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United States Patent 3,769,631
Hill ,   et al. October 30, 1973

INCREASING THROUGHPUT IN INK JET PRINTING BY DROP SKIPPING AND REDUCING INK JET MERGING AND SPLATTER USING A STAIRSTEP GENERATOR

Abstract

The present case concerns various techniques for increasing the printing rate of an ink jet printing apparatus. As one example, drops not required for printing, that is, drop intervals, are skipped. This reduces the time required to form individual characters. Another technique modifies the normal generation of drops which is sequential in a column by column fashion and instead imposes non-sequential drop generation and deflection thereby reducing drop merging and splatter that otherwise might occur.


Inventors: Hill; James D. (Lexington, KY), Naylor, III; Hugh E. (Lexington, KY), West; Donald L. (Lexington, KY), Williams; Thomas H. (Lexington, KY)
Assignee: International Business Machines Corporation (Armonk, NY)
Appl. No.: 05/297,546
Filed: October 13, 1972


Current U.S. Class: 347/10 ; 347/76; 347/89
Current International Class: B41J 2/075 (20060101); B41J 2/08 (20060101); G01d 015/18 ()
Field of Search: 346/75

References Cited

U.S. Patent Documents
3298030 January 1967 Lewis et al.
3562757 February 1971 Bischoff
3596275 July 1971 Sweet
Primary Examiner: Hartary; Joseph W.

Parent Case Text



PATENT APPLICATION OF INTEREST

U.S. Pat. application Ser. No. 266,790 filed June 27, 1972, entitled "Ink Jet Synchronization and Failure Detection System," and having James D. Hill, et al., as inventors.
Claims



What is claimed is:

1. In an ink jet printing apparatus, an arrangement for increasing throughput during printing of information on a record medium, comprising:

nozzle means for forming and propelling a plurality of ink jet drops toward said record medium, means for charging said ink drops with code representations, means for deflecting said ink drops in accordance with the charge on said drops, said drops being deflected in successive vertical columns, each column comprising a predetermined number of drop locations;

a data source for providing code representations for information to be printed, each code representation being indicative of a drop to be printed or not to be printed;

register means connected to said data source for storing said code representations;

means interconnecting said register means to said charging means for controlling said charging means to charge said drops in accordance with said code representations for the purpose of printing said information;

control means operable in one mode to supply control signals to said nozzle means, said data source, said register means and said interconnecting means at a predetermined rate during printing of information and operable in another mode to supply control signals at a higher rate;

means connected to said register means for monitoring the code representations therein and for developing a skip signal when code representations indicative of no information to be printed are detected; and

skip means for connecting said predetermined rate control signals to said register means and said interconnecting means at said predetermined rate during the printing of information and responsive to said skip signal for connecting said higher rate control signals to said register means and said interconnecting means in order to skip non-information code representations.

2. The apparatus of claim 1, wherein said control means includes:

a clock circuit, said clock circuit providing a pulse output at a first predetermined relatively slower rate for activating said circuits during printing of information and said clock circuit further providing pulse signals at a relatively higher rate for activating said circuits in order to skip non-information representations.

3. The apparatus of claim 2, wherein the predetermined rate provides a 100 KHz pulse output for printing and said higher rate provides 800 KHz pulse output for non-information sequences.

4. The apparatus of claim 2, wherein said interconnecting means comprises a counter for keeping track of drop locations within each vertical column; and

a digital to analog convertor for converting said code representations to potentials required for charging said drops.

5. The apparatus of claim 4, wherein said skip means includes:

gate means driving said counter and said register means;

means connecting said predetermined and said higher rate pulse signals from said clock circuit to said gate means; and

skip logic connected to said skip means for controlling said gate means and thereby controlling said register means and counter means to operate at a relatively low rate during printing of information and a relatively high rate when non-information code representations are detected.

