pixel. 
         FIG. 8  is a plot of waveform and drop sequences for print and non-print pixels when the number of base drop periods per pixel varies between three and four. 
         FIG. 9  is a table of pixel waveform sequences when the number of base drop periods per pixel varies between three and four. 
         FIG. 10  is a schematic view of a drop forming mechanism control circuit. 
         FIG. 11  is a timing diagram illustrating the operation of a drop forming mechanism control circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. 
     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. 
     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. 
     Referring to  FIG. 1 , a continuous inkjet 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. An image synchronization controller  25  receives data from the image memory and synchronization signals for the paper transport control  36  to align the image data with the movement of the recording medium  32 . The drop forming mechanism control circuit  26  receives the synchronized image data from image synchronization controller  25  and applies time-varying electrical pulses to the 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 inkjet stream will form spots on a recording medium  32  in the appropriate position designated by the data in the image memory. 
     Recording medium  32  is moved relative to printhead  30  by a recording medium transport system  34 , which is electronically controlled by a paper transport control  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. 
     Ink is contained in an ink reservoir  40  under pressure. In the non-printing state, continuous inkjet 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 . 
     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, also commonly called a nozzle plate, of printhead  30  to its front surface, where a plurality of nozzles and drop forming mechanisms, for example, heaters, are situated. When the nozzle plate of the printhead  30  is fabricated from silicon, the drop forming mechanism control circuit  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 . 
     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 . 
     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. 
     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 . 
     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. Nos. 6,457,807 (Hawkins et al.); 6,491,362 (Jeanmaire); 6,505,921 (Chwalek et al.); 6,554,410 (Jeanmaire et al.); 6,575,566 (Jeanmaire et al.); 6,588,888 (Jeanmaire et al.); 6,793,328 (Jeanmaire); 6,827,429 (Jeanmaire et al.); and 6,851,796 (Jeanmaire et al.). 
     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. When the drop forming device(s) is integrated into nozzle plate  49 , which is fabricated from silicon, a portion of the drop forming mechanism control circuit  26  can be integrated with the nozzle plate. This portion of the drop forming mechanism control circuit is referred to as nozzle plate control circuit  53 . Other portions of the drop forming mechanism control circuit, as well as the image synchronization controller  25 , can reside on a separate circuit board that is also part of the printhead. These are referred to as jetting module electronics  55 . The nozzle plate control circuit  53  is connected to the jetting module electronics  55  by means of an interconnect  59 . 
     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 . 
     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 . 
     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 ink 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 ink catcher  42  while drops following the other trajectory bypass the catcher and impinge a recording medium  32  (shown in  FIG. 3 ). 
     When ink catcher  42  is positioned to intercept small drop trajectory  66 , large drops  56  are deflected sufficiently to avoid contact with ink catcher  42  and strike the print media. When ink 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. 
     Jetting module  48  includes an array or a plurality of nozzles  50 . Liquid, for example, ink, supplied through ink 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. 
     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 . 
     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)  84  provides an air seal between jetting module  48  and upper wall  76  of gas flow duct  72 . 
     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 . 
     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)  84  provides an air seal between jetting module  48  and upper wall  82 . 
     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 . 
     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 ink catcher  42 . Small drops  54  contact face  90  and flow down face  90  and into a liquid return duct  86  located or formed between ink 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 ink catcher  42  and travel on to recording medium  32 . Alternatively, ink catcher  42  can be positioned to intercept large drop trajectory  68 . Large drops  56  contact ink catcher  42  and flow into a liquid return duct located or formed in ink catcher  42 . Collected liquid is either recycled for reuse or discarded. Small drops  54  bypass ink catcher  42  and travel on to recording medium  32 . 
     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 (Chwalek et al.). 
     Referring to  FIG. 4 , there is shown a sequence of waveforms  100  for the creation of a sequence of drops from a nozzle. The waveform sequence  100  shows the waveforms used to create drops for a sequence of four pixels: a first print pixel  102 , a second print pixel  104 , a first non-print pixel  106 , and a second non-print pixel  108 . The drops resulting from the waveform sequence  100  are shown as large print drops  110  and  111  and small non-print drops  112  and  113 . The waveform sequence shows the case when the print speed is such that the number of base drop periods per pixel is three and the volume ratio of large print drops to small non-print drops is three. The waveform sequence assumes that the pixel preceding the first print pixel  102  is a non-print pixel. 
