JP2974318B2 - Image processing device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus for forming a pulse width modulation signal from an image signal and a pattern signal having a predetermined period. [Prior art] When forming an image with a laser beam printer or the like, a multi-valued digital image signal is converted into an analog image signal in order to obtain a halftone gradation, and the converted signal is converted into a periodic signal such as a triangular wave. There has been proposed a method of generating a binary signal subjected to pulse width modulation by comparing with a pattern signal. FIG. 2 shows an example of this technique. The 8-bit digital video signal is latched by a video clock from a reader unit (not shown) in a latch 1 and is synchronized. This 8-bit video signal is converted by a D / A converter 2 into an analog video signal. The output of the D / A converter 2 is input to one input terminal of two comparators 5 and 6 after the output level is adjusted to an optimum value by the variable resistors 3 and 4. On the other hand, two reference pattern generation clocks PHCLK and TXCLK synchronized with the video clock sent from a reader unit (not shown) are supplied by J / K flip-flops 7 and 8 to generate two reference pattern clocks PHCLK and TXCLK.
Clock signal PHCLK2, TXCLK divided by 50% and duty ratio 50%
Form 2. Here, the frequency of the TXCLK is for generating a pattern signal for a line drawing original such as a character, so that the resolving power is important. Therefore, the frequency is twice as high as that of the video signal and a pattern signal is generated for each pixel. On the other hand, the frequency of the PHCLK is for generating a pattern signal for a halftone image, so that it is set to be 1/3 the frequency of the TXCLK signal in order to increase the gradation. These divided clocks of 50% duty are input to a pattern generation circuit A9 and a pattern generation circuit B10 to be compared with an analog video signal.
W1 and SAW2 are generated. In the circuit shown in FIG. 2, the pattern signal is a triangular wave, so that both of the pattern generating circuits A and B are constituted by integrating circuits composed of resistors and capacitors. The bias components of the triangular wave signals SAW1 and SAW2 are each connected to a variable resistor 11
The voltage is adjusted by the variable resistor 12 and is input to the other input terminals of the comparators 5 and 6 to be compared with an analog video signal to be two systems of pulse width modulation signals PWMA and PWMB. Here, in order to obtain high gradation, it is desirable that the level of the analog video signal and the level of the triangular wave have a relationship as shown in FIG. Therefore, in order to satisfy such a relationship, the bias of the triangular wave is adjusted by the variable resistors 11 and 12 of the pattern generation circuits A and B, and output from the D / A converter 2 by the variable resistors 3 and 4. The amplitude of the analog video signal is adjusted. FIGS. 4 (a) and 4 (b) show examples of triangular waves SAW2 and SAW1 with different periods and adjusted optimally. The two-system video signals PWMA and PWMB whose pulse widths have been converted by the triangular waves SAW1 and SAW2 are used as switching signals 15 according to the image tone (characteristics or characteristics of the image) on the original from a reader unit (not shown). Is selected and output. In other words, in the case of the halftone area, an "L" signal is output from the reader unit as the switching signal 15, and the pulse width modulation signal PWMA is a laser drive signal (not shown). Conversely, in the line drawing area on the document, an "H" signal is output as the switching signal 15 from the reader unit, and the pulse width modulation signal PWHB becomes the laser driving signal. By selecting the optimum pulse width modulation signal according to the image tone of the original in this manner, a high quality image can be provided. However, as described with reference to the circuit of FIG. 2, when two clock signals, that is, TXCLK and PHCLK, are received in parallel from a reader unit (not shown) through an interface cable (not shown), the clock signal is very high in the cable. Because of the high-speed signal, crosstalk is generated and interferes with each other at the clock frequency. As a result, as shown in FIGS. Disadvantageously occurs. The effect of such a waveform on image output will be described with reference to FIG. FIG. 6A shows an example of an ideal triangular wave and a pulse width modulation signal. The value of the output level of the D / A converter 2 and the pulse width modulation signal need to be linear. That is, from the input value OOH of the D / A converter 2 (H represents a hexadecimal system) to FFH
For each value up to, the pulse width W must change continuously. However, as shown in FIG.
