JPH06278320A - Electrophotographic apparatus - Google Patents

Abstract

PURPOSE:To realize an electrophotographic apparatus performing edge smoothing (ESTP) to enhance image quality and changing the pulse width of a laser driving pulse by a pulse width modulating system to perform gradation expression. CONSTITUTION:An electrophotographic apparatus is equipped with a D/A conversion part, 1 for converting image data consisting of a plurality of bits to an analogue image signal, a saw-tooth wave generating part 2 generating a saw- tooth wave, a comparison circuit part 5 comparing the image signal with a saw-tooth wave voltage level and a selection part 6 selecting two signals (PWM signals 1, 2) formed by comparison and mutually reversed in logic on the basis of a selection signal. A laser driving pulse is formed so that the degree of a dot added or eliminated from the left end of one pixel region to the right end thereof or from the right end to the left end is changed by the value of image data and the logic of the selection signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic apparatus which expresses gradation of an image by varying the pulse width of a laser driving pulse according to the data value of image data consisting of a plurality of bits. .

[0002]

2. Description of the Related Art In recent years, electrophotographic devices typified by laser beam printers have been increasing in value added in terms of functions such as colorization and high image quality. Especially regarding colorization,
Expectations are high as a growth market in the future, and research and development of color electrophotographic devices are also active. Color gradation expression technology is one of the notable technologies of the color electrophotographic apparatus. As one example thereof, there is a method of varying the pulse width of a laser drive pulse by a pulse width modulation method to perform gradation expression.

A laser drive pulse forming circuit of a conventional electrophotographic apparatus will be described below with reference to FIGS. 4 to 7. 4 is a block diagram showing a configuration of a laser drive pulse forming circuit of a conventional electrophotographic apparatus, FIG. 5 is a waveform diagram showing a laser drive pulse forming process in FIG. 4, and FIG.
(a) and (b) are ESPs obtained by performing edge smoothing processing (hereinafter referred to as ESP processing) described below on the original image.
FIGS. 7A and 7B are diagrams showing an example of the processed image, and FIGS. 7A and 7B are diagrams showing the image obtained when the ESP processing is performed by the conventional laser drive pulse forming circuit.

As shown in FIG. 4, the laser drive pulse forming circuit of the electrophotographic apparatus is composed of a D / A converting section 1, a sawtooth wave generating section 2, a comparing circuit section 3 and a laser diode 4. ing. The D / A converter 1 converts image data composed of a plurality of bits into an analog level and outputs it as an image signal. A clock signal is input to the D / A converter 1, for example, image data is latched at the rising edge of the clock signal, the latched image data is converted into an analog value, and the image signal is output. Generally, the number of gradations of an image that can be expressed is determined by the number of bits of image data. For example, when the image data is 8 bits, it is possible to represent 256 gradation numbers.

In the sawtooth wave generator 2, the voltage is once lowered to the minimum value in synchronization with the rising edge of the clock signal,
After that, a sawtooth wave whose voltage gradually increases is generated. The comparison circuit section 3 compares the image signal and the voltage level of the sawtooth wave, and outputs a PWM (pulse width modulation) signal whose output changes at a binary level of high or low depending on the magnitude relationship. Then, this PWM signal becomes a laser drive pulse. The laser diode 4 is a main element of image formation, and is turned on / off according to a laser drive pulse. When the laser diode 4 is turned on, a laser is emitted to form an image. For example, when the laser drive pulse is high, the laser diode 4 turns on and emits light, and when the laser drive pulse is low, the laser diode 4 turns off. A drive circuit for driving the laser diode 4 is not shown in FIG.

FIG. 5 is a waveform diagram showing the laser drive pulse forming process in the laser drive pulse forming circuit configured as described above. Compare the voltage level of the image signal and the sawtooth wave, when the voltage of the image signal is higher than the voltage of the sawtooth wave, the laser drive pulse becomes high, when the voltage of the image signal is lower than the voltage of the sawtooth wave, The laser drive pulse goes low. When the laser drive pulse is high, the laser diode 4 is turned on and emits light, and dots (images) are formed during that period.
When the laser driving pulse is low, the laser diode 4 is turned off. For example, when forming dots completely within one pixel,
The laser drive pulse is turned on 100% within that pixel, and when forming a dot of 50% area within one pixel, the onduty of the laser drive pulse is set to 50% depending on the image data so that the onduty of the laser drive pulse is 50%. Control. Further, the dots grow from the left end to the right end within one pixel according to the on-duty of the laser drive pulse. The growth direction of the dots depends on the comparison logic of the sawtooth wave and the comparison circuit unit 3. If a sawtooth wave is used in which the voltage once rises to the maximum value and then gradually drops in synchronization with the rising edge of the clock signal, and the comparison logic in the comparison circuit unit 3 is the same as above, Within one pixel, dots grow from the right end to the left end according to the on-duty of the laser drive pulse.