     Since there are three base drop periods per pixel in the waveform sequence  100 , there are three waveforms per pixel. The first print pixel  102  is comprised of waveforms  114   a ,  114   b  and  114   c . These waveforms act together to form a single large print drop  110 . Similarly, the second print pixel  104  is comprised of waveforms  114   d ,  114   b  and  114   c  which result in a single large print drop  111 . The waveform sequence for the second print pixel  104  is distinguished from the waveform sequence for the first print pixel  102  due to changes in the desired activation pattern of the drop forming device  28  required to account for the second large print drop  111  following immediately after the first large print drop  110  and being affected by that preceding large drop. The first non-print pixel  106  is comprised of waveforms  116   a ,  116   b  and  116   c . These waveforms are distinguished from each other due to the variations in the activation pattern of the drop forming device  28  necessary to ensure the three drops remain separate as they follow the preceding large print drop  111 . The second non-print pixel  108  is composed of waveform  116   d  repeated three times. The waveform sequence for the second non-print pixel  108  is distinguished from the waveform sequence for the first non-print pixel  106  due to changes in the desired activation pattern of the drop forming device  28  because the first small non-print drops  112  are following large print drop  111  and are affected by the preceding large drop as they travel from the nozzle  50  to the recording medium  32 . After the first non-print pixel  106  completes, the effects of large print drop  111  have dissipated and the second non-print pixel  108  is composed by repeating the steady-state waveform  116   d.    
     The number and relative size of the stimulus pulses in waveforms  114   a - 114   d  and  116   a - 116   d  in  FIG. 4  are shown for illustrative purposes only. The duration and number of stimulus pulses in each waveform may vary in order to improve drop formation, drop spacing, reduce satellite drops, or otherwise improve print quality. Such variations are understood to be within the scope of the invention. 
       FIG. 4  shows that there are four possible waveform sequences which correspond to combinations of print and non-print pixels. These sequences are: a printing pixel preceded by a non-printing pixel, as shown in first printing pixel  102 , a printing pixel preceded by another printing pixel, as shown in second printing pixel  104 , a non-printing pixel preceded by a printing pixel, as shown in first non-printing pixel  106 , and a non-printing pixel preceded by another non-printing pixel, as shown in second non-printing pixel  108 . As each line of pixels is printed by printhead  30 , one of these four waveform sequences is selectively used to activate each drop forming device  28  to create the desired pattern of small non-print drops and large print drops. The table in  FIG. 5  shows the waveform sequences for each of the four combinations of print and non-print pixels. 
     Referring to  FIG. 6 , there is shown a sequence of waveforms  120  for the creation of a sequence of drops from a nozzle for the case when the print speed is such that the number of base drop periods per pixel is four and the ratio of large print drops to small non-print drops is three. The waveform sequence  120  shows the waveforms used to create drops for a sequence of four pixels: a first print pixel  122 , a second print pixel  124 , a first non-print pixel  126 , and a second non-print pixel  128 . The drops resulting from the waveform sequence  100  are shown as large print drops  130  and  134  and small non-print drops  132 ,  136 ,  138  and  140 . The waveform sequence assumes that the pixel preceding the first print pixel  122  is a non-print pixel. 
     Since there are four base drop periods per pixel in the waveform sequence  120 , there are four waveforms per pixel. The first print pixel  122  is comprised of waveforms  114   a ,  114   b ,  114   c  and  116   a . These waveforms are distinguished from each other due to the variations in the activation pattern of the drop forming device  28  necessary to form a single large print drop  130  and to cause the creation of a separate small non-print drop  132 . Similarly, the second print pixel  124  is comprised of waveforms  114   a ,  114   b ,  114   c  and  116   a  which result in forming a single large print drop  134  and a separate small non-print drop  136 . In this case, the waveform sequence for the second print pixel  124  is the same as the waveform sequence for the first print pixel  122  since, in both cases, the large print drop is following a small non-print drop. 