6. The apparatus of claim 2, where said interconnecting means includes:

a stairstep generator providing stepped output potential levels respectively representative of drop locations within each of said vertical columns, said stairstep generator being stepped at a low rate during printing and at a relatively high rate when non-information code representations are detected.

7. In an ink jet printing apparatus, an arrangement for reducing drop merging and splatter, comprising:

nozzle means for forming and propelling a plurality of ink jet drops toward said record medium, means for charging said ink drops with code representations, means for deflecting said ink drops in accordance with the charge on said drops, said drops being deflected in successive vertical columns, each column on said record medium comprising a set of a predetermined number of drop locations;

cycling means operable in at least two cycles for each column of printing for controlling said nozzle means and said charging means; said cycling means being operable in at least a first cycle of operation for controlling said nozzle means and said charging means to selectively propel said ink jet drops toward selected ones only of said drop locations on said record medium comprising less than a set of drop locations, and said cycling means being further operable in another of said at least two cycles for controlling said nozzle means and charging means to selectively propel said ink jet drops toward other drop locations on said record medium not previously selected in said first cycle of said at least two cycles of operation, said other drop locations comprising the remainder of a set of drop locations for each said column.

8. The apparatus of claim 7, wherein said cycling means include:

nozzle control means for activating said nozzle means and said charging means during a first drop printing cycle for each column to direct drops toward odd numbered drop locations on said record medium and during a second drop printing cycle for each column to direct drops toward even numbered drop location on said record medium, the skipping of alternate drop locations thereby minimizing merging and splatter effects.

9. The apparatus of claim 7, wherein:

said cycling means comprises a stairstep generator.

10. The apparatus of claim 7, wherein:

said cycling means comprises a digital to analog convertor and associated counter means.
Description



BACKGROUND OF THE INVENTION AND PRIOR ART

While ink jet printing is noted for high speed operation, increased throughput is gained in the present machine by minimizing the number of drop time intervals encountered depending upon the character shapes and the number of drops required to print characters. Thus, characters requiring more drops take somewhat more time, while characters requiring few drops take a proportionately less amount of time to print. A typical prior device is described in the U.S. Pat. No. 3,298,030 involving facilities for printing of information on a record medium using ink jet techniques taught in the Sweet U.S. Pat. No. 3,596,275. Both of these patents are directed to the printing of information by forming, propelling, charging, and deflecting drops to desired locations on a record medium. The teachings are based on a variable drop charging and constant potential deflection technique. Individual character boxes comprise a number of drops in width times a number of drops in height, that is, a matrix format. Ordinarily, drops are produced during each of the drop time intervals represented by the coordinate locations of the matrix. The Hill, et al., application referred to above also produces drops for each coordinate location. In such devices, all drops not required for actual printing on the record medium are directed to a gutter and drops required are directed to the record medium. Thus, drops are produced in each drop time interval regardless of whether they are needed or not and a great amount of time is wasted in proportion to the amount of time required for printing of information.

The U.S. Pat. No. 3,298,030 at column 5, lines 68-74 describes only in a general way the possibility of saving time but no techniques are set forth for doing so.

SUMMARY OF THE INVENTION

In accordance with a first embodiment of the invention, a charge electrode driver is controlled to produce varying amounts of charges on drops passing therethrough on the way from the ink jet nozzle to a record medium. The driver is driven by a digital to analog convertor that provides a potential indicative of the amount of charge to be placed on the drop and accordingly the amount of deflection for the individual drops as they proceed toward the paper. Also, of course, when drops are not required, no charge is produced. A counter keeps track of the vertical drop location and controls the digital to analog convertor to produce a voltage corresponding to such location. The digital to analog converter also receives an output from a shift register having a binary zero or one representative of a drop not to be charged or a drop to be charged, respectively. The system provides two control pulses to drive the shift register and counter in one mode, referred to as a "normal drop formation and charging" mode at a relatively lower speed, such as 100 kiloHertz and further provides pulses at a much higher rate such as 800 kiloHertz in a second or "skip" mode of operation to step the counter and shift register through unused drop positions when no information (that needs to be printed) is present. During such skipping, the digital to analog convertor provides a zero level output to the charge electrode driver to insure that any drops that are formed during this interval proceed to the gutter. Thus, skipping takes place at a high rate of speed.