In the case of a color electrophotographic apparatus, it is possible to form a color image by appropriately combining and combining the three color levels of yellow, magenta and cyan which are color components. When forming a black image, it is common to form a color image using four color toners including black for reasons such as securing absolute black density and increasing contrast. By changing the on-duty of the laser drive pulse within one pixel and changing the dot area of each color to an appropriate level and combining, the hue and gradation can be expressed by the area effect for human vision. This is a basic concept of a method of driving a laser by a pulse width modulation method in an electrophotographic apparatus.

Although not directly related to the gradation expression technique by the pulse width modulation method described above, one of the other remarkable techniques of the recent electrophotographic apparatus is the edge smoothing process (ESP process). . The fonts installed in the electrophotographic device are bitmap fonts (the actual font image is stored as a dot image in the memory as it is, and the microprocessor etc. develops it in the image memory as it is) to outline fonts (each stored in the memory). Microprocessors read vector information and other information necessary to form fonts and perform calculations to develop actual dot images on the image memory.) Let's do it. When the characters are enlarged, the outline font eliminates the so-called jagged feeling associated with bitmap fonts, giving the human eye an impression of good image quality. Therefore,
Further, as a technique for impressing the good image quality, a technique has been devised that not only the characters but also various image patterns such as a line pattern are subjected to a smoothing process on an edge portion to impress the good image quality to the human eye. This is the ESP process. Since a detailed description of the ESP processing is outside the scope of the present invention, the description thereof will be omitted here.
The basic concept of the P process will be briefly described with reference to FIGS. 6 (a) and 6 (b). An image is formed by collecting tens of thousands of pixels one by one like a checkerboard, and deciding whether or not to form dots for each pixel to form a desired image. . The size of one pixel is a factor such as the above-mentioned clock signal in the main scanning direction, the number of faces and the number of revolutions of the polygon mirror (the description of which is omitted in the present invention) for scanning the laser beam in the main scanning direction, and the laser power. The sub-scanning direction is determined by factors such as the process speed of the electrophotographic apparatus. In order to simplify and understand the description, the dot change process in the sub-scanning direction is ignored, and the dot change process is described and illustrated only in the main scanning direction. Figure 6
In (a) and (b), a part of the entire image is extracted and shown. For example, in the case of a line pattern such as a straight line, an edge portion appears as shown in FIG. 6A due to the inclination of the line, which gives a strong impression to the human eye in the output image, and it is as if the image quality is poor. It can make an impression. When this edge portion is smoothed by the ESP processing, for example, it becomes as shown in FIG. That is, dots are added to or deleted from the target pixel of the edge portion. In the example of FIG. 6B, if one pixel is considered to be one dot, 1/3 dot is added or deleted to the four pixels in the central portion. The degree of addition and deletion of dots is often determined by the characteristics of the image forming process of the electrophotographic apparatus and circuit factors. FIG. 6B is an example of ESP processing in which one pixel is divided into three in the main scanning direction and 1/3 dot and 2/3 dot can be added and deleted. This dot addition / deletion can be performed by driving the laser with timing of dot addition / deletion by a signal having a frequency several times that of the clock signal, for example, and data processing and a circuit for that are required. Become. If too many levels of dot addition / deletion are set, there are problems such as complicated data processing, increased circuit scale, and difficulty in timing itself. .

Now, let us consider a case where the ESP processing is functionally mounted on the electrophotographic apparatus having the laser driving pulse forming circuit shown in FIG. 7A and 7B exemplify the formed image and the laser driving pulse forming process for the original image and the ESP processed image. The waveform is noted with attention to the second line of the image. In the case of the original image of FIG. 7 (a), dots are formed only in the second pixel from the left, so the image signal based on the image data has a higher voltage level than the sawtooth wave in that pixel and the laser drive pulse It goes high and the laser diode 4 is turned on to form dots. Similar to FIG. 6, the image obtained by ESP processing the original image is FIG. 7B, but there is a clear difference from FIG. 6B. Explaining the reason, for the second pixel from the left in the second line, the left side 1
/ 3 dots are deleted from the original image and 1/3 dots on the left side are added to the original image for the third pixel from the left. Therefore, 2/3 is added to the second pixel from the left on the second line.
The on-duty of the laser driving pulse is controlled by making the image data correspond to the third dot and the third pixel to form 1/3 dot. However, as described above, the dots grow from the left end to the right end of the pixel in accordance with the on-duty of the laser drive pulse, so that the E as shown in FIG.
It becomes an SP processed image.

