GB2268355A - Non-impact printer for printing a video signal of higher resolution - Google Patents

Abstract

When a video signal (11) received at a printing control unit (1) has a higher resolution than an LED head (19), the control unit (1) carries out a logical AND operation between successive bit data of the video signal (11) to convert the video signal (11) to a first signal. Corresponding to a line timing signal, this first signal is transferred as a head data signal (18) to the LED head (19) and is then printed on a basic raster scanning line. Further, the control unit (1) also carries out an exclusive-OR operation between the successive bit data of the video signal (11) so that the video signal (11) is converted to a second signal and outputted to a line buffer. Corresponding to an additional line timing signal, the second signal is transferred as another head data signal (18) to the LED head (19) and is printed on an additional raster scanning line. The LED head drive energy for performing printing along the two raster scanning lines is set independently; that of the additional raster scaning line may be lower. <IMAGE>

Description

"NON-IMPACT PRINTER" This invention relates to a non-impact printer, and more specifically to a non-impact printer making use of an LED head as a print head.

Non-impact printers, primarily electrophotographic printers, receive print information for a host controller such as a microcomputer or a work station and perform printing by means of a print head such as an LED head.

The LED head employed in such non-impact printers is provided with light emitting diodes arranged with a predetermined density in a raster scanning direction. Depending on which level the density is set at, the resolution of print to be obtained is determined. When the resolution is low, there are fewer dots per inch in the raster scanning direction. A picture so printed hence presents a zig-zag image at oblique lines so that the quality of the picture is low. To improve the quality of a print, an LED head having light emitting diodes at a high concentration is needed. Such an LED head is, however, difficult to manufacture so that its price is very high.

The present invention seeks to provide a nonimpact printer which can reduce a zig-zag image at oblique lines of a printed picture and hence improve the quality of the print bq receiving print information at a resolution higher than that of an LED head itself and making the apparent dot density higher than the resolution of the LED head.

A non-impact printer according to this invention is provided with means for producing basic line timing signals adapted to perform printing along a basic raster scanning line and also with means for producing additional line timing signals between the successive basic line timing signals. These additional line timing signals are adapted to perform printing along an additional raster scanning line.

When received video signals have a resolution higher than that of an LED head, a logical AND operation between successive bit data of the video signals is carried out so that the video signals are converted to first signals formed of a data row corresponding to the resolution of the LED head.

Further means is also provided to transferring the first signals, as head data signals for performing the printing along the basic raster scanning line, to the LED head in correspondence to the respective basic line timing signals.

An exclusive-OR operation between the successive bit data of the video signals is carried out to convert the video signals to second signals formed of another data row. The second signals are outputted to a line buffer. Still further means is provided to transfer the second signals in the line buffer, as head data signals for performing the printing along the additional raster scanning line, to the LED head in correspondence with the respective additional line timing signals.

LED head drive energy for performing the printing along the basic raster scanning line and that for performing the printing along the additional raster scanning line are set independently.

It is also possible to print the logical AND between successive bit data of video signal on the additional raster scanning line and the exclusive-OR between the successive bit data on the basic raster scanning line.

As an alternative, it is also feasible to receive video signals on the same line twice. First, a logical AND operation between successive bit data of the video signals received first is carried out and is then printed. Next, an exclusive-OR operation between successive bit data of the video signals received second is carried out, followed by printing.

Further, LED head drive energy upon performing printing with the first signals obtained by the logical AND can be set greater than LED head drive energy upon performing printing with the second signals obtained by the exclusive-OR.

In addition, a non-impact printer according to this invention can comprise an LED head, an optical system for allowing light, which has been emitted from each light emitting diode of the LED head, to converge on a photoconductor drum, and means for transferring, as head data signals for performing printing along a raster scanning line, successive bit data of received video signals to the LED head in correspondence with line timing signals. In this case, by the LED head and the optical system, the size of each of the luminescent points on the raster scanning line and the resolution are set to make the highest one of the emission intensities higher than the sensitivity of the photoconductor drum.The emission intensity distribution is controlled to be narrow so that, when the intensities of light at adjacent luminescent points are combined, a valley is formed between the adjacent luminescent points in an emission intensity distribution diagram.

In the non-impact printer according to this invention, a logical AND operation between successive bit data of received video signals is carried out when the video signals so received have a resolution higher than that of an LED head. The video signals are converted to first signals and, in correspondence with line timing signals, the first signals are transferred to the LED head as head data signals to be printed along the basic raster scanning line.

