JPS59223070A - Laser printer - Google Patents

Abstract

PURPOSE:To modulate the laser lighting time and to reproduce a half-tone picture, by weighting an n-bit digital signal and performing pulse width modulation of 2<n> tone wedge. CONSTITUTION:A 2-bit digital signal DATAO is supplied to ABD gates 46-1 and 46-2. At this time, the signal DATAO is brought to the opposite phase. Both transfer clock signals CLK gated at the gates 46-1 and 46-2 have the same period T but different phases which are different by 180 deg. from each other. The A3 and B3 of the figure show opposite phase pulses of a width T/2 gated by the gates 46. Outputs of the gates 46 are delayed by 1/6 period by means of delay elements 47 and 48 and become the signal of 4. A gate 49 modulates the signal DATAO having a width of T/3 as shown by A5 of the figure. At this time, the signal DATAO has pulse widths from 1/6T to 1/2T when counted from the rising time of the clock CLK. On the other hand, the signal DATAO is modulated into a signal (B5) having a width of 2T/3 through a gate 50. The signal of this time has pulse widths from T/2 to 7T/6. Therefore, a laser output which is modulated in terms of pulse width is obtained from a laser diode 41 in correspondence to a signal DATA1.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention provides two
``Regarding a laser printer that performs gradation laser modulation.

(Prior Art) Laser printers have been known that emit laser light in response to a DigiMole image signal and scan a photoreceptor with a rotating polygon mirror to obtain a digital image.An example of this is shown in FIG. Here, ■ is a laser device that emits a semiconductor laser, 2 is a rotating polygon mirror, and 3 is a photosensitive drum. A laser beam is incident on the rotary polygon mirror 2 in the direction of the arrow to scan the photoconductor 3. At the same time, the photoconductor 3 is also rotated in the direction of the arrow and a digital image is transferred onto the photoconductor 3. It forms a latent image.

Conventionally, this type of laser printer has controlled laser emission using binary even numbers of 1 and 0 to form images.

The reason for this is that it is difficult to modulate the density level so that it has gradation because the laser has a very steep rising light emission characteristic. Generally, 2
Dithering is known as a method for achieving gradation from individual numbers. However, since the dither method expresses shading based on the area ratio of black pixels in a minute area, it has the disadvantage that not only the resolution decreases but also the circuit becomes complicated.

Therefore, a method has been adopted to express gradation by pulse width modulating the laser. Figure 2 shows an example of 3AI! 13>
zt) vJ3Bffit:, Tht.

FIG. 2 shows an example of a circuit for pulse width modulating a 2-bit, 4-value digital signal. Here, 21 is a laser diode that emits a laser beam, 22 and 23 are terminals for inputting 2-bit digital signals DATAO and DATAI, 24 is an input terminal for a transfer lock CLK that transfers signals DATAO and DATAI, and 25 is a transfer lock CLK input terminal for transferring signals DATAO and DATAI. Rock C
This is a terminal for inputting a clock signal CLKH having a frequency three times that of LK. 26 is a decoder that decodes a 2-bit digital signal into 4 bits; 27 is a pulse width modulation circuit that generates clock signals CLKHI, CLKH2, and CLKH3 with different pulse widths based on the triple clock signal CLKH; and 28 is a decoded signal. 29 is an AND gate, 30 is an OR gate, and 31 is a transistor for causing the laser diode 21 to emit light.

First, two bits input to terminals 22 and 23 are latched by latch circuit 28. On the other hand, a clock signal CL having a frequency three times that of the clock signal CLK
KH is generated by the pulse width modulation circuit 27 into three clock signals CLK with different duty cycles)11. CLK)
12. Converted to CLKH3.

FIG. 3A shows the waveforms of each of these clocks. here,(
1) indicates a clock signal CLK, which has a period of T.

Duty cycle is 50%. (2) is (1)
The period synchronized with T73. This is a clock signal CLKH with a duty cycle of 50%. The pulse width modulation circuit 27 generates signals with a period T and a duty cycle of 1/3.2/3.4, respectively, as shown in (3), (4), and (5). That is, (2), (3), and (4) are each 2-bit digital signals (DATA I, DA
(0.1), (i, o), , ('
1.1). These signals and the decoded signal latched by the latch circuit 28 are
・Introduced into 29, a pulse width corresponding to the digital signal can be obtained.

Then, the pulse width modulated upper 3 bit decode signal is converted into 1 bit by the OR gate 30, and the laser diode 21 is driven to emit light by the transistor 31.

2-bit digital signal (DATAI, [)ATAO
) +: The corresponding laser emission pulse width (lighting time) is as shown in FIG. 3B. That is, the 2-bit digital numbers (o, o), (o, i), (
1, o) and (1, 1), 0.
A pulse-width modulated laser as shown in Nos. 1 and 2.3 is emitted.

