JPS6365769A - Picture processor - Google Patents

Abstract

PURPOSE:To obtain a picture output with high gradation by converting once a digital picture signal into an analog picture signal and comparing the signal with a triangle wave signal of a prescribed period thereby applying nearly continuous pulse width modulation. CONSTITUTION:A digital picture data outputted from a digital data output device is converted into an analog signal at each picture element by a D/A converter 1 and one picture element is inputted sequentially to one terminal of a comparator circuit 3. Simultaneously, a triangle wave analog reference pattern signal is generated at a period corresponding to a desired pitch of a halftone screen from the signal generator 2 and inputted to the other terminal of the comparator circuit 3. The signal is counted down in synchronism with a BD signal generated at each line from a horizontal syncrhonizing signal generating circuit 23 and used for latch timing. A pulse width modulation signal Pw is outputted from the comparator circuit 3 and inputted to the laser modulation circuit. As a result, the laser beam is turned on/off in response to the pulse width and the halftone picture is formed on a recording medium 16.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to an image processing device for obtaining high-quality reproduced images.

[Prior art]

Nowadays, laser beam printers are popular due to their high image quality and high speed. FIG. 3 is a diagram showing the basic configuration of a conventional laser beam printer. In the figure, bit 0N
The image input signal Vin expressed at the 10FF level is amplified by the laser driver circuit lo and converted into an 0N10FF level signal LDR for controlling the lighting of the laser unit 11. In this example, the laser unit 11 is a semiconductor laser. The light emitted from the semiconductor laser is transmitted through a collimator lens 12, a cylindrical lens 13, and an imaging lens 1.
4 and a mirror 15, an image is formed so that the spot diameter is constant in any axial direction on the surface of the drum 16. On the other hand, a scanner motor 18 driven by a scanner driver circuit 17 rotates a decahedral mirror 19 to scan the laser beam in the direction of the drum axis. In this way, an electrostatic latent image is formed on the surface of the photosensitive drum in accordance with the image input signal. Then, in the subsequent development process, toner (black) is deposited on this area, thus making the electrostatic latent image visible. In this case, the amount of change in drum surface potential correlates with the irradiation time and beam intensity (power) of the laser beam, but in the laser beam printer mentioned above, the irradiation width per dot is constant and the beam intensity is constant, so it has a binary value. This means that only black and white images can be played back. Therefore, when reproducing continuous density images such as photographs, the so-called dithering method is used to express halftones, but in any case, since it is a collection of black and white dots, it is sufficient as a continuous density expression method. I can't say that.

〔the purpose〕

The aim of the invention is to eliminate the above-mentioned drawbacks.

Another object of the present invention is to improve an image processing device.

A further object of the present invention is to provide an image processing device that can obtain high-quality reproduced images.

Another object of the present invention is to provide an image processing device that can obtain excellent reproduced images with a simple device configuration.

A further object of the present invention is to provide an image processing device that can reproduce high-quality images at high speed.

A further object of the present invention is to provide an image processing device capable of obtaining high-quality reproduced images with high resolution and good gradation.

Another object of the present invention is to provide an image processing device that can easily reproduce the density of an original image faithfully, and that can also easily perform density corrections caused by differences between models and devices. It is about providing.

Another object of the present invention is to provide an image processing device that is capable of not only faithfully reproducing the density of a document, but also easily emphasizing a certain density or weakening a certain density. be.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