Further, an exclusive-OR operation between successive bit data of the video signals is carried out so that the video signals are converted to second signals. Corresponding to the additional timing signals, these second signals are then transferred to the LED head as head data signals to be printed along the additional raster scanning line.

LED head drive energy for performing printing along the basis raster scanning line and LED head drive energy for performing printing along the additional raster scanning line are set independently.

Accordingly, even when the resolution of video signals from a host computer is higher than that of a printing unit, it is still possible to receive the video signals. Based on the video signals so received, printing is performed along an additional raster scanning line so that zig-zag image of oblique lines in a picture can be reduced.

Furthermore, a still further non-impact printer according to this invention comprises an LED head, an optical system for allowing light, which has been emitted from each light-emitting diode of the LED head, to converge on a photoconductor drum, and means for transferring, as head data signals for performing printing along a raster scanning line, successive bit data of received video signals to the LED head in correspondence to line timing signals. In this case, by the LED head and the optical system, the size of each of the luminescent points on the raster scanning line and the resolution of the optical system are set to make the highest one of the emission intensities higher than the sensitivity of the photoconductor drum. The emission intensity distribution is controlled to be narrow so that, when the intensities of light at adjacent luminescent points are combined, a valley is formed between the adjacent luminescent points in an emission intensity distribution diagram.

In this case, it is possible not only to print fine lines but also to reduce variations in print density compared with variations in emission intensity, thereby making it possible to provide a print with improved quality.

In order that the invention may be better understood, several embodiments thereof will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 is a block diagram of a control circuit of a non-impact printer according to a first embodiment of the present invention; Figure 2 is a block diagram of a print data receiving circuit of the non-impact printer; Figure 3 is a time chart of the print data receiving circuit; Figure 4 is an enlarged diagram of the time chart; Figure 5 is a block diagram of an LED head useful in the non-impact printer according to the first embodiment of the present invention; Figure 6 diagrammatically shows bit data of video signals received by the non-impact printer according to the first embodiment of the present invention;; Figure 7 diagrammatically illustrates bit data of head data signals in the non-impact printer according to the first embodiment of the present invention; Figures 8(a), 8(b) and 8(c) diagrammatically illustrate the appearance of a print by the nonimpact printer according to the first embodiment of the present invention Figures 9(a), 9(b) and 9(c) diagrammatically illustrate the appearance of another print by the non-impact printer according to the first embodiment of the present invention; Figure 10 diagrammatically shows dots printed on a raster scanning line by a first method and emission intensities corresponding to the printed dots; Figure 11 diagrammatically depicts dots printed on a raster scanning line by a second method and emission intensities corresponding to the printed dots;; Figures 12(a), 12(b) and 12(c) diagrammatically show emission intensities in the actually printed image of Figure 8; Figure 13 is a block diagram of a print data receiver in a non-impact printer according to a second embodiment of the present invention; Figure 14 is an enlarged time chart of the print data receiver in a non-impact printer according to a second embodiment of the present invention; Figure 15 is a plot of the relationship between emission intensity versus print density where the emission intensity distribution is broad; Figure 16 is a plot of the relationship between emission intensity versus print density where the emission intensity distribution is narrow; and Figures 17(a) and 17(b) diagrammatically illustrate an emission intensity when printing is performed by a non-impact printer according to a third embodiment of the present invention.

Referring first to Figure 1, the non-impact printer according to the first embodiment of the present invention has a printing control unit 1 and an LED head 19. Although not specifically illustrated, the printing control unit 1 can be constructed of ROM, RAM, I/O port, timer circuit and the like or, as an alternative, can also be constructed a logical circuit having equivalent functions thereto. The printing control unit 1 receives, via an unillustrated interface, control signals 10 and video signals 11 from a host controller such as a personal computer or a work station. Through a print data converter 40, the printing control unit 1 outputs timing signals 12 to the host controller and print head drive signals 13, actual print data signals, i.e. head data signals 18, clock signals 18a and latching signals to the LED head 19.As a consequence, the LED head 19 is selectively actuated to form an electrostatic latent image on a developer 27.

In addition to receiving the control signals 10 and video signals 11 outputted from the host controller, the printing control unit 1 detects the condition of the non-impact printer itself on the basis of signals from sensors arranged in the non-impact printer itself such as a paper inlet sensor 6, a paper output sensor 7, a paper remaining sensor 8, a paper size sensor 9 and a fixing device temperature sensor 29, whereby the printing control unit 1 determines whether the non-impact printer is in condition ready for printing. If the temperature of a fixing device 22 is lower than a preset level, for example, the printing control unit 1 outputs a heater drive signal 21 to turn on a heater 22a, so that the heater 22a is heated to a temperature at which printing is feasible.

