WO1987006529A1

WO1987006529A1 - Double pass printing in dot matrix printer

Info

Publication number: WO1987006529A1
Application number: PCT/US1987/000787
Authority: WO
Inventors: Michael Joseph Piatt
Original assignee: Eastman Kodak Company
Priority date: 1986-04-24
Filing date: 1987-04-09
Publication date: 1987-11-05

Abstract

In a dot matrix printer of the type having a dot matrix print head (10) with a linear array of dot generators, improved quality double pass printing is achieved by employing both vertical and horizontal dot interlacing on the second pass, so that the dot placement on the second pass is staggered with respect to dot placement on the first pass.

Description

DOUBLE PASS PRINTING IN DOT MATRIX PRINTER Technical Field

The invention relates to dot matrix printers of the type having a print head with a linear array of dot generating elements that is moved in a direction perpendicular to the array to form a line of printing. More particularly, the invention relates to a scheme for interlacing the ink drops during double pass printing to improve the quality and resolution of the printing. Background Art

Dot matrix printers, such as ink jet printers, impact printers, and thermal printers of the type having a print head with a linear array of dot generating elements that are moved in a direction perpendicular to the array to produce a line of print are well known. It is also known in such a dot matrix printer to make a double pass with the print- head over a line of characters or a portion of a graphic to improve the resolution and quality of the characters or graphics that are printed.

Generally, the minimum horizontal dot spacing in the dot matrix printer is determined by effecting a trade off between print quality and printing speed. The dot generating elements of the print head can be operated repeatedly at some maximum rate determined by the physics of the print head. With the print head driven at this maximum rate, a horizontal print head velocity is selected that produces the trade off between dot spacing and printing speed. The faster the head is moved, the faster the printing speed, but the wider the dot spacing. Rather than reduce the head speed for higher resolution printing (i.e. closer dot spacing), it is the practice to make more than one pass with the print head. In a technique called "interlacing," the dot placement is shifted between the first pass and the second pass of the print head to increase the e fective resolution of the dot matrix printer in the vertical or horizontal direction. For example in prior art dot matrix printers employing vertical interlacing, the paper is advanced by an amount equal to one half the vertical dot spacing between the first pass of the print head and the second pass. Thris technique effectively doubles the vertical ceHClution of the printer. The effect is shown in Fig-. 3a where the dots show the placement of dots on the page during the first pass of the head, and the X's show the placement of dots on the page on the second pass of the head. The row numbers are the rows of dots generated by the respective dot generating elements of the print head.

Other prigr art dot matrix printers employ horizontal interlacing. In this technique, the paper is not moved between the first and second pass of the print head, but the dot placement is shifted horizontally during the second pass with respect to the first pass by one half a horizontal dot pitch. This is achieved by adjusting the timing of the drive pulses sent to the dot matrix print head, thereby ef-Ξβ-ctively doubling the resolution in the horizontal direction. This effect is shown in Fig. 3b. Although vertical and horizontal interlacing improves the appearance and resolution of vertical and horizontal lines respectively, the appearance of diagonal lines suffers from either of the two procedures, thereby making improvements in the appearance of some aspects of characters and graphics produced by the printers, while failing to improve other aspects. A further problem is encountered in printing graphics where the higher dot resolution in one direction or the other gives rise to objectionable artifacts in shaded areas. It is therefore the object of the present iπ»eiϊt.ion to provide improved quality and resolution in characters and graphics produced by a dot matrix printer employing double pass printing. 5 Disclosure of the Invention

The objects of the invention are achieved according to one aspect of the present invention by providing both vertical interlacing by advancing the page one half a vertical dot pitch between the first 10⁾ and. second passes of the print head, and horizontal interlacing by providing a horizontal phase shift of 180° during the second pass of double pass printing, thereby staggering the dot placement between the first and second pass. This staggering improves the 15 ability of the printer to produce graphics, particularly gray scales and shading without introducing objectionable artifacts. According to a second aspect of the invention, vertical interlacing is provided as described above, and the dot placement 20 is selectively phase shifted by 180° during both the first pass and the second pass of the print head, • thereby enabling higher quality characters to be generated.

Brief Description of the Drawings 25 Fig. 1 is a perspective view of the paper drum and print head of a dot matrix printer useful with the present invention;

Fig. 2 is a schematic diagram of the microcomputer control system of the dot matrix 30 printer according to the present invention;

Fig. 3a is a diagram useful in describing the prior art technique of double pass vertical interlacing;

Fig. 3b is a diagram useful in describing 35 the prior art technique of double pass horizontal interlacing; Fig. 3c is a diagram useful in describing the technique of staggered interlacing according to one aspect of the present invention;

Fig. 4a is a diagram showing the character "A" to be produced by a dot matrix printer;

Fig. 4b is a diagram showing a possible dot pattern using the prior art technique of double pass vertical interlace to form the character A;

Fig. 4c is a diagram showing a possible dot pattern! using the double pass staggered interlacing accora__-uιg to one aspect of the present invention to produc the character A;