In another version, the digital to analog convertor and counter are replaced by a stairstep generator that provides incremental stepped potentials to the charge electrode driver and that steps normally for normal printing and at high speeds for skipping in a manner similar to the operation just described.

In accordance with another version, the stairstep generator or digital to analog convertor is utilized to produce non-sequential drop placement with appropriate charging as the drops proceed toward the record medium to establish proper deflection in each vertical column of the character box. No drop skipping or high speed skip logic is involved in this technique but it does offer an advantage in reducing the merging of one drop with another as they travel through the air toward the record medium or a splatter effect resulting when one drop impacts on the record medium and partially overlaps a preceding drop.

OBJECTS

An object of the present invention is to provide an ink jet printing system having provision for skipping drop intervals when no drops are required for printing and essentially propelling, charging, and deflecting drops only when information is required.

Another object of the present invention is to reduce ink jet drop merging and splatter.

The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of the preferred embodiments of the invention as illustrated in the accompanying drawings.

DRAWINGS

In the drawings:

FIG. 1a illustrates an ink jet printing system incorporating skip logic and including a digital to analog convertor, shift register, and counter together with a typical nozzle arrangement. FIG. 1b is intended for substitution in FIG. 1a and comprises a stairstep generator serving a function comparable to that of the digital to analog convertor and the counter in FIG. 1a.

FIG. 2 shows a typical character, such as a capital "T," illustrating some of the drop skipping techniques in the present case.

FIGS. 3a-3c and FIG. 4a represent various waveforms encountered during the operation of the stairstep generator of FIG. 5. FIG. 4b illustrates timing intervals related to FIG. 4a.

FIG. 5 is a circuit diagram of a stairstep generator corresponding to that shown in FIG. 1b, while FIG. 6 illustrates a digital to analog convertor corresponding to that shown in FIG. 1a.

FIG. 7 shows a capital "T" showing a non-sequential drop formation and propulsion technique, while FIGS. 8a and 8b illustrate outputs of the stairstep generator to produce the non-sequential type of operation.

DETAILED DESCRIPTION

Referring to FIG. 1a, a record medium, such as a sheet of paper 1 is positioned for receipt of drops 2a for printing of a character while unused drops 2b are directed to a gutter 35. Drops are formed by a nozzle assembly 3 having a nozzle 10 and initially forming a stream 2c that passes through a charge electrode 18. Charge electrode 18 is driven by charge electrode driver 21. As is known, nozzle assembly 3 includes a crystal 8 that is driven by crystal driver 15 at high frequencies in order to cause drop formation. Ink is provided from a supply tank 5 to nozzle assembly 3 by means of a pump 6. Ink received in gutter 35 is returned by conduit 27 and pump 30 to tank 5. As drops pass through charge electrode 18, they receive a variable charge or no charge at all depending upon whether information is to be printed or whether they are to be directed to gutter 35. Deflection plate 22 is supplied with a high potential from terminal 25 which remains constant. Plate 23 is grounded at 7. The drops are deflected between the plates on their way to paper 1 in proportion to the charge that they carry. Master clock 11 provides timing signals to machine logic 13 and to crystal driver 15 as well as a character generator 14 during operation of the system. Certain synchronizing signals such as those described in the Hill, et al., application are provided from sync control 40 by line 62. As such vertical column of a character is encountered during printing, character generator 14 provides a drop configuration to shift register No. 1 designated 44. At an appropriate time, the information in register 44 is shifted to shift register number No. 2 designated 46. The information in the shift registers 44 and 46 is represented in a familiar binary 0 and 1 configuration. The circuits further include a digital to analog convertor 42, a counter 43 and skip logic 47, as well as a gate 52. Counter 43 provides a 6 bit output on bus 54 to convertor 42 representative of the vertical drop location within any given column. Convertor 42 in turn provides a potential to charge electrode driver 21 for charging of the individual drops in order to achieve a degree of deflection between plates 22 and 23 which will insure that the individual drops strike paper 1 in the proper vertical coordinate location. Whether a drop is charged at all or not is determined by whether or not the right-most position 46a of register 46 carries a zero or 1 configuration.