     The first non-print pixel  126  is comprised of waveforms  116   b ,  116   c ,  116   d  and  116   d . These waveforms are distinguished from each other due to the variations in the activation pattern of the drop forming device  28  necessary to cause the four drops to remain separate as they follow the preceding large print drop  134 . The second non-print pixel  128  is composed of waveform  116   d  repeated four times. The waveform sequence for the second non-print pixel  128  is distinguished from the waveform sequence for the first non-print pixel  126  due to changes in the desired activation pattern of the drop forming device  28  because the first small non-print drops  138  are following large print drop  134  and are affected by the preceding large drop as they travel from the nozzle  50  to the recording medium  32 . After the first non-print pixel  126  completes, the effects of large print drop  134  have dissipated and the second non-print pixel  128  is composed by repeating the steady-state waveform  116   d.    
     The number and relative size of the stimulus pulses in waveforms  114   a - 114   d  and  116   a - 116   d  in  FIG. 6  are shown for illustrative purposes only. Furthermore, while the waveforms  114   a - 114   d  and  116   a - 116   d  in  FIG. 6  are shown to be the same as the waveforms shown in  FIG. 4 , they may be different. The duration and number of stimulus pulses in each waveform may vary in order to improve drop formation, drop spacing, reduce satellite drops or otherwise improve print quality. Such variations are understood to be within the scope of the invention. 
     As in  FIG. 4 ,  FIG. 6  shows that there are four possible waveform sequences which correspond to combinations of print and non-print pixels. As each line of pixels is printed by printhead  30 , each of the plurality of nozzles  50  will use one of these four waveform sequences to activate the drop forming device  28  to create the desired pattern of small non-print drops and large print drops. The table in  FIG. 7  shows the waveform sequences for each of the four combinations of print and non-print pixels. 
     The printing system  20  needs to be able to print at multiple speeds, not just at those print speeds at which there are a constant integer number of base drop periods per pixel. At such intermediate print speeds, the time between successive print drops is not fixed. For example, the number of base drop periods per pixel may be three for some of pixels, while other pixels have four base drop periods per pixel.  FIG. 8  illustrates a waveform sequence for five pixels in which the pixels have a length of three base drop periods per pixel, except for the second pixel, which has a length of four base drop periods. The waveform sequence  160  shows the waveforms used to create drops for the five pixels: a first print pixel  162 , a second print pixel  163 , a third print pixel  164 , a first non-print pixel  166 , and a second non-print pixel  168 . The drops resulting from the waveform sequence  160  are shown as large print drops  170 ,  172  and  174  and small non-print drops  173 ,  176  and  178 . The waveform sequence assumes that the pixel preceding the first print pixel  162  is a non-print pixel. 
     In  FIG. 8 , there are three waveforms per pixel for the first, third, fourth and fifth pixels, and four waveforms for the second pixel. The length of the waveform sequence for the second pixel includes one additional waveform, which produces a small non-print drop  173 , to accommodate a slightly slower print speed than shown in  FIG. 4 . The determination of which pixel(s) require additional base drops is made by image synchronization controller  25 , based on synchronization signals received from paper transport control  36 . The synchronization controller  25  inserts additional base drop periods as required to keep large print drops aligned with the movement of the recording medium  32 . 
     When an additional base drop period is added to a pixel, the waveforms of the following pixel may be altered. Referring to  FIG. 8 , this is shown in the second print pixel  163  and third print pixel  164 . For both pixels, the preceding pixel was a print pixel, but the waveform sequence differs. The second print pixel  163  is comprised of waveforms  114   d ,  114   b ,  114   c  and  116   a , with waveforms  114   d ,  114   b  and  114   c  forming the large print drop  172 . The third print pixel  164  is comprised of waveforms  114   a ,  114   b  and  114   c  which together form the large print drop  174 . The waveform sequence for large print drop  174  differs from the waveform sequence for large print drop  172  due to the intervening small non-print drop  173  inserted at the end of the second print pixel  163 . 
       FIG. 9  shows an expanded table of waveform sequences for combinations of print and non-print pixels and whether the preceding pixel was three or four base drop periods in length. For pixels in which only three base drop periods are needed, the fourth waveform in the table is skipped. While the table shows eight possible waveform sequences, only four of them are applicable during the printing of any given row of pixels, since for the preceding row of pixels, all of the pixels would have been printed with either three or four base drop periods. 
     The preceding examples have shown four waveforms used for generating large print drops and four waveforms used for generating small non-print drops. Implementations using a greater or fewer number of waveforms for either large print drops or small non-print drops are understood to be within the scope of the invention. Similarly, implementations that use fewer than three or more than four base drop periods per pixel are also understood to be within the scope of the invention. 