Fig. 4d is a diagram showing a possible dot pattern using a combination of vertical interlace and selective horizontal phase shifting during the first and second passes of double pass printing according to another aspect of the present invention to produce the letter A; and

Fig. 5 is a diagram illustrating dot placement by an ink jet printer capable of being temporarily overdriven, useful for effecting the 180° phase shift according to the second aspect of the present invention. Modes of Carrying Out the Invention Referring now to Fig. 1, there is shown a dot matrix printer for practicing double pass printing according to the present invention. A dot matrix print head 10, comprising for example, an ink jet print head having a vertical linear array of drop—on— emand type ink jets, is slidably mounted on a support 12. A stepper motor 14 is coupled to the print head 10 through a pair of pulleys 16, 18 and a cable 20 to advance the print head parallel to a paper drum 22. A stepper motor 24 is coupled to the paper drum 22 and rotates the paper drum to advance paper in the paper drum in a vertical direction with respect to the print head. Control electronics 26 receives print data from a personal computer (not shown) and controls the operation of the paper drum drive motor 24, the print head advance motor 14, and the print head 10. In a draft printing mode, the dot matrix print head 10 is advanced in the direction of arrow A to print one line of characters. The paper drum 22 is then rotated in the direction of arrow B to advance the paper one line, and the next line of characters is printed.

In the double pass printing mode for high quality text and graphics, the dot matrix print head 10 is advanced in a first pass in the direction of arrow A to partially form a line of characters or graphics, the print head is then returned in a second pass over the same line to complete the printing of the line. According to one aspect of the present invention, the paper drum is advanced by one half a vertical dot spacing and the timing or "phase" of the dot placement on the second pass is shifted by one half of a horizontal dot spacing (i.e. 180°) from the placement on the first pass to provide staggered interlacing as shown in Fig. 3c. This staggered interlacing is useful in printing graphics, where shaded areas produced by the staggered interlacing technique will be relatively free from artifacts.

According to a second aspect of the present invention, the paper drum is advanced by one half a vertical dot spacing between the first and second pass, and the phase of dot spacing on the first and second pass can be selectively shifted by 180° to locally provide either the effect of pure vertical interlace, as shown in Fig. 3a, or staggered interlace as shown in Fig. 3c, as called for by the shape of characters to be produced.

The operation of the control electronics 26 will now be described in more detail with reference to Fig. 2. The control electronics 26 includes a microprocessor 28, a read only memory (ROM) 30 for storing the control program for the microprocessor and the dot matrix patterns for the character fonts employed in the dot matrix printer, and a random access memory (RAM) 32 that functions as an input buffer for temporarily holding data representing the characters to be printed.

The microprocessor 28 and RAM 32 receive printing instructions and character data from a microcomputer via a standard computer interface 34. The paper drum stepper motor 24 and the print head stepper motor 14 are controlled by drivers 36 and 38 respectively. Motor phase information is latched into the drivers 36 and 38 by a latch 40, and then power is applied to the drivers by the microprocessor 28 to effect stepping of the motors.

The individual dots produced by the dot matrix print head 10 are controlled,by a set of drivers 42, one driver for each dot generating element in the print head (e.g. one driver for each ink jet in an ink jet printer, or one driver for each pin in a dot matrix impact printer) under control of the microprocessor 28. During printing, the microprocessor retrieves the data representing a line of characters to be printed from RAM 32. The microprofessor employs the character data to address a character font in the ROM 30 to retrieve the dot matrix pattern for driving the print head 10. The dot matrix pattern for each character is stored in a rectangular array having rows and columns of binary bits, each bit indicating whether or not a dot should be formed at a corresponding point in the array. The binary bits are supplied one column at a time to the print head drivers 42. The generation of print commands sent to the drivers 42 is coordinated with the command sent to the stepper motor 14 so that the dots are regularly spaced along the page in the direction of print head movement.

Separate fonts are stored in the ROM 30 for the draft printing mode, and the double pass printing mode. In the double pass printing mode with staggered interlace according to the present invention, on the first pass, alternate lines of the dot matrix pattern are sent to the drivers. Next, the paper is advanced by one half a dot spacing by activating drive motor 24, and the rest of the dot matrix pattern is read out on the second pass of the print head with a delay of 180° in the horizontal direction.

Fig. 4a shows an example of the character "A" on a grid. Each §quare on the grid represents a possible dot location. Alternate rows on the grid are addressed by successive passes of the print head. Fig. 4b shows a possible double pass printing pattern for forming the character "A" using pure vertical interlacing as in the prior art. The O's in Fig. 4b represent the location of dots generated on the first pass of the print head, and the X's represent the location of dots generated on the second pass. As can be seen from Fig. 4b, the diagonals in the A are not well rendered. Fig. 4c shows a possible double pass printing pattern for the character A using staggered interlace according to the present invention. As can be seen by comparing Fig. 4c with 4b, some improvement has been achieved in the rendering of the diagonals of the character A but there is still room for more improvement. According to a further aspect of the present invention, the phase of the drops on the first or second pass can be selectively shifted by 180°, thereby achieving the local effect of either pure vertical interlace, or staggered interlace as required by the shape of the character. This provides the font designer with access to the best of both schemes. Fig. 4d shows a possible double pass printing pattern for the character A using the selective phase shifting on the first and second pass. As can be seen from Fig. 4d, this mode of interlacing yields a further improvement in the quality of character reproduction.