During normal printing of information, counter 43 and shift register 46 are stepped by output pulses from gate 52 on line 55. Gate 52 is conditioned by skip logic 47 under these conditions to pass pulses at the 100 kiloHertz rate on line 60. Whether or not this speed of operation, that is, the normal character printing speed of 100 kiloHertz is maintained is determined by whether or not position 46a of shift register 46 has a one bit in it. In any case where position 46a of shift register 46 has a zero bit, skip logic 47 changes the condition of gate 52 to pass a higher frequency pulse input from line 65 to both shift register 46 and counter 43 in order to rapidly skip such drop intervals since no printing is required.

In accordance with another mode of operation, stairstep generator 50, FIG. 1b is substituted in FIG. 1a for digital to analog convertor 42 and counter 43 by appropriate substitution at interconnecting points 70, 71, and 72 involving the corresponding points 70a, 71a, and 72a in FIG. 1b. Thus, stairstep generator 50 is substituted in its entirety for the convertor circuit 42 and counter 43. In this case, generator 50 is stepped under normal character printing conditions on a drop by drop basis, that is, over a fixed drop voltage interval while in the event of no information in shift register position 46a, stairstep generator 50 is rapidly stepped over a large number of drop intervals corresponding to the number of intervals to be skipped. Charge electrode driver 21 receives a potential from generator 50 representative of the amount of charge to be imparted to the individual drops as they pass through charge electrode 18. Following is more detailed description of the various techniques.

DROP SKIPPING TO INCREASE THROUGHPUT

An ink jet printer producing typewriter quality print requires a large matrix or character box. A 40 high by 24 wide character box and an example of a capital "T" is shown in FIG. 2. In order to allow for underscores and letters with "tails" such as a small g or small j, the character box must include some area below the "T" shown in FIG. 2. Similarly, some area above the T is required for accents, the spanish tilde, etc.

Drops are placed on the paper by moving an ink jet head from left to right across a page and vertically deflecting the drops as required. If the ink jet operates at a 100 KHz drop formation rate (10 .mu.s per drop) then 40 .times. 24 = 960 drops are formed in 9.6 ms to create one character. Note, however, that in the case of the "T" shown in FIG. 2, only 124 of those 960 drops are actually used. More than 85 percent of the drops in the character box are not used.

The present techniques enable a reduction in the number of drops that are not used and, hence, less time required to form a character. For a given ink drop formation rate, then, more characters can be formed. Usually no more than 31 drops need to be placed on the paper during one vertical scan for a maximum size character. Two methods have been described above for reducing the number of drops required to print a character. These schemes allow for skipping over the blank space within a vertical stroke, thus using the drops that would normally be directed to the gutter.

FIG. 6 is a schematic diagram of a digital to analog convertor which is an efficient method of driving the charge electrode. Digital logic, shown in FIG. 1a, presents a sequence of digital codes to the D to A convertor which then produce an appropriate voltage on the charge electrode. For example, a code of 000000 (all switches closed) might produce a 20 volt output at the charge electrode. A drop charged by that voltage would impact at the bottom of the character box--position 40 in FIG. 2. A code of 101000 (40) might then cause the drop to impact at position 1 at the top of the character box (200 volts on charge electrode). Intermediate digital codes produce intermediate voltages in (200-20)/39 = 4.61 volt steps.

Operation of the digital to analog convertor FIG. 6 is as follows. Switches C1, C2, C3 . . . C6 are operated by bits one through six respectively of the digital input code. Resistors R1, R2 . . . R6 are binarily related to each other, such that, assuming that the same voltage appears across each resistor, with switch C2 closed, twice as much current flows through R2 as through R1 with switch C1 closed, etc.