If the non-impact printer is in condition ready for printing, the printing control unit 1 outputs a drive signal to a driver 2 to turn on a motor 3 for developing and transfer processes. At the same time, the printing control unit 1 outputs a charging signal 23 to a charging high-voltage power supply 25 to charge the developer 27 and, at the same time, another charging signal 24 to a transferring high-voltage power supply 26 so that a toner image formed on the developer 27 is transferred onto a paper sheet by a transferring device 28.

The paper sheet is fed by outputting a drive signal to a driver through the printing control unit 1 and turning on a paper feed motor 5. By the paper remaining sensor 8 and the paper size sensor 9, the printing control unit 1 detects the kind of paper sheets which are set in the non-impact printer, followed by the initiation of paper feeding as needed for the paper sheets. Described specifically, the printing control unit 1 can drive through the driver 4 the paper feed motor 5 in both directions. A paper sheet, which has been set by first driving the paper feed motor 5 in reverse direction, is fed over a preset distance until the paper inlet sensor 6 detects the paper sheet. An unillustrated gear is next changed over to drive the paper feed motor 5 in normal direction, whereby the paper sheet is conveyed to a printing section of the non-impact printer.As soon as the paper sheet reaches a position where printing is feasible, the printing control unit 1 sends timing signals 12 (including both line timing signals and video signal transferring clock signals) to the host controller and receives the video signals 11 in correspondence to these timing signals 12. The video signals, which were compiled page by page by the host controller and have been received by the printing control unit 1, are converted to actual print data signal 18 by the print data converter 40 and are then delivered to the LED head 19. By the LED head 19, an electrostatic latent image is formed on the developer 27 as described above.

The printing control unit 1 fixes the transferred toner image on the thus-conveyed paper sheet by the heat of the fixing device 22 and, by the paper feed motor 5, conveys the paper sheet past the paper outlet sensor 7 to the outside of the non-impact printer.

A description is now made of a printing operation under the following conditions: Resolution of the LED head 19 in the printing section of the non-impact printer (the pitch of arrangement of light emitting diodes): 300 DPI (raster scanning direction); resolution of the video signals 11 to be received: 600 DPI; resolution of an actually printed image: 300 DPI (raster scanning direction) and 1200 DPI (pseudo-600 DPI, printing direction, that is, direction of conveyance of paper sheet).

Reference is now had to FIG. 2. When a paper sheet reaches the position where printing is feasible, the print data converter 40 transmits the timing signals 12, including both line timing signals 12a and video signal transferring clock signals 12b, to the host controller and receives the video signals 11. The video signals 11 of 600 DPI in both vertical and horizontal directions, said video signals 11 having been compiled page by page at the host controller, are converted to first signals 53a by D flip-flops 51,52 and an AND circuit of a resolution conversion block as illustrated in FIGS. 2 through 4. To print the first signals 53a along a basic raster scanning line, the first signals 53a are transferred as head data signals 18 to the LED head 19 via a selector 57.Further, second signals 54a which have been converted at the same time by an exclusive-OR circuit 54 are stored in a line buffer 56 via a selector 55 so that the second signals 54a will be printed along an additional raster scanning line formed between basic raster scanning lines.

FIG. .2 also shows a clock signal generator 58, a delay circuit 60, a line timing signal generator 62, a multiplier 61 for doubling the frequency of the line timing signals 12a generated by the line timing signal generator 62, and an AND circuit 63. By the multiplier 61, additional line timing signals 12c are generated to perform printing along the additional raster scanning line between the successive line timing signals 12a.

Different from the video signal transfer clock signals 12b outputted from the printing section to the host controller, the clock signals 18a have been obtained by dividing the clock signals, which were generated by the clock signal generator 58, into two at a D flip-flop 59. The clock signals 18a are sent via the delay circuit 60 to the LED head 19 shown in FIG. 5.

Referring next to FIG. 5, the LED head 19 is constructed of a shift register 19a adapted to input the head data signals 18 and the clock signals 18a, a latch circuit 19b for holding output signals of the shift register 19a by latch signals 17, and a driver 19d for receiving the print head drive signals 13 and supplying electric power to an LED array 19c.

The head data signals 18 and the clock signals 18a, which are both inputted to the LED head 19, are outputted from the print data converter 40 as will be described next.

Each first signal 53a is the logical AND of outputs from the D flip-flop 51 and the D flip-flop 52.