[0010]

However, the above-mentioned conventional electrophotographic apparatus having the ESP processing function has the following problems to be solved. That is, in the ESP-processed image as shown in FIG. 7B, as a result of the ESP processing, a dot discontinuity (non-printing area) as shown in the figure is formed between the ESP-processing target pixels. When an image is actually output, the non-printed area may appear to the human eye as, for example, a line break, and as a result of the ESP processing, the image quality may be deteriorated.

The present invention solves the above-mentioned conventional problems, and an object of the present invention is to provide an electrophotographic apparatus provided with a laser driving pulse forming circuit by a pulse width modulation method capable of realizing ESP processing for effectively improving image quality. It is what

[0012]

In order to achieve the above-mentioned object, the present invention comprises a D / A converter for converting a plurality of bits of image data into an analog image signal, and a main factor for image formation. A sawtooth wave generator for generating a sawtooth wave for forming a laser drive pulse for turning on and off a certain laser diode, an image signal output from the D / A converter, and a voltage level of the sawtooth wave. The comparison circuit unit for comparison and the selection unit for selecting two signals whose logics formed by this comparison are mutually inverted by a selection signal are configured.

[0013]

According to the present invention, with the above configuration, dots can be added to or deleted from the target pixel of the ESP processing from the left end to the right end or from the right end to the left end of the pixel, and the dot can be added depending on the image data. The degree of deletion can be easily and freely changed.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A laser drive pulse forming circuit according to an embodiment of the present invention will be described below with reference to FIGS. 1 is a configuration diagram of a laser drive pulse forming circuit of an electrophotographic apparatus according to an embodiment of the present invention, FIG. 2 is a waveform diagram showing a laser drive pulse forming process in FIG. 1, and FIG.
(a), (b) is the figure which compared the formation image and the waveform diagram showing the laser drive pulse formation process in a present Example. In addition,
In the configuration diagram of the laser drive pulse forming circuit shown in FIG. 1, the same parts as those in the conventional example shown in FIG. 4 will be denoted by the same reference numerals and the description thereof will be omitted.

In FIG. 1, the comparison circuit section 5 compares the image signal and the voltage level of the sawtooth wave, and outputs the PWM signals 1 and PW.
Form the M signal 2. In this embodiment, when the voltage of the image signal is higher than the voltage of the sawtooth wave, the PWM signal 1 is high and the PWM signal 2 is low, and conversely when the voltage of the image signal is lower than the voltage of the sawtooth wave. , The PWM signal 1 is low, and the PWM signal 2 is high. That is, the PWM signal 1
And PWM signal 2 are signals whose logics are opposite to each other. The selection unit 6 selects either the PWM signal 1 or the PWM signal 2 according to the logic of the selection signal to form a laser drive pulse.
In this embodiment, the PWM signal 1 is selected when the selection signal is low, and the PWM signal 2 is selected when the selection signal is high to form the laser drive pulse. The selection signal is sent from the controller unit (not shown in FIG. 1) in synchronization with the image data. The controller section is a section in the electrophotographic apparatus that processes image information sent from a host computer or the like to form image data. The ESP processing is processed by the controller unit based on the formed image data. The processing is pixel information of the target pixel and surrounding pixels (for example, pixels in the main and sub-scanning 3 × 3 areas with respect to the target pixel) (whether or not dots are formed).
Image that decides how to add or delete dots to the target pixel, which part of the pixel is to add or delete dots, and the degree of addition or deletion of dots to the target pixel The data value is determined, the logic of the selection signal is determined based on the processing result, and the selection signal is sent to the laser drive pulse forming circuit side in synchronization with the image data of the pixel. In normal printing without ESP processing, one of two PWM signals is used to form a laser drive pulse. In this embodiment, the PWM signal 1
To use. That is, the selection signal is low in normal printing. The PWM signal 2 is used by a pixel that forms dots from the right end to the left end of the pixel, and the selection signal is high in this pixel.

FIG. 2 shows an example of the laser driving pulse forming process in this embodiment. As a result of comparing the voltage levels of the image signal and the sawtooth wave, the PWM signal 1 and the PWM signal 2 are as shown in FIG. The selection signal is set for each pixel similarly to the image data. In the example of FIG. 2, only two pixels are high, and the PWM signal 2 is selected as the laser driving pulse in that pixel, and the PWM signal 1 is selected in the remaining pixels as low, so that the laser driving pulse of FIG. 2 is formed. It

The formed image and the corresponding laser drive pulse forming process in this embodiment will be described with reference to FIG. Note that the waveforms in FIG. 3 are noted focusing on the second line of the image as in the case of the conventional example. Figure 3 (a)
When the original image of is formed, ESP processing is not performed.
The select signal is low. Therefore, the PWM signal 1 becomes the laser drive pulse as it is. In the second line, dots are formed only in the second pixel from the left, so that the image data is supplied to the D / A conversion unit 1 so that the voltage of the image signal is higher than that of the sawtooth wave in this pixel, and the PWM is applied. Set the signal 1 onduty to 100%.

