JP3049754B2 - Image signal processing device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image signal processing apparatus having a function of recording and reproducing a halftone image such as a photograph with high quality.

2. Description of the Related Art In recent years, in an electrophotographic system typified by a laser beam printer, in order to smoothly record and reproduce a halftone image such as a photograph or a halftone photograph, a dither method or an error diffusion method is used. From the pseudo-gradation expression by area gradation or density modulation of binary dots, recording by analog density gradation expression in which an input image signal is pulse-width modulated together with the improvement of the characteristics of the photoconductor is used. (For example, Tanaka, Hoshino: High quality recording of color images by electrophotography, IEICE Transactions, J69-C, 9, 117
9-1184, 1986) FIGS. 7 (a) and 7 (b) show a conventional typical method of pulse width modulating an input image signal in the main scanning direction. GCLK in FIG. 3A is a pixel clock, which represents the processing time of one pixel of the input image signal in one cycle (T) of the clock. SW2 in FIG. 7B is an example in which two pixels constitute one cycle with a triangular wave. IDT is an input image signal whose level changes for each pixel in the main scanning direction. Now switch these image signals to SW
Comparing with the triangular wave threshold value of 2, a pulse width modulation signal PW2 corresponding to the input image signal level is obtained. If the laser beam is turned on when the pulse width modulation signal is "1" and is not turned on when the pulse width modulation signal is "0", a pattern corresponding to the pulse width can be recorded.

As can be seen from FIG. 7, in such a modulation method, the area of the pixel changes in the main scanning direction according to the input image signal level, and connected lines (stripes) are formed in the sub-scanning direction.

Problems to be Solved by the Invention In general, in the electrophotographic method using such a modulation method, the dynamic range of pulse width modulation of an image having a pixel density of 16 dots / mm is narrow and unstable. Therefore, in order to reproduce a stable gradation, the dynamic range of the pulse width is increased by expanding the triangular wave by two pixels as shown in FIG. However, even in this case, a small image signal level in the highlight region cannot be recorded due to the characteristics of the photoreceptor, resulting in a discontinuous tone reproduction image. In order to solve such a problem, there is a method of further expanding the dynamic range to three to four pixels. In this case, the resolution is reduced.

The present invention solves the above problem of the pulse width modulation method by using a threshold signal having a long time constant to perform pulse width modulation at the time of a highlight image signal level, thereby reproducing a highlight region and producing a continuous smooth output image. It is an object of the present invention to provide an image signal processing device that enables the image signal processing.

Means for Solving the Problems The present invention determines a highlight signal level of an input image signal, generates a threshold signal having a different time constant from the determination result, and compares the threshold signal with the input image signal level. The above-mentioned object is achieved by using a means for outputting a pulse width modulation signal.

Function The present invention has the above-described configuration, and generates a threshold signal having a long time constant in accordance with the image signal level of highlight, and performs pulse width modulation by the threshold signal to improve the gradation characteristics of the highlight region, thereby improving the continuous tone. Thus, a smooth output image can be obtained.

Embodiment FIG. 1 is a block diagram of an image signal processing apparatus according to an embodiment of the present invention, and FIG. 2 is a timing chart of the main part thereof.

In FIG. 1, reference numeral 101 denotes a gradation correction table, which is composed of a ROM or a RAM. 102 is a digital-analog converter,
104 is an image signal level determiner, 105 is a CR integrator, 106 is a triangular wave generator, 107 is a waveform converter, 108 and 109 are selectors, 1
10 is a comparator.

In this embodiment, the dynamic range of the pulse width modulation is set to two pixels.

 Hereinafter, the operation will be described in detail with reference to FIG.

The input image signal IDT 901 in the main scanning direction in FIG. 2C synchronized with the pixel clock is input to a gradation correction table 101 to correct the gradation, and then converted into an analog signal by a digital-analog converter 102. . The converted analog signal is input to a comparator 110, compared with a threshold signal 909, which is an output signal from a selector 109 described later, and outputs a pulse width signal 910 corresponding to the analog image signal level.