With the non-inverting input of operational amplifier 47 at ground, its inverting input is at virtual ground. Therefore, with any switch (C1, C2 . . . C6) closed, the voltage across its load resistor is the negative reference voltage from source 36. The sum of the currents through each of the switches for any digital input, I.sub.1, must be drawn from the circuit node at the inverting input of amplifier 47. The input impedance of the amplifier is large so the current can not be supplied by the amplifier's input. Therefore the output of the amplifier rises in voltage until the voltage V.sub.E at the emitter of transistor 80 is large enough to allow a current I2 to flow through resistor R1 such that I.sub.2 is exactly equal to I.sub.1. This allows the voltage at the inverting input of amplifier 47 to remain at virtual ground.

In order for the voltage at the emitter of transistor 80 to rise to the value I2 .times. Rf, some current from transistor 80 must also flow through resistor 83 to ground. If the gain of the transistor is large then the emitter current, which is the sum of the currents in resistors 81 and 83, is essentially identical to the collector current. The output voltage is then developed across resistor 85. Transistor 80 thus serves as an output buffer for amplifier 47 and allows amplifier 47 to control a load connected to a high voltage power source 87 (+200 V DC).

Since the digital to analog convertor responds to whatever digital code is presented at its input and is not dependent upon charging a capacitor, it is not necessary to waste or throw away any drops to skip over white space in a vertical scan. The input code is merely changed. In the numerical example above, only 31 drops per vertical scan are required. The character box thus needs only 31 .times. 24 = 744 drops. With a 10 .mu.s drop time, only 7.44 ms are required per character. This results in a possible maximum character formation rate of 134 cps, significantly faster than the normal 104 cps.

FIG. 5 is a diagram of a modified stairstep generator 50. Timing logic 90, operates analog switch 91. When switch 91 is closed for a precisely controlled interval, a current flows through a resistor 93 to the inverting input of operational amplifier 94. However, since the amplifier input is at a virtual ground, the current through resistor 93 flows through capacitor 96, resulting in a charge stored on the capacitor 96 with polarity as shown in FIG. 5. If switch 91 is closed during the first 1.25 .mu.s of each 10 .mu.s drop time, the output voltage of amplifier 94 is a stairstep as shown in FIG. 3a.

With the non-inverting input of operational amplifier 98 grounded, its inverting input is at virtual ground. With analog switch 99 open, a current proportional to the output voltage of amplifier 94 flows through resistors 100 and 101. Because the input to amplifier 98 is a high impedance the current through resistors 100 and 101 flows through resistor 102 to the emitter of transistor 105, connected as an emitter follower output buffer for amplifier 98. The voltage at the emitter of transistor 105 is determined by the product of resistance 102 and the current through it (switch 99 open).

If transistor 105 has a large current gain, then its collector current is essentially identical to its emitter current. Resistor 108 in parallel with resistor 102, to ground, determines the total emitter current and hence the collector current. The output voltage appears across resistor 110, and is determined by the collector current of transistor 105. FIG. 3c is a sketch of the output voltage of the circuit at terminal 111.

Closing the switch 99 in FIG. 5 during any number of drop times does not affect the output of amplifier 94, which continues to step toward a negative voltage. However, a large current I flows through resistors 101 and 102 causing the collector of transistor 105 to approach zero volts. This causes drops formed during this time to receive a minimal charge. Therefore, they are not deflected and impact on the gutter 35, FIG. 1a.

By charging capacitor 96 for 1.25 .mu.s during each 10 .mu.s drop time a series of 31 drops can be placed one above the other in positions one through 31 in each vertical scan of the character box shown in FIG. 2. Each drop formed while switch 99 is closed receives minimal charge and does not reach the paper.