In Fig. 4d, the rows of dots are numbered with pairs of numbers. The first number of the pair indicates the dot generating element on the print head, and the second number of the pair indicates whether the dot is produced on the first or second pass of the print head. Comparing Fig. 4b with 4d, it is seen that in the first row on the first pass (1—1), the dots occur in their ordinary positions. In the next row (1—2), the dots are phase shifted by 180° with respect to the first row. This is an example of a local staggered interlace. In row 2-2, the dots are also phase shifted 180° which produces staggered interlacing with row 2—1. However, row 3—1 has also been phase shifted 180° so that row 3—1 is vertically interlaced with respect to row 2—2.

The dot matrix patterns for the character fonts are stored in the ROM 30 in the form of a grid having horizontal and vertical dimensions of one half a normal dot spacing. It is recognized during font generation that two dot printing commands in a row cannot be sent to a dot generating element closer than one full dot spacing apart, since to do so would exceed the frequency response capability of the printing head. However, there is no problem with shifting the phase of the dot placement by 180° from time to time as long as the phase shifts do not result in two dots being placed adjacent to each other on a single line. In some types of thermal drop-on-demand ink jet print heads however, the individual jets may be momentarily (i.e. for one or several drops) driven past their steady state capacity. In such dot matrix printers, the 180° phase shift can be provided, where required, by calling for two consecutive drops separated by only half the normal drop spacing time. Fig. 5 illustrates a dot pattern produced by such a print head. The normal dot spacing P is compressed to P/2 for one drop, thereby phase shifting the following drops by 180°. This technique provides even greater flexibility for the font designer, providing the ability to further achieve a local effect of pure horizontal interlace where required in generation of characters.

A compact ink jet dot matrix printer employing double pass printing according to the present invention was constructed. The printer includes a drop-on-demand thermal ink jet print head io having twelve jets arranged in a vertical line. The microprocessor 28 is a μPD 78C10-36 CMOS 8 bit microprocessor with an 8 port, 8 bit on board A/D converter, manufactured by the NEC Company in Japan. The ROM 30 is 256 of CMOS ROM and the RAM 32 is 64K of 100 ns CMOS static RAM. The computer interface 34 is a standard Centronics parallel interface for personal computers. Stepper motors 24 and 14 are 4—phase stepper motors with 7.5% resolution (48 steps per revolution). Stepper motor 24 is geared to advance the paper drum one half vertical dot spacing per step. Stepper motor 14 is geared to advance the print head four ordinary dot spacings per step. The print head is moved at 12.5 inches per second and 96 dots per inch are produced by the print head. A font of characters was generated using the double pass vertical interlace with selective phase shifting during the first and second pass according to the present invention. The printed characters displayed visual improvement over characters produced by comparable prior art ink jet printers. Industrial Applicability and Advantages The present invention is useful in dot matrix printers. The invention is advantageous in that more rounded and fully formed characters can be printed, diagonal lines are more sharply defined and easier to generate, and gray scales and shading may be produced without annoying artifacts due to higher dot resolution in one direction or another.

Claims

Claims :

1. In a dot matrix printer of the type employing a dot matrix print head (10) having a linear array of dot generators, the dot matrix print 5 head being movable in a direction (A) perpendicular to the linear array to produce a line of printing, said printer employing double pass printing with dot interlacing to increase the quality and resolution of the printing, characterized by:

10 control means (26) for providing both horizontal and vertical interlacing during said double pass printing, whereby the dot placement on the second pass of the double pass printing is staggered with respect to the first pass.

15 2. The dot matrix printer claimed in claim

1, characterized in that the control means (26)

' includes, means (24, 36) for advancing the paper by one half a vertical drop spacing between the first . pass and the second pass of the print head (10) and

20 means (14, 38) for shifting the phase of the dot placement during the second pass of the print head by 180° from the dot placement on the first pass, thereby staggering the placement of drops between the first and second pass.

25 3. The dot matrix printer claimed in claim

2, characterized in that the control means (26) further includes means (14, 38) for selectively shifting the phase of the drop placement during the first pass by 180°, and wherein said means for

30 shifting the phase of the drop placement during the second pass by 180° is selectively actuable.

4. The dot matrix printer claimed in claim

3, wherein said dot matrix print head (10) is an ink jet print head capable of being temporarily

35 overdriven to produce two drops separated by one half the normal drop pitch, and wherein said means (14, 38) for selectively shifting the phase of drop placement by 180° during the first and second pass comprises means (42) for producing two sequential drops separated by one half the ordinary drop pitch.

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