As is shown in FIG. 4, the first bit datum is the logical AND of the first and second bit data of the video signal 11 of 600 DPI, the second bit datum is the logical AND of the second and third bit data, .... The first signals 53 are transferred as head data signals 18 tithe LED head 19 via the selector 57. Since each clock signal 18a is outputted in synchronization with an (n+l)th datum (n: even number) of the head data signals 18 owing to the provision of the flip-flop 59 and the delay circuit 60, the bit data which are successively stored in the shift register 19a disposed in the LED head 19 shown in FIG. 5 are only (n+l)th bit data (n: even numbers) of the head data signals 18.Signals to be printed along a basic raster scanning line are therefore the logical AND of the 1St and 2nd bit data of the video signals 11, the logical AND of the 3rd and 4th bit data of the video signals 11, ...., the logical AND of the (2m+1)th and (2m+2)th bit data of the video signals 11 (m = 0, 1, 2 ...., 2559), .....

Since each second signal 54a is the exclusive-OR of signals outputted from the D flip-flop 51 and the D flip-flop 52, respectively, the first bit datum is the exclusive-OR of the 1St and 2nd bit data of the 600 DPI video signals 11, the 2nd bit datum is the exclusive-QR of the 2nd and 3rd bit data of the 600 DPI video signals 11, ..... The second signal 54a is stored in the line buffer 56 via the selector 55 and, upon performing printing along the additional raster scanning line, is read as the third signal 55a via the selector 55 and then transferred as the head data signal 18 to the LED head 19 via the selector 57.Similarly to the printing along the basic raster scanning line, it is designed that each clock signal laa is sent to the LED head 19 in synchronization with the (n+l)th (n: even number) bit datum of the head data signals 18. The bit data successively stored in the shift register 19a are only the (n+l)th (n: even numbers) bit data of the head data signals 18. Accordingly, those to be printed along the additional raster scanning line are the exclusive-OR of the 1st and 2nd bit data of the video signals 11, the exclusive-OR of the 3rd and 4th bit data of the video signals 11, ...., the logical AND of the (2m+l)th and (2m+2)th bit data of the video signals 11 (m = 0, 1, 2, ...., 2559), .....

Regarding, for example, the video signals 11 received at 600 DPI as shown in FIG. 6, bit data of the logical ANDs of the (n+1)th (n: even numbers) bit data and the nth (n: even numbers) bit data of the video signals 11 are printed along the basic raster scanning line as shown in FIG. 7, whereas bit data of the exclusive-ORs of the (n+l)th (n: even numbers) bit data and the nth (n: even numbers) bit data of the video signals 11 are printed along the additional raster scanning line. Namely, the bit data in sl shown in FIG. 6 are converted to the bit data in S2 illustrated in FIG. 7.

After transferring one line of the head data sig nal 18 to LED head 19, the printing control unit 1 sends the latch signal 17 to the LED head 19 so that the (n+l)th bit data of the converted head data signals 18 are held in the LED head 19. The LED head 19 has a number of light emitting diodes arranged in the raster scanning direction. Upon receipt of the print head drive signal 13, the LED head 19 is driven with LED head drive energy El by the bit data held therein and forms an electrostatic latent image on the photoconductor drum.

When the paper sheet reaches a position 1/1200 inch apart in the paper conveying direction, the printing control unit 1 then changes over the selectors 55,57 to take out the bit data (the third signals 55a) stored in the line buffer 56. By the clock signals 18a, the third signals 55a are transferred as the head data signals 18 to the LED head 19. At this time, the line timing signals 12a are not generated to the host controller and the above operation is practiced by the printing section alone.

The printing control unit 1 then delivers a latching signal 17 to the LED head 19, whereby the above bit data signals are held in the LED head 19.

Upon receipt of the print head drive signal 13, the LED head 19 is driven with LED head drive energy E2 by the bit data held therein ard forms an electrostatic latent image on the photoconductor drum.

At the developer 27, picture-forming toner charged negative is attracted to the individual dots by electrically attracting force to form a toner image.

The toner image is sent to the transferring device 28.

On the other hand, the transferring high-voltage power supply 26 of positive potential is turned on by the charge signal 24, and the transferring device 28 transfers the toner image onto a paper sheet which is passing through a gap between the photoconductor drum and the transferring device 28.

The paper sheet carrying the toner image transferred thereon is conveyed while being maintained in contact with the fixing device 22 having the heater 22a accommodated therein. As a result, the toner image is fixed by the heat of the fixing device 22. The paper sheet with the toner image so fixed is conveyed further from the printing section and past the paper outlet sensor 7 and is then ejected to the outside of the nonimpact printer.