As a method for improving the drawbacks of halftone expression such as the dither method described above, it is possible to use, for example, a pulse width modulator to express laser light as a continuous time difference in pulse width depending on the density. FIG. 4(a) shows a block diagram of a possible pulse width modulator. Here, image density data V
Do-VD5 is expressed in 6-bit parallel, and as shown in Figure (b), image density is determined by a total of 64 bit combinations from 0 to 63, for example, "0" - white density, "
63"=expressed as black density. This image data is D
/A (digital/analog) converter 1 converts to a corresponding DC level, and the converted levels are white = OV and black = 5V. The converted DC level signal DA is input to one input terminal of the comparator (CMP) 3, and the other input terminal of the comparator 3 is input to the signal generator (CMP).
A triangular wave signal S A W generated by SG) 2 is input. In this way, the levels of the triangular wave signal SAW and the image signal DA are compared by the comparator 3, and FIGS.
) is converted into a pulse width modulated signal Pw as shown in FIG. That is, when the image signal is close to the white level, the pulse width of the signal P.multidot.,v becomes wide, and the laser diode LD is turned on for a relatively long time. Also, when the black level is close to the black level, the pulse width becomes narrower, and the laser diode LD
lights up. By doing this, the bit parallel data is converted into an image signal Pw whose pulse width changes continuously, so that the expression of gradation becomes smoother and the output characteristics of photographs and the like are improved.

However, the image processing apparatus shown in FIG. 4 does not sufficiently take into account various actual influences on the gradation of reproduced images. For example, if the gradation density characteristics of input image data are not linear to begin with, this must be combined with complex correction in the pulse width modulation system, which makes the device structure complicated and expensive. There is a risk of becoming Even if the gradation density characteristics of the input image data are linear, if there are variations in the sensitivity characteristics of the photosensitive drum, variations in the developer concentration or conditions during development, variations in the conditions when transferring to paper, etc. There is a risk that the desired reproduced image will not be obtained when the output is output, but it is extremely difficult to correct such characteristics in conjunction with the pulse width modulation system. We are trying to solve the problem by putting them together.

In this embodiment, the various corrections described above are performed with a simple configuration.

FIG. 1 is a block diagram showing the density conversion section of the image processing apparatus of this embodiment. Note that the same components as in FIG. 4(a) are given the same numbers and their explanations are omitted.

In the figure, LD is a laser diode, which is driven by a switching transistor Q with 0N10FF. Furthermore, the switching transistor Q is operated by a pulse width modulation signal Pw. The peak current value IA flowing through the laser diode LD can be determined by the constant current source circuit 4, and the constant current value II can be determined by setting the voltage level signal Vc to the control terminal of the constant current source circuit 4.
! can be freely controlled. FIG. 2(a) shows an example of the input/output Vc-Ij7 characteristics. Here, when the level signal Vc is high, a relatively large current is passed through the laser diode LD to increase the beam power, and when the level signal Vc is low, a relatively small current is passed through the laser diode LD to reduce the beam power. In a relationship. Thus, the pulse width 1-I at the base of switching transistor Q
When the I level is applied, the peak current value II! as shown in FIG. 1(b) is reached. , a drive current waveform with a pulse width Pwj2 is obtained. 6 is a D/A converter, and ROM 5
An analog level signal V proportional to the read signal R8-R5 of
Output C. 5 is a ROM (memory) from which predetermined conversion outputs R8-R5 are read out using the input bit parallel image signals VDo-VD5 as addresses.

Moreover, this conversion can take various forms depending on the purpose. Examples of the conversion table are shown in FIGS. 2(b) and 2(c). Table A is input data VDo-
This is a case where VD5 is output as is, and since there is a relationship in which a strong peak current Ij7 flows for image data with a black density, white on a black background is represented as good (Table A' is the opposite). Table B is an example in which the linearity of the conversion is poor, and the white output is strengthened for intermediate density data.Table C is the opposite.Of course, the above relationship applies to black It depends on whether you develop with toner (+) or white toner (-).
It can be freely determined depending on the relationship with various configurations of the image forming apparatus. Furthermore, in table D, the laser power 11 is constant for image data VDo-VD5. This corresponds to the case where no density correction is performed in the apparatus of this embodiment. Similarly, in the case of table E, the laser power 1j7 becomes large at intermediate density data, and table F
The opposite is true for .

FIG. 2(d) shows the beam lighting pulse width PwI! The relationship between Vd and the surface potential Vd on the photosensitive drum is shown. The potential of a portion of the photosensitive drum where the laser beam is irradiated for a long time per certain area is relatively low, and if the beam power is constant, it is approximately inversely proportional to the irradiation pulse width. However, since this surface potential Vd is a function of the beam power as well as the beam lighting time width, it is easily possible for one to correct the nonlinearity of the other, or forcibly cause nonlinearity. become.