     Referring to  FIG. 10 , drop forming mechanism control circuit  26  is shown. The DATA, CLOCK, LATCH, WAVEFORM and ENABLE signals are inputs to the control circuit generated by image synchronization controller  25 , which may be a microprocessor, application-specific integrated circuit, field programmable gate array, or similar device. Image data, consisting of print/non-print values, is provided via the DATA signal which drives the input to shift register bit  200 , the first element of the array of shift register bits  202 . The number of elements, N, in shift register  202  corresponds to the number of nozzles  50  in nozzle plate  49 . Image data is serially loaded into shift register bit  200  and subsequently shifted into successive shift register bits according to the CLOCK signal from image synchronization controller  25 . After N clock pulses of the CLOCK signal, shift register  202  holds the complete set of print/non-print data for the pixels in the next print line. 
     Once shift register  202  is loaded with the print/non-print data for the next print line and image synchronization controller  25  receives an indication from paper transport control  36  that recording medium  32  is in position to receive the next line of image data, image synchronization controller  25  pulses the LATCH signal. The LATCH pulse causes first latch bit  204 , the first element in the array of current line latch  206 , to store the contents of first register bit  200 . There are N elements in current line latch  206 , and each bit is loaded from the corresponding bit in shift register  202 . The LATCH pulse also causes first latch bit  208 , the first element in the array of previous line latch  210 , to store the contents of first latch bit  204 . There are N elements in previous line latch  210 , and each bit is loaded from the corresponding bit in current line latch  206 . Latch synchronization logic  216  receives the LATCH input from image synchronization controller  25  and produces the LATCH 1 _EN and LATCH 2 _EN signals such that the previous line latch  210  captures the data stored in current line latch  206  before the current line latch  206  captures the data stored in shift register  202 . This timing sequence is illustrated in  FIG. 11 . 
     After image synchronization controller  25  pulses the LATCH signal, the print/non-print data for the current line and previous line of the image is stored in current line latch  206  and previous line latch  210  respectively. The outputs of first latch bits  204  and  208  are used as selector inputs for 4-to-1 multiplexer  212 . Multiplexer  212  uses these selector inputs to select one of the four WAVEFORM signals to pass through to the output of the multiplexer. The four WAVEFORM input signals from image synchronization controller  25  are the set of pixel waveform sequences, such as described in  FIGS. 5 ,  7  and  9 . There are N 4-to-1 multiplexers, with one multiplexer associated with each nozzle of nozzle plate  49 . 
     The output of multiplexer  212  passes through latch bit  214  which is controlled by latch synchronization logic  216 . Latch bit  214 , the first element of an array of N latch bits, is operated such that the output of multiplexer  214  is stored while current line latch  206  and previous line latch  210  are being updated. Once the current line latch  206  and previous line latch  210  have been updated, latch bit  214  is returned to its transparent state. This operation ensures that no spurious transitions occur on the output while current line latch  206  and previous line latch  210  are being updated. Latch bit  214  is controlled by the LATCH 3 _EN signal generated by latch synchronization logic  216  and inverter  218 . The timing sequence for the LATCH 3 _EN signal is illustrated in  FIG. 11 . 
     The output of latch bit  214  is combined with the ENABLE signal from image synchronization controller  25  in AND gate  220 . The output of AND gate  220  is connected to drop forming device  28 . There are N AND gates, with one AND gate associated with each nozzle of nozzle plate  49 . The ENABLE signal provides a global means to disable all outputs of drop forming mechanism control circuit  26 . 
     Line latches  206  and  210  enable image synchronization controller  25  to load the next line of image data into shift register  202  at the same time that image synchronization controller  25  is providing the pixel waveform sequences to print the current line of image data. This operation is illustrated in  FIG. 11 . 
     The circuit shown in  FIG. 10  is one embodiment of a drop forming mechanism control circuit, and those skilled in the art will understand that other embodiments are possible. For example, the latch synchronization logic could be implemented as a synchronous state machine, the current and previous line latches could be implemented using registers, the interface could be expanded to support the loading of more than one image data bit per clock pulse, or the interface could be expanded to support more lines of print/non-print data used to select from more waveforms. These and similar variations are understood to be within the scope of the invention. 