Responsive to the detection of the paper sheet by the paper size sensor 9 and the paper inlet sensor 6, the printing control unit 1 applies a voltage to the transferring device 28 from the transferring high voltage power supply 26 only for the period during which the paper sheet passes by the transferring device 28. When the printing is completed and the paper sheet is conveyed past the paper output sensor 7, the application of the voltage to the developer 27 by the charging high-voltage power supply 25 is stopped and, at the same, the rotation of the motor 3 for developing and transferring processes is stopped.

After that, the above operation is repeated.

Here, the LED head drive energies E1,E2 per line are set so that, as illustrated in FIGS. 8 and 9, a dot image formed by printing at pseudo-600 DPI becomes equivalent to a dot image printed at standard 300 DPI.

As is shown in FIG. 8(b), printing with the LED head drive energy El is performed along basic raster scanning lines on lines L1 and L2 1/300 inch apart from each other and printing with the LED head drive energy E2 is effected along additional raster scanning lines.

The image actually printed here are formed of print portions such as A-A and C-C on the basic raster scanning lines and print portions such as B-B on the additional raster scanning lines.

In the first embodiment, conversion is effected to increase data at positions 1200 DIP apart from each other. The LED head drive energies E1,E2 are therefore set to satisfy the follcwing inequality: El > E2 The required LED head drive energies E1,E2 are set as follows on the basis of the LED head drive energy E for standard 3000 DPI, although they vary depending on the characteristics of the developer 27, lens and toner: El = (0.4 to 0.6) x E E2 = (0.15 to 0.25) -x E Referring next to FIGS. 10 and 11, a description will be made of a method for performing printing at a resolution higher than that of the LED head 19.

In FIG. 10, the emission intensity at a luminescent point by each light emitting diode of the LED head 19 (see FIG. 1) sufficiently exceeds the sensitivity of the photoconductor drum so that printed dots are formed at the positions of individual luminescent points.

At this time, the emission intensity at an intermediate part between adjacent fluorescent points is increased because of lighting of the adjacent luminescent points, and exceeds the sensitivity of the photoconductor drum. Such a high emission intensity is needed to form an image. Therefore, printing is also conducted at thil"intermediate part so that the adjacent dots are connected via the dot at the intermediate part.

In FIG. 11, the emission intensity at each single luminescent point does not exceed the sensitivity of the photoconductor drum although an emission intensity higher than the sensitivity is required for the formation of an image. No printed dot is therefore formed. When adjacent luminescent points are lit up at the same time, on the other hand, the combined emission intensity exceeds the sensitivity of the photoconductor drum at the intermediate part. It is therefore possible to form a printed dot at the intermediate part. Namely, it is possible to perform printing at a resolution twice as high as that of the printing section.

FIGS. 12(a), 12(b) and 12(c) diagrammatically illustrate emission intensities in an actually-printed image shown in FIG. 8(c). FIG. 12(a) shows the emission intensity along a print portion A-A, FIG. 12(b) the emission intensity along a print portion B-B, and FIG. 12(c) the mission intensity along a print portion C-C.

FIG. 12 (a) indicates an emission intensity when printing was performed with the LED head drive energy El [FIG. 8(b)) on the basic raster scanning line. At the portion where the emission intensity exceeds the sensitivity of the photoconductor drum, the print portion A-A in the actually-printed image of FIG. 8(c) is formed.

FIG. 12(b) indicates an emission intensity when printing was performed with the LED head drive energy E2 on the additional raster scanning line. At the portion where the emission intensity exceeds the sensitivity of the photoconductor drum, the print portion B-B in the actually-printed image of FIG. 8(c) is formed. In this case, the emission intensity becomes highest on a line L3 located between lines L1 and L2 so that a printed dot is also formed on the line L3 (pseudo-600 DPI).

FIG. 12(c) indicates an emission intensity when printing was performed with the LED head drive energy El on the basic raster scanning line. At the portion where the emission intensity exceeds the sensitivity of the photoconductor drum, the print portion C-C in the actually-printed image of FIG. 8(c) is formed.

In the first embodiment described above, the logical AND between successive bit data is carried out by the print data receiver and is printed on a basic raster scanning line and, at the same time, the exclusive-OR between the successive bit data is carried out by the print data receiver and is printed on an additional raster scanning line. As an alternative, it is also possible to carry out the logical AND between successive bit data by the print data receiver and to print it on an additional raster scanning line and, at the same time, to carry out the exclusive-OR between the successive bit data by the print data receiver and to print it on a basic raster scanning line.

When dots are printed between individual luminescent points as shown in FIG. 11 described above, signals to be printed on a basic raster scanning line and an additional raster scanning line can be generated in the following manner.