In the above-described embodiment, a case was described in which the reproducibility of image density was improved, but conversely, it is also possible to use the method as active density control to emphasize or weaken a certain density. Of course, when achieving such multiple purposes, ROM
5, a plurality of conversion tables may be prepared and the selection means may be used to switch between them.

Further, although a triangular wave signal is used for pulse width modulation in this embodiment, a sawtooth wave, a sine wave, a trapezoidal wave, etc. may also be used.

The image processing apparatus shown in FIG. 1 can also be applied to the laser beam printer shown in FIG. An image is formed thereon.

The printer shown in Fig. 3 is of the type that scans the beam line by line to form an image, but the scanning position of the beam must be detected for each line (a 13D mirror is used as the beam detection means).
0 and fiber 21 are provided.

The beam direct signal (BD double signal) output from this beam detection means serves as a signal for defining the recording start position (left margin) of an image on the photosensitive drum 16.

By the way, the signal generator (SG) 2 in FIG. 1 generates a triangular wave S A W for each scanning line, but if this triangular wave S A W is output in synchronization with the BD double signal, a reproduced image of even better quality can be obtained. This is something that can be obtained.

Below, the relationship between the triangular wave S A "vV and the BD double signal will be explained in more detail using Figures 5 and 6. Figure 5 shows a part of the apparatus shown in Figure 1 in more detail. Components with the same function are given the same reference numerals.

In the figure, 22 is a digital data output device, which A/D (analog/digital) converts analog image data from a CCD sensor or video camera (not shown), and outputs a digital video signal VDo-VD5 of predetermined bits having density information. Output. This digital video signal may be temporarily stored in a memory, or may be input from an external device via communication or the like. The digital video signal output from the digital data output device 22 is D
/A converter l and is also input to ROM5.

The digital image data output from the digital data output device is converted into an analog signal for each pixel by a digital crew analog converter (D/A converter) 1.
The two picture elements are sequentially input to one terminal of the comparator circuit 3.

At the same time, a triangular wave analog reference pattern signal is generated from the signal generator (SG) 2 at a period corresponding to the desired pitch of the halftone screen and inputted to the other terminal of the comparator circuit 3. Further, the horizontal synchronizing signal generating circuit 23 has the above-mentioned beam detecting means, and synchronizes with the BD double signal generated for each line from the horizontal synchronizing signal generating circuit 23, and outputs a reference clock ( mas
terclock) is the timing signal generation circuit 2
5 counts down to a quarter period, for example,
It is used for digital video signal transfer lock and D/A converter 1 latch timing. The comparator circuit 3 compares the level of the analog video signal converted to analog with the level of the triangular wave pattern signal, and outputs a pulse width modulation signal Pw. This pulse width modulation signal Pw is then input to a laser modulation circuit consisting of a transistor Q and the like shown in FIG. As a result, the laser beam is turned on and off according to the pulse width, and a halftone image is formed on the recording medium 16.

FIG. 6 is a diagram for explaining signal waveforms of each part of the apparatus shown in FIG. FIG. 6(a) shows the reference clock signal of the oscillator 24, and FIG. 6(b) shows the BD multiplied signal described above.

Further, FIG. 6(C) shows a pixel clock obtained by counting down the reference clock of the oscillator 24 by the timing signal generation circuit 25. In other words, the pixel clock in FIG. 6(c) is a signal obtained by synchronizing the BD double signal and counting down the reference clock to one-fourth period by the timing signal generation circuit 25, and is input to the D/A converter 1 and converted into a digital video signal. Used as a transfer lock. Figure 6 (d
) shows a pattern signal synchronization clock (screen clock) with a period of once every three image clocks obtained by synchronizing the BD double signal and counting down the reference tark to one-twelfth period by the timing signal generation circuit 25. That is, the screen clock shown in FIG. 6(d) is used as a synchronizing signal for triangular wave generation, and is input to the signal generator (SG) 2.