If the noise is on the triangular wave, the pulse width will be shorter than the ideal pulse width W by the time of a + b,
There is a disadvantage that the pulse width may be reversed with respect to the output value of the D / A converter 2. Such disturbance of the pulse width causes fatal deterioration of image quality such as reversal of density in a delicate area with respect to output of a halftone image, image roughness, and false contour. The present invention has been made in view of the above points, and provides an image processing apparatus capable of suppressing generation of noise in a pattern signal and obtaining a high-quality image. [Means for Solving the Problems] In order to achieve the above object, an image processing apparatus of the present invention comprises:
Input means for inputting a digital image signal represented by a plurality of bits for each pixel, receiving means for receiving a reference clock signal of a predetermined cycle, and a digital image signal input by the input means, having a predetermined cycle, A pulse width modulation signal for selectively outputting a pulse width modulation signal modulated according to a first pattern signal having an extreme value in one cycle or a second pattern signal having a cycle different from the first pattern signal Generating means, wherein the pulse width modulation signal generating means includes a first pattern signal generating circuit for generating the first pattern signal, and a second pattern signal generating circuit for generating the second pattern signal Wherein the first and second pattern signal generation circuits generate the first and second pattern signals based on a reference clock signal of a predetermined cycle received by the receiving means. Raw Then and said. Embodiment An embodiment of the present invention will be described below in detail with reference to FIG. In FIG. 1, components having the same functions as those in FIG. 2 are denoted by the same reference numerals. 1 is a latch circuit for latching a video signal in synchronization with a video clock sent from a reader unit (not shown), 2 is a D / A converter for converting a digital image signal into an analog image signal, and 3 is an analog output for line drawing. A variable resistor for adjusting the level, 4 is a variable resistor for adjusting the analog output level for halftone, 5 is a comparator for comparing the analog signal for halftone and the triangular wave signal SAW1, and outputting a pulse width modulation signal, 6
Is a line drawing comparator for inputting a triangular wave signal SAW2, and 8 is a reference clock (hereinafter referred to as a screen clock) for generating a pattern signal having a predetermined period synchronized with a video clock transmitted from a reader (not shown). A 1/2 frequency divider that divides the frequency by 2 to form a clock signal TXCLK2 with a duty ratio of 50%, and 7 is a clock that is also frequency-divided by a 1/3 frequency divider 13 to generate a halftone pattern signal. More 1/2
1/2 divider 9 for generating a clock signal PHCLK2 having a duty ratio of 50% by dividing the frequency of the clock signal into a halftone pattern generating circuit A for generating a halftone triangular wave SAW1 having three pixels as one cycle. A line drawing pattern generator B, which generates a line drawing triangular wave SAW2 with one cycle of pixels, a variable resistor 11 that adjusts the DC bias component of the halftone triangular wave signal, and 12 also adjusts the bias component of the line drawing triangular wave signal. A variable resistor 13 for dividing the screen clock into a 1/3 frequency divider for generating a triangular wave signal for halftone, and a line drawing 14 sent from a reader (not shown). And a switching circuit for selecting the pulse width modulation signals PWMA and PWMB based on the switching signal 15 specifying the halftone area. As described above, the pattern generation circuit is formed by an integrator using a resistor and a capacitor. Next, the operation in the above configuration will be described step by step. Those having the same numbers as in FIG. 2 perform the same operations. That is, a screen clock sent from a reader unit (not shown) becomes a pattern generation reference clock TXCLK2 for a 50% duty line drawing by the 1/2 frequency divider 8. Further, in order to generate a pattern for the halftone, the frequency of the screen clock is converted into a frequency of 1/3 using the 1/3 frequency divider 13, and the clock output from the 1/3 frequency divider 13 is further reduced by one. A half frequency dividing circuit 7 divides the frequency by 1/2 to form a halftone reference clock PHCLK2 having a duty ratio of 50%. As described below with reference to the circuit of FIG. 2, the digital image signal transmitted from the reader is D / A
The analog image signal is converted into an analog image signal by a converter 2, and the analog image signal is compared with two triangular waves having different periods by comparators 5 and 6, and a pulse width modulation signal B for line drawing
Is converted into a halftone pulse width modulation signal A. These pulse width modulation signals are selected and output by a switching signal 15 corresponding to the image tone on the document from a reader unit (not shown). That is, in the case of the halftone area, the switching signal is sent from the reader unit.