First, every second data signals are selected from 600 DPI video signals inputted, whereby first head data signals of 300 DPI are generate and printed on a basic raster scanning line. The generation of the first head data signals can be effected by selecting video signals upon output of the video signal transferring clock signals 12b. Since these first head data signals correspond to the positions of the respective luminescent points, the printing is controlled so that the emission intensity at each single luminescent point exceeds the sensitivity of the photoconductor drum.

Next, the remaining data signals are selected to generate the above 300 DIP signals from the 600 DPI signals inputted, followed by the generation of second head data signals of 300 DPI in a similar manner.

These data signals can be selected in a similar manner to the above-described first head data signals by using clock signals which have the same frequency as the above video signal transferring clock signals 12b but are shifted in phase by 1 clock of the video signals.

Each second head data signal corresponds to a dot between each two adjacent luminescent points of the LED head. To print the dot, the luminescent points on the opposite sides are lit. The emission intensity at this time is adjusted so that, as illustrated in FIG. 11, light is emitted with an emission intensity not reaching the sensitivity of the photoconductor drum at each single luminescent point but exceeding the sensitivity at each part where light beams emitted from two adjacent pixels, respectively, are overlapped.

The non-impact printer according to the second embodiment of the present invention will be described next.

The non-impact printer of the second embodiment is not provided with any line buffer adapted to store bit data to be printed along an additional raster scanning line. To perform printing along the additional raster scanning line, line timing signals 12a are sent to a host controller to receive, as video signals 11, the same 600 DPI data as those printed along a basic raster scanning line. The 600 DPI data are then converted to head data signals 18.

In FIGS. 13 and 14, there are illustrated D flipflops 51,52,59, an AND circuit 53, an exclusive-OR circuit 54, a selector 57, a clock signal generator 58, a delay circuit 60, a multiplier 61, and a line timing signal generator 62.

To perform printing along a basic raster scanning line, line timing signals 12a conforming with timing signals 12 are sent to an unillustrated host controller. Bit data on the mth line of 600 DPI data are transmitted as video signals 11. The AND circuit 53 carries out the logical AND between outputs from the D flip-flops 51,52 and the logical AND is outputted to the selector 57. Here, the selector 57 selects the first signal 53a, the output from the AND circuit 53, and transfers it as a head data signal 18 to the LED head 19 (FIG. 1). Like the first embodiment, each clock signal 18a is obtained by dividing a clock, which has been generated by the clock signal generator 58, into two by the D flip-flop 59. The clock signal 18a is delivered to the LED head 19 through the delay circuit 60.The clock signal 18a is synchronized with the (n+l)th (n: even number) bit datum of the first signal 53a. Bit data similar to those obtained in the first embodiment are therefore sent to the shift register l9a of the LED head 19 (see FIG. 5) and are then printed.

To perform printing along an additional raster scanning line, line timing signals 12a are sent to the host controller. At this time, the same bit data on the mth line as the bit data for the basic raster scanning line are transmitted as video signals 11 from the host controller. The exclusive-OR circuit 54 carries out the exclusive-OR between outputs from the D flipflops 51,52, followed by the output of the exclusive-OR to the selector 57. At this time, the selector 57 selects the second signal 54a, the output from the exclusive-OR circuit 54 and transfers it as a head data signal 18 to the LED head 19. Since the clock signal 18a is sent as in the case of the basic raster scanning line, printing is performed as in the first embodiment.

Even when the line buffer 56 (FIG. 2) is not provided in the printing control unit 1, the same printing results can be obtained as in the first embodiment by sending data on the same mth line twice from the host controller.

In the second embodiment described above, the logical AND between successive bit data is carried out by the print data receiver and is printed on a basic raster scanning line and, at the same time, the exclusive-OR between the successive bit data is carried out by the print data receiver and is printed on an additional raster scanning line. As an alternative, it is also possible to carry out the logical AND between successive bit data by the print data receiver and to print it on an additional raster scanning line and, at the same time, to carry out the exclusive-OR between the successive bit data by the print data receiver and to print it on a basic raster scanning line.

In the second embodiment described above, each adjacent two luminescent points are lit at the same time. Their emission intensities are combined to perform printing at an intermediate part between the luminescent points. A broad emission intensity distribution is therefore needed. It is therefore necessary to use light having a broad emission intensity distribution. Use of light having a broad emission intensity distribution, however, results in greater variations in print density compared with variations in emission intensity.