By adding a logical "Or" capability to timing logic block 90 in FIG. 5, one drop can be sent to the gutter and capacitor 96 charged for up to 10 .mu.s. In this manner, capacitor 96 steps eight drop positions during this particular drop time. The next drop thus will appear eight drop positions above the last printed drop (assuming that this subsequent drop time also includes a 1.25 .mu.s charging time for capacitor 96).

Timing logic 90 of FIG. 5 can charge capacitor 96 for intervals less than 10 .mu.s but more than 1.25 .mu.s. But each such interval wastes one drop so, preferably, no more than one or two such intervals are used per vertical scan.

Allowing two special charging times per vertical scan costs two drops and allowing up to 31 drops for printing means that only 33 drops per vertical scan are required. Thus 33 .times. 24 = 792 drops are required per character. At an assumed 100 KHz (10 .mu.s) drop rate, only 7.92 ms are required to print a character. This technique improves printing speed from 104 cps to 126 cps without any reduction in print quality.

The timing logic for the stairstep generator shown as block 90 in FIG. 5, can be easily implemented using the assumed timing signals produced by logic (not shown) in FIG. 4a. The four fundamental timing sequences and their inverses provide eight 100 KHz signals shifted in phase by 45.degree. (1.25 .mu.s) increments.

Eight possible timing intervals are derived from these signals. They are shown in FIG. 4b. Upon generation of these terms, they are then controlled by external logic so that the desired number of skips are obtained.

Prevention of Drop Merging and Splatter by Non-Sequential Operation

Deflected drops tend to merge as they travel through air due to drag and slip stream effects. Also, splatter problems occur when a drop impacts on the printing surface and partly overlaps the preceeding drop. This causes small amounts of ink to splash back on and contaminate the deflection plate assembly. It also causes ink to be splashed into the white space around the character reducing print quality.

Drops that have nearly the same charge, such as successive drops in a vertical scan, are sufficiently close that the greater resistance to movement through air experienced by the leading drop causes the leading drop to lose velocity at a sufficiently high rate to be intercepted by the following drop ("drop merging").

Greater spacing between successive drops, achieved by having a greater charge difference between successive drops, reduces this effect. One approach is to throw away every other drop. This, however, reduces the data throughput by 50 percent. Alternatively several stairstep charging cycles can be provided per vertical scan of the character box. This causes greater charge differences between successive drops and thereby greater separation between them. In addition to eliminating the problem of drop merging several other benefits result:

1. Ink droplets tend to bounce back from the surface of impact. This is reduced by (a) reducing the drop velocity and (b) by allowing drops to strike a dry or nearly dry surface. This technique permits as much as 20 drop times between adjacent drops thereby allowing more time for the first drops to spread over and be absorbed before adjacent drops strike the paper.

2. Elimination of the merging problem allows the nozzle to be further away from the paper printing surface. This in turn allows more flexibility in the other elements of the drop formation and deflection mechanism: (a) deflection voltage can be lowered if the deflection plates are lengthened so drops experience a smaller deflection force but for a longer time. (b) The ink pressure can be lowered, reducing impact velocity and hence reducing the splatter problem described above. (c) Merging does not impose a constraint on minimum spacing between drops. Higher drop formation rates are now possible for any given pressure since no two successive drops travel similar paths.

The following is a description of one implementation. Reference is again made to FIG. 5, which is a schematic diagram of the stairstep generator. FIG. 4a shows the timing signals provided by external logic to implement the drop skipping technique previously described. Changes in timing control circuitry allow the circuit of FIG. 5 to operate with multiple cycles per vertical scan.

Consider an example of using two stairstep generator cycles per vertical scan. For simplicity, the drop skipping technique for increasing throughput described is not considered, although it can be applied here to improve printing speed.