As “15”, an “L” signal is output, and the pulse width modulation signal PWMA becomes a laser drive signal (not shown). Conversely, in the line drawing area on the original, an "H" signal is output from the reader unit as the switching signal 15, and the pulse width modulation signal PWMB is a laser driving signal. The laser beam is turned on / off based on the pulse width modulation signal, and an image is formed on a photoconductor (not shown). In this way, the pattern signal generation reference clock from the reader unit is reduced to one, and two pattern signal generation reference clocks are generated in the image forming apparatus, as shown in FIGS. 5 (a) and 5 (b). Since the noise component of the pattern signal under the influence of the crosstalk in the interface cable can be removed, it is possible to provide a high quality image. Although a triangular wave signal is used as a pattern signal in this embodiment, a pattern signal such as a trapezoidal wave may be used. As described above, the image forming apparatus receives the area switching signal from the reader unit and the only reference generation clock, and generates a line drawing pattern signal having a duty ratio of 50%, and
This is to generate a halftone pattern signal. Therefore, the two reference clocks for generating a pattern signal can contribute to the generation of the pattern signal without interfering with each other. This makes it possible to remove noise components generated in the pattern signal, and a continuous pulse width W can be obtained for each value from the digital value OOH ₁ to FFH which is the input of the D / A converter. Was. In addition, it has become possible to obtain high quality images for both halftone images and line drawing images. [Effects] As described above, according to the present invention, a high-quality image can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of an image forming apparatus according to the present embodiment, FIG. 2 is a circuit diagram of an image forming apparatus that outputs a pulse width modulation signal, and FIG. FIGS. 4 (a) and 4 (b) show the relationship between the analog video signal obtained by the present embodiment and the levels of triangular waves having different periods, and FIGS. 5 (a) and 5 (a). FIGS. 6A and 6B are diagrams showing an example of interference between two patterns of pattern signals. FIG. 6A is a diagram showing the relationship between an ideal triangular wave signal and a pulse width modulation signal. (b) is a diagram showing that interference between two pattern signals affects a pulse width modulation signal. 1 is a latch circuit, 2 is a D / A converter, 3 is a variable resistor for adjusting the analog signal amplitude for line drawing, 4 is a variable resistor for adjusting the analog signal amplitude for half tone, 5 is a comparator for half tone, 6 is a comparator for line drawing, 7 , 8 is a 1/2 frequency divider, 9 is a halftone pattern generating circuit, 10 is a line drawing pattern generating circuit, 13 is a 1/3 frequency dividing circuit, and 14 is a line drawing and halftone switching circuit.

Claims (1)

(57) [Claims] Input means for inputting a digital image signal represented by a plurality of bits for each pixel; receiving means for receiving a reference clock signal having a predetermined period; and a digital image signal input by the input means.
A first pattern signal having a predetermined period and having an extreme value within one period or a pulse width modulation signal modulated in accordance with a second pattern signal having a period different from the first pattern signal is selectively provided. And a pulse width modulation signal generating means for outputting the first pattern signal, and a pulse width modulation signal generating means for generating the first pattern signal, and a pulse width modulation signal generating circuit for generating the second pattern signal. And the first and second pattern signal generation circuits generate the first and second pattern signals based on a reference clock signal of a predetermined cycle received by the receiving means. An image processing apparatus characterized by the above-mentioned.