A broad emission intensity distribution can be expressed by the following formula (1): P(x,y) Ae-2(x + y )/# (1) The formula (1) is generally known as a formula representing the emission intensity distribution of a laser beam. P(x,y) is the emission intensity at coordinates (x,y), A is a parameter associated with the emission intensity P(x,y), and r is a parameter indicating the breadth of the emission intensity distribution. The greater the parameters A and t, the greater the emission intensity P(x,y) and the broader the distribution.When printing is performed as a part where the emission intensity P(x,y) is equal to or higher than the sensitivity Eth of the photoconductor drum, the print area S can be expressed by the following formula (2): S=irr2/2.(log A - log Eth) ..... (2) Partially differentiating the formula (2) with respect to a parameter A in order to investigate the rate of a change of the print area S relative to a change in the parameter A, OS/8A = (rr2/2)X(1/A) ...-- (3) From the formula (3), it is appreciated that the print area S undergoes a greater change as the rate of a change of the parameter r relative to a change of the parameter A increases, namely, as the emission intensity distribution becomes broader.As described above, the print density varies greater compared to variations of the emission intensity P(x,y).

A description will next be made of an embodiment which can reduce variations in print density relative to variations in the emission intensity P(x,y).

Reference is now had to FIGS. 15 and 16, which shows the emission intensity in the raster scanning direction only. In each of these drawings, Eth indicates the sensitivity of a photoconductor drum, the solid curve a standard emission intensity, the dashed curve an emission intensity higher than the standard emission intensity, w1,w2 print widths at the standard emission intensity, and W1,W2 print widths at the higher emission intensity.

When the emission intensity varies and becomes greater than the standard emission intensity, for example, the print density varies by Wl/wl when the emission intensity distribution is broad, while the print density varies by W2/w2 when the emission intensity distribution is narrow.

Here, Wl/wl > W2/w2.

It is hence understood that a narrower emission intensity distribution leads to smaller variations in print density even if the emission intensity varies.

Reference is next had to FIGS. 17(a) and 17(b), in which FIG. 17 (a) indicates an emission intensity upon printing at an intermediate part between two luminescent points while FIG. 17(b) shows an emission intensity upon printing at a luminescent point.

In each of these drawings, P,Q are luminescent points on a raster scanning line, Eth the sensitivity of a photoconductor drum, and H the distance between the luminescent points. The solid curves indicate the luminescent intensities at the respective luminescent points P,Q, while the dashed curve shows a combined emission intensity.

In this case, compared with the emission intensity distributions shown in FIGS. 12(a) to 12(c), the breadth of the emission intensity distribution at each of the luminescent points P,Q is set narrower.

Further, the maximum value of the emission intensity is set greater than the sensitivity Eth of the photoconductor drum. In the combined emission intensity distribution, a valley is formed at an intermediate part between the luminescent points P and Q. The combined emission intensity at the intermediate part is higher than the sensitivity Eth of the photoconductor drum so that printing is feasible there. Even if the combined emission intensity at the intermediate part between the luminescent points P-and Q is lower than the sensitivity Eth of the photoconductor drum, no problem arises in the quality of the resulting print as long as the width at the low emission intensity is so small that the width is not visible to the eye.

To make the breadth of the emission intensity distribution smaller as described above, it is necessary to use such light emitting diodes as enabling to make the individual luminescent points P,Q sufficient small compared with the distance H between the luminescent points and, forming a valley at an intermediate part between the luminescent points P and Q in the combined emission intensity distribution, that is, at the center of prints. Further, an optical system of high resolution is used so that the adjacent two luminescent points P,Q can be sufficiently distinguished. Where the individual luminescent points P,Q are extremely small or the resolution of the optical system is extremely high, however, printed dots are connected upon printing and the resulting print has low quality.

In the case of an LED head 19 having the resolution of 300 DPI (FIG. 1), for example, it is therefore desired to set the size of the individual luminescent points at 48 x 48 (pom) and also to use "SLA20B" (trade name; rod lens array having high resolution MTF and manufactured by Nippon Sheet Glass Co., Ltd.) as an optical system for causing light, which have been emitted from light emitting diodes, to converge on a photo conductor drum. In this case, the breadth of the emission intensity distribution at each of the luminescent points P,Q is narrow so that the emission intensity has to be increased. It is however easy to make a print width w3 at such a combined emission intensity as shown in Figure 17(a) equal to a print width w4 at the emission intensity at the luminescent point Q as shown in Figure 17(b).