Next, in order to determine a highlight image signal level area, and to obtain a threshold signal corresponding to the highlight image signal in an integrator 105 described later, a digital-analog converter is used.
The output of 102 is input to an input image signal level determiner 104, and a signal obtained by comparing several types of preset highlight image signal levels as threshold values is input to an integrator 105 and a selector 108 described later. For example, when the output of the input image signal level determiner 104 is in the highlight image signal level region, the selector 108 selects the threshold signal SW1 908 in FIG. 2 (h), and the output of the determiner 104 is highlighted. If it is not in the image signal level area, 906 in FIG. 2 (e) which is the threshold signal SW2 is selected.

On the other hand, threshold signals SW1 and SW2 for obtaining a pulse width signal
Is obtained as follows.

The pixel clock GCLK of 902 shown in FIG. 2 (a) is input to, for example, a JK flip-flop to generate a reference signal 904 (FIG. 2 (d)) for two pixels in one cycle. The reference signal 904 is, for example, C as shown in FIG.
₁ , a CR composed of R _{1 to} R ₃ and an integrator 1 composed of an operational amplifier 31
05, and a signal 905 in which CR is selected and integrated according to the signal range of the image signal level determining unit 104 (FIG. 2 (f))
The reference signal 904 shown in FIG. 2 (d) is input to the triangular wave generator 106, and outputs a conventional triangular wave signal SW2 906. This triangular wave signal SW2 is used to obtain a pulse width modulation signal when the image signal is not an image signal in a highlight area.
In order to convert the signal of 905, which is the integration output of 105, to the waveform,
Input to waveform converter 107. The waveform converter 107 is configured by operational amplifiers 41 and 42 as shown in FIG. 2 (g) is output. The signal 907 is input to the selector 108, and the signal of one pixel of the signal 905 and the signal of the second pixel of the signal 908 are synthesized by switching the signal of one input 905 with the pixel clock GCLK and 902. 908 is obtained.

FIG. 5 shows another method of obtaining the signal SW1, which comprises an operational amplifier, R, C, and a diode and rectifies a sine wave. In this case, the threshold signal is fixed because it cannot be changed by the time constant.

Thus, the signal 910 outputs a pulse width modulation signal according to the input level according to the threshold signal of 909. FIG. 6 is a diagram showing a state of pulse width modulation by a threshold value in this embodiment. With respect to the input image signal IDT, different pulse width signals can be obtained by threshold signals having different time constants.

As described above, the present invention improves the tone characteristics of the highlight area of the pulse width modulation system in a copying machine such as a laser beam printer. By performing pulse width modulation with a threshold signal having a long time constant from the triangular wave threshold, the lighting time of the laser beam can be made longer, and the gradation characteristics in the highlight region can be improved.

[Brief description of the drawings]

FIG. 1 is a block connection diagram for realizing pulse width modulation of an image signal processing apparatus according to one embodiment of the present invention, FIG. 2 is a timing chart of a main part of the configuration of FIG. 1, and FIG. FIG. 4 is a block connection diagram of the same waveform conversion circuit, FIG. 5 is a block connection diagram of the same threshold signal generation circuit, and FIG. 6 is the same input image. FIG. 7 is a relationship diagram of a pulse width modulation signal based on a threshold signal of a signal,
FIG. 1 is a diagram showing a relation of conventional pulse width modulation. 101: gradation correction table, 102: digital / analog converter, 103: JK flip-flop, 104: input image signal level determiner, 105: integrator, 106: triangular wave corrector, 107 ... Waveform converter, 108, 109 ... Selector, 1
10 ... Comparator.

Claims (1)

(57) [Claims] An input unit for inputting an input image signal; a determination unit for determining a highlight signal level of the input image signal; and a threshold generation unit for generating a threshold signal having a different time constant from the determination result of the determination unit. And an output means for comparing the threshold signal with the input image signal level and outputting a pulse width modulation signal.