However, in this type of pulse width modulation, as the number of bits of the digital signal increases, the frequency of the high-frequency clock whose duty cycle is to be changed deteriorates, so it is not suitable for increasing speed. Another disadvantage is that the pulse width modulation circuit becomes complicated.

(Objective) In view of the above-mentioned points, an object of the present invention is to weight an n-bit digital signal and perform 2T1 gradation pulse width modulation, thereby modulating the laser lighting time, thereby creating a halftone image. It is an object of the present invention to provide a laser printer that can reproduce images.

Example Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 4 shows an embodiment of the invention. In this figure, 4
1 is a laser diode, 42 and 43 are terminals for inputting 2-bit digital signals DATAO and DATAI, 44 is a terminal for inputting a transfer lock signal CLK for transferring signals DATAO and DATAI, 45 is an inverter, 46-1 and 4 to 2 is an and gate, 4
7 and 48 are delay elements that delay the transfer lock signal CLK by 1/6 period, 48 is an AND gate, 50 and 51 are OR gates, and 52 is a transistor that lights up the laser diode I. 2-bit digital signals DATAO and DATAI from input terminals 42 and 43
input and transfer lock signal CLK to the anime game)
413-1 and 46-2. At this time, the signal DATAO is converted into an opposite phase by the inverter 5 and converted into an antegrated signal (4E!). -1.

FIGS. 5A and 5B show signal waveforms at each part of the fourth circuit. The transfer lock signals CLK gated by AND gates 4B-1 and 46-2 both have a period of T and a phase of 180 degrees, as shown in FIG. 5A (1) and (2).
°Different. FIG. 5A (2) and FIG. 5B (2) respectively show waveforms (period T) when the signals DATAO and DATAI are l'. FIG. 5A (3) and FIG. 5B (3) show anti-phase pulses of T/2 width generated by AND gates 4B-1 and 48-2.

In addition, the outputs of 4B-1 and 48-2 are delayed by 1/8 period by delay elements 47 and 48, respectively, resulting in signals as shown in FIG. 5A (4) and FIG. 5B (4). . Antogame) 48-1 output (5th A
(See Figure 5A (3)) and the output of the delay element 47 (See Figure 5A (4)) are gated by an AND gate 48.
As shown in 5), the signal DATAO has a pulse width of 717z and g.
Modulation is performed.

At this time, if the rise of the transfer lock CLK is taken as time O, the pulse width will be from 1/8T to 1/2T.

On the other hand, the output of the AND gate 46-2 (see FIG. 5B (3)) and the output of the delay element 48 (see FIG. 5B (4)) are transmitted via the OR gate 50 to a pulse signal with a pulse width of 2773 (see FIG. 5B). (5)). At this time, the pulse generation times for the signals DATAO and DAjAl- are different from each other and are continuous.

The OR gate 51 is used to convert these outputs into pulse width modulated signals, and the transistor 52 causes the laser diode 41 to emit light. FIG. 6 shows the lighting time of the laser diode 41. That is, the digital signal (DA
TAI, DATAO), respectively (0, 0
) , (0,1) , (1,0) , (1,1)
A pulse width modulated laser output can be obtained as shown in FIG. Note that the shaded area A shown in FIG. 6 is a pulse signal caused by the signal DATAO, and the shaded area B shown in the figure is a pulse signal caused by the signal 'DATAI. In this way, each bit of the digital signal can be weighted by the delay element, thereby performing pulse width modulation.

In addition, in this embodiment, a 2-bit digital signal is pulse-width modulated, but even in the case of 3 or more bits, each bit can be weighted using a delay element to perform pulse-width modulation. can.

(Effects) As explained above, according to the present invention, pulse width modulation can be performed easily and quickly without the need for a high-frequency clock, so a laser printer that can reproduce halftones can be made inexpensively and compactly. can be converted into

[Brief explanation of the drawing]

FIG. 4 is a diagram illustrating a conventional method of performing modulation. DESCRIPTION OF SYMBOLS 1... Laser device, 2... Rotating polygon mirror, 3... Photosensitive drum, 21... Laser diode, 22-25... Terminal, 28... Decoder, 27... Pulse width Modulation circuit, 28... Latch circuit, 29... AND gate, 30... OR gate, 31... Transistor, 41... Laser diode, 42-44... Terminal, 45... Inverter, 46-1, 413-2, 411-51...gate, 4
7.48...Delay element, 52...Transistor. -needle →riT-=toT □alsoToI/2r
r o//2-r-r 6th
figure

Claims (1)

[Claims] l) In a laser printer that forms an image by irradiating a photoreceptor with modulated laser light, each n-bit digital image signal is weighted to perform 2'' gradation laser modulation. 2) A laser printer according to claim 1, characterized in that the weighting is performed using a delay element.