Claims (8)

1. A non-impact printer comprising: (a) a print head in the form of an LED head; (b) a main signal generator for producing basic line timing signals adapted to perform printing along a basic raster scanning line; (c) an auxiliary signal generator for producing additional line timing signals between the successive basic line timing signals, said additional line timing signals being adapted to perform printing along an additional raster scanning line; and (d) means for independently setting the LED head drive energy for performing printing along the basic raster scanning line and the LED head drive energy for performing printing along the additional raster scanning line.

2. A printer as claimed in claim 1, further comprising: (e) a converter for carrying out a logical AND operation between successive bit data of video signals received, whereby the video signals are converted to first signals; (f) a first circuit for transferring the first signals, as head data signals for performing the printing along the basic raster scanning line, to the LED head in correspondence to the respective basic line timing signals; (g) a second circuit for carrying out an exclusive-OR operation between the successive bit data of the video signals to convert the video signals to second signals and outputting the second signals to a line buffer; and (h) a third circuit for transferring the second signals in the line buffer, as head data signals for performing the printing along the additional raster scanning line, to the LED head in correspondence to the respective additional line timing signals.

3. A printer as claimed in claim 1, further comprising: (e) a converter for carrying out a logical AND operation between successive bit data of video signals received, whereby the video signals are converted to first signals; (f) a first circuit for transferring the first signals, as head data signals for performing the printing along the additional raster scanning line, to the LED head in correspondence to the respective additional line timing signals; (g) a second circuit for carrying out an exclusive-OR operation between the successive bit data of the video signals to convert the video signals to second signals and outputting the second signals to a line buffer; and (h) a third circuit for transferring the second signals in the line buffer, as head data signals for performing the printing along the basic raster scanning line, to the LED head in correspondence to the respective basic line.timing signals.

4. A printer as claimed in claim 1, further comprising: (e) a converter for receiving first video signals at a first timing and carrying out a logical AND operation between successive bit data of the first video signals, whereby the video signals are converted to first signals; (f) a first circuit for transferring the first signals, as head data signals for performing the printing along the basic raster scanning line, to the LED head in correspondence to the respective basic line timing signals; (g) a second circuit for receiving the same video signals at a second timing different from the first timing and carrying out an exclusive-OR operation between successive bit data of the same video signals to convert the same video signals to second signals; and (h) a third circuit for transferring the second signals, as head data signals for performing the printing along the additional raster scanning line, to the LED head in correspondence to the respective additional line timing signals.

5. A printer as claimed in claim 1, further comprising: (e) a converter for receiving video signals at a first timing and carrying out a logical AND operation between successive bit data of the video signals, whereby the video signals are converted to first signals; (f) a first circuit for transferring the first signals, as head data signals for performing the printing along the additional raster scanning line, to the LED head in correspondence to the respective additional line timing signals; (g) a second circuit for receiving the same video signals at a second timing different from the first timing and carrying out an exclusive-OR operation between successive bit data of the same video signals to convert the same video signals to second signals; and (h) a third circuit for transferring the second signals, as head data signals for performing the printing along the basic raster scanning line, to the LED head in correspondence to the respective basic line timing signals.

6. A non-impact printer comprising: (a) an LED head; (b) an optical system for allowing light1 which has been emitted from each light emitting diode of the LED head, to converge on a photoconductor drum; (c) a circuit for transferring, as head data signals for performing printing along a raster scanning line, successive bit data of received video signals to the LED head in correspondence to line timing signals; and (d) means for setting the size of each of luminescent points on the raster scanning line and the resolution of the optical system to make the highest one of the intensities of light at the individual luminescent points higher than the sensitivity of the photoconductor drum and, when the intensities of light at adjacent luminescent points are combined, forming a valley between the adjacent luminescent points in a composite emission intensity distribution diagram.

7. A process for performing printing by means of a non-impact printer equipped with an LED head, comprising the following steps: (a) inputting video signals having a resolution as much as twice the resolution of the LED head; (b) selecting every second bit data of the video signals and outputting first head data signals; (c) selecting the remaining bit data of the video signals and outputting second head data signals; (d) making the first head data signal correspond to individual pixels of the LED head and driving the LED head in correspondence to basic line timing signals adapted to perform printing along a basic raster scanning line, whereby the LED head is caused to emit light of an intensity higher than the sensitivity of a photoconductor drum; and (e) making the second head data signals correspond to adjacent two pixels of the LED head and driving the LED head in correspondence to additional line timing signals between the basic line timing signals, thereby emitting light of an intensity not higher than the sensitivity when taken as light emitted from each pixel of the LED head but higher than the sensitivity when measured at an overlapped part of light emitted from the adjacent two pixels.

8. A process as claimed in claim 7, wherein the basic line timing signals and the additional line timing signals are outputted at mutually close timings, respectively.