Further, FIG. 6(e) shows a digital video signal (code data), which is output from the digital data output device 22. FIG. 6(f) shows the D/A converter 1
This shows a converted analog video signal, and as shown in the figure, each pixel data at an analog level is output in synchronization with the pixel clock.

On the other hand, the output of the signal generator (SG) 2 is generated in synchronization with the screen clock of FIG. 6(d), as shown by the solid line in FIG. 6(g), and is input to the comparator circuit 2.

The broken line in FIG. 6(g) is the analogized image data (analog video signal) in FIG. 6(f), and this analog video signal is converted into a triangular wave from the signal generator (SG) 2 by the comparator circuit 3. (pattern signal) and the sixth
The signal is converted into a pulse width modulated signal Pw as shown in Figure (h).

In this way, in this embodiment, by first converting the digital image signal into an analog image signal and then comparing it with a triangular wave signal of a predetermined period, almost continuous pulse width modulation becomes possible, and a high-gradation image output can be obtained. It is something.

Further, according to this embodiment, a reference clock having a frequency higher than the frequency of the pattern signal synchronization clock for generating a pattern signal (for example, a triangular wave) is used to generate a horizontal synchronization signal (a pattern signal synchronization clock (screen clock) synchronized with the BD double signal). ), the fluctuation (jitter) of the pattern signal generated from the pattern signal generation circuit 3, for example 1
In this embodiment, the deviation (offset) between the pattern signals on the second line and the second line is less than 1/12 of the period of the pattern signal. This precision is required to ensure high-quality halftone reproduction in which line screens are formed evenly and smoothly for each line.

Therefore, since the gradation information is accurately pulse width modulated using a pattern signal with little fluctuation, a high-quality reproduced image can be obtained.

〔effect〕

As explained above, according to the present invention, high-quality reproduced images can be obtained.

[Brief explanation of the drawing]

FIG. 1(a) is a block configuration diagram showing the density conversion section of the image processing apparatus of this embodiment, FIG. 1(b) is a timing chart showing an example of the driving current waveform of the laser diode LD, and FIG. ) is a diagram showing the relationship between the current control voltage Vc and the constant current 11 in the constant current source 4, and FIGS. 2(d) is a diagram showing the relationship between the beam lighting pulse width PwI and the potential Vd on the photosensitive drum, and FIG. 3 is a diagram showing the basic configuration of a conventional laser beam printer. Figure 4(a) is a block configuration diagram of a pulse width modulator that can be used in a laser beam printer, and Figure 4(b) is image density data VDo to VD5 and image density signals obtained by D/A conversion. 4(C) and 4(d) are timing charts showing pulse width modulation by a triangular wave signal; FIG. 5 is a diagram showing a part of FIG. 1 in detail; FIG. 6 is a waveform diagram of each part of FIG. 5. Here, l is a D/A converter, 2 is a signal generator, 3 is a comparator, 4 is a constant current source circuit, 5 is a ROM, 6 is a D/A converter, 10 is a laser driver circuit, 11 is a laser unit, 12 is a collimator lens. (b) P'dl (d) ' vc. (b) Second haze (d) Figure 3 No. 1 (b) σ VDo, VDs −= 63 ■Mark (d)

Claims (1)

[Claims] A recording device that modulates and scans a beam in accordance with an image signal to form an image on a recording medium, comprising: means for generating a position signal indicating a scanning position of the beam; and an image signal generating means for generating the image signal. pulse width modulation signal generation means for outputting a pulse width modulation signal for modulating the beam according to the image signal generated by the image signal generation means; and controlling the intensity of the beam according to the image signal generated from the image signal generation means. and a control signal generating means for generating a control signal for the predetermined pattern signal, the pulse width modulation signal generating means processing the image signal based on a predetermined pattern signal and outputting a pulse width modulation signal. An image processing apparatus characterized in that the image processing apparatus is configured to generate the following based on the position signal.