FIGS. 2 and 7 represent 40 high by 24 wide character boxes with a capital "T" shown. Consider the vertical scan in column 12 of these figures which makes up half of the center of the "T". With 40 drops and two stairstep cycles per vertical scan 20 drops in 200 .mu.s would be needed for each stairstep cycle. In order to complete the vertical scan for the letter "T" in column 12 of FIG. 7, the stairstep circuit in FIG. 5 operates as follows:

A. first Cycle: Capacitor 96 has zero charge causing amplifier 94 to have zero output voltage. Since the first drop would normally go to row 40, column 12, in the character box (FIG. 7), and that must be blank, switch 99 is closed and the output of the circuit is approximately zero volts. Drop No. 1 goes to the gutter. For the second drop, switch 91 is closed for 2.5 .mu.s and capacitor 96 charges through resistor 93. The voltage on capacitor 96 would normally be enough to cause drop No. 2 to impact on row 38 in column 12 of FIG. 3. However, this drop is not needed to print the capital "T," so it is sent to the gutter.

Capacitor 96 is charged 2.5 .mu.s for drop No. 3 and again for drop No. 4. They would normally impact in column 12, rows 36 and 34, respectively. But since they are not needed for the "T," these drops are sent to the gutter by closing switch 99.

Drops No. 5 through No. 16 all impact on paper 1 and drops No. 17, through No. 20 are not used. At that point, switch 115 is closed and the charge on capacitor 96 is reduced to zero. A partially completed scan for the letter "T" results upon completion of the first stairstep cycle. FIG. 8a shows the output voltage waveforms of amplifier 94 and the stairstep circuit output for the first cycle.

B. second Cycle: At the beginning of the second cycle capacitor 96, FIG. 5, which has been discharged by switch 115, is charged by switch 91 for 1.25 .mu.s. The output voltage of amplifier 94, shown in FIG. 8b, applied to amplifier 98 would normally send drop No. 21 to row 39 of column 12. But since this drop is not needed it is sent to the gutter. Capacitor 96 is charged in 2.5 .mu.s intervals for all subsequent drops in this scan. Drops Nos. 22 and 23 are not used for printing. Drops Nos. 24-35 are printed and fill in the remainder of that portion of the "T" in column 12. Using this technique there is a delay of 20 drop times (200 .times. 10 .mu.s = 200 .mu.s) between the time a drop hits the paper adjacent to another, previously printed drop. This is a 20 times longer interval than the 10 .mu.s delay when adjacent drops are deposited sequentially. The extra time allows the first of two adjacent drops to spread out over and be more fully absorbed by the paper and thus significantly reduces splatter.

Although this example utilized two stairstep cycles per vertical scan, three, four or five cycles per scan could be used if desired. Another advantage of this technique is a less stringent accuracy requirement on the integrator operational amplifier formed by capacitor 96, Resistor 93 and amplifier 94. With all drops charged sequentially, the charge on capacitor 96 must very accurately reflect the sum of the charges stored for each drop over a full 400 .mu.s scan time. This imposes a significant leakage current requirement on the circuit. Using the two cycle per scan method above, the sum of the charges applied to capacitor 96 for only 20 drops need be held for only 200 .mu.s. This allows the use of lower cost components in the circuit.

The digital to analog convertor of FIG. 6 also offers a solution to the problem of drop merging. As explained above, merging occurs when two successive drops carry very similar charges and follow very similar flight paths because one must appear above the other on the printing surface. Using the digital to analog convertor, described above, it is not necessary to send two successive drops to adjacent positions on the paper. This has been described in connection with the vertical scan in column 12 for the letter "T" shown in FIG. 7. With no two successive drops following similar flight paths, drop merging does not occur.

A third advantage of using the digital to analog convertor, as described above to avoid merging is that the "splatter" problem is also reduced. All drops overlap about 20 percent, so when one drop overlaps and impacts on the paper, just above the preceding drop, tiny droplets of ink are splattered in the white space around the character and back toward the high voltage electrodes where they cause contamination. If no two successive drops impact adjacent to each other then more time is available for the ink from the first drops to be absorbed by the paper before the adjacent drops arrive.

While the invention has been particularly shown and described with reference to several embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention.

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