     The drop forming mechanism control circuit shown in  FIG. 10  has been described as having N elements of shift register bits, latches, multiplexers, and AND gates, where N is the number of nozzles in the nozzle plate. In an alternative embodiment of the invention, the nozzle plate may be divided into segments of nozzles, with each segment having an independent drop forming mechanism control circuit. For example, a nozzle plate with 2560 nozzles may be divided into five segments of 512 nozzles each. Dividing the nozzle plate into segments may be done to reduce timing delays or to improve the manufacturing process for the nozzle plate. Those skilled in the art will understand that using multiple segments in a nozzle plate is within the scope of the invention. 
     As discussed in U.S. Pat. No. 7,758,171 (Brost), the print quality can be improved by employing a phase shift or stagger in the data between adjacent nozzles. When employing such a phase shift or stagger, it can also be advantageous to employ different sets of waveforms, one set for the odd numbered nozzles and one set for the even numbered nozzles. The architecture discussed herein can accommodate such odd-even waveform differentiation by providing the two sets of waveform inputs to the drop forming mechanism control circuit. The multiplexers associated with the odd nozzles would then use the current and previous line data to select one waveform from the odd set of waveforms, while the multiplexers associated with the even nozzles would use the current and previous line data to select one waveform from the even set of waveforms. In addition, it may be desirable to separate the shift register, current line latch and previous line latch into odd and even components with separate data, clock, and latch control interfaces. The use of multiple sets of waveforms to introduce a phase shift between nozzles or otherwise improve print quality is understood to be within the scope of the invention. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
     PARTS LIST 
     
         
           20  continuous printer system 
           22  image source 
           24  image processing unit 
           25  image synchronization controller 
           26  drop forming mechanism control circuit 
           28  drop forming device 
           30  printhead 
           32  recording medium 
           34  recording medium transport system 
           36  paper transport control 
           38  micro-controller 
           40  ink reservoir 
           42  ink catcher 
           44  ink recycling unit 
           46  ink pressure regulator 
           47  ink channel 
           48  jetting module 
           49  nozzle plate 
           50  plurality of nozzles 
           51  heater 
           52  liquid 
           53  nozzle plate control circuit 
           54  drops 
           55  jetting module electronics 
           56  drops 
           57  trajectory 
           58  drop stream 
           59  interconnect 
           60  gas flow deflection mechanism 
           61  positive pressure gas flow structure 
           62  gas flow 
           63  negative pressure gas flow structure 
           64  deflection zone 
           66  small drop trajectory 
           68  large drop trajectory 
           72  first gas flow duct 
           74  lower wall 
           76  upper wall 
           78  second gas flow duct 
           82  upper wall 
           84  seal 
           86  liquid return duct 
           88  plate 
           90  front face 
           92  positive pressure source 
           94  negative pressure source 
           96  wall 
           100  waveform sequence 
           102  first print pixel 
           104  second print pixel 
           106  first non-print pixel 
           108  second non-print pixel 
           110  large drop 
           111  large drop 
           112  small drop 
           113  small drop 
           114   a - 114   d  waveforms for large drop 
           116   a - 116   d  waveforms for small drop 
           120  waveform sequence 
           122  first print pixel 
           124  second print pixel 
           126  first non-print pixel 
           128  second non-print pixel 
           130  large drop 
           132  small drop 
           134  large drop 
           136  small drop 
           138  small drop 
           140  small drop 
           160  waveform sequence 
           162  first print pixel 
           163  second print pixel 
           164  third print pixel 
           166  first non-print pixel 
           168  second non-print pixel 
           170  large drop 
           172  large drop 
           173  small drop 
           174  large drop 
           176  small drop 
           178  small drop 
           200  shift register bit 
           202  shift register 
           204  latch bit 
           206  current line latch 
           208  latch bit 
           210  previous line latch 
           212  4-to-1 multiplexer 
           214  latch bit 
           216  latch synchronization logic 
           218  inverter 
           220  AND gate
A method of producing ink drops ( 54, 56) in a printing apparatus ( 20) sends print-nonprint data from a controller ( 38) to at least one inkjet nozzle ( 28). The print-nonprint data includes data on a current ink drop and data on at least one previous ink drop. A set of waveforms ( 114, 116) is provided to the at least one nozzle and a waveform based on the print-nonprint data is selected. The selected waveform is supplied to an ink droplet formation device associated with the at least one nozzle and an ink drop is produced from the at least one nozzle.