Next, a laser drive pulse forming process for forming the ESP processed image shown in FIG. 3B will be described. In FIG. 3B, the selection signals are low because the first and fourth pixels from the left of the second line are not the target pixels for ESP processing. Therefore, as in FIG. 3A, the PWM signal 1 becomes the laser drive pulse as it is. Since this pixel does not form a dot, image data is supplied to the D / A conversion unit 1 so that the voltage of the image signal becomes lower than that of the sawtooth wave, and P
The WM signal 1 is set low, and as a result, the laser drive pulse becomes low and dots are not formed. On the other hand, the second and third pixels from the left of the second line are the target pixels for ESP processing.

First, the second pixel from the left of the second line will be described. With respect to this pixel, the 1/3 dot on the left side of the original image is deleted by ESP processing. That is, 2/3 dots are formed from the right edge to the left edge of the pixel. When dots are formed from the right edge to the left edge of the pixel, PWM
Since the signal 2 is selected, the selection signal becomes high. In this pixel, the image data is supplied to the D / A conversion unit 1 so that the image signal has the voltage level shown in the figure, so that the PW
The M signal 2 becomes high in the portion (1) shown in FIG. 3 (b), which turns on the laser driving pulse as shown by the arrow in FIG. 3 (b).

Next, the third pixel from the left of the second line will be described. This pixel has 1/3 dot added to the left side of the original image by ESP processing. Since the dots are formed from the left end to the right end of the pixel, the PWM signal 1 is selected. Therefore, the selection signal is low. In this pixel, the same data as the image data of the second pixel from the left of the second line is given to the D / A conversion unit 1, so that the PWM signal 1 is only in the portion (2) shown in FIG. 3B. Goes high, which turns on the laser drive pulse as indicated by the arrow in FIG. 3 (b). As a result, the laser drive pulse has the waveform shown in FIG. 3B for the second line. Therefore, for the image of the second line, the image shown in FIG. 3B is obtained after the ESP processing.

3rd line 2nd and 3rd ESP from the left
Similarly, for the pixel to be processed, a laser drive pulse that produces the image shown in FIG. 3B is formed.
By the way, in the second pixel from the left on the third line, 1/3 dot is added from the right end to the left end of the pixel, so P
The controller unit sends image data so that the selection signal is high to select the WM signal 2 and the on-duty of the PWM signal 2 corresponds to 1/3 dot. In the third pixel from the left, 1/3 dots are deleted from the right end to the left end of the pixel, that is, 2/3 dots are formed from the left end to the right end of the pixel. Therefore, the selection signal for selecting the PWM signal 1 is selected. Is low, the image data is sent from the controller unit so that the on-duty of the PWM signal 1 corresponds to 2/3 dots. In this way, the ESP-processed image shown in FIG. 3B can be formed, and the image quality can be improved by the ESP process which is effective even in the electrophotographic apparatus in which the pulse width of the laser drive pulse is changed by the pulse width modulation method. Can be planned.

[0022]

As is apparent from the above embodiment, according to the present invention, the image data sent from the controller section for performing image data formation and ESP processing according to the processing result,
By forming the laser drive pulse based on the selection signal,
It is possible to form an ESP-processed image with good image quality, and it is possible to improve the image quality by effective ESP processing even in an electrophotographic apparatus in which the pulse width of the laser drive pulse is changed by the pulse width modulation method. In addition, since the degree of addition and deletion of dots in the ESP processing target image can be easily and freely changed only by changing the image data, a smoothing effect can be obtained for various images as compared with the conventional ESP processing. It is possible to easily realize high ESP processing and further improve the image quality.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a laser drive pulse forming circuit of an electrophotographic apparatus according to an embodiment of the present invention.

FIG. 2 is a waveform diagram showing a laser drive pulse forming process in the laser drive pulse forming circuit of the electrophotographic apparatus of one embodiment of the present invention.

FIG. 3 is a diagram showing, in contrast, a waveform image showing a formed image and a laser drive pulse forming process in a laser drive pulse forming circuit of an electrophotographic apparatus according to an embodiment of the present invention.

FIG. 4 is a block diagram showing a configuration of a laser drive pulse forming circuit of a conventional electrophotographic apparatus.

FIG. 5 is a waveform diagram showing a laser drive pulse forming process in a laser drive pulse forming circuit of a conventional electrophotographic apparatus.

FIG. 6 is a diagram showing an example of an ESP-processed image obtained by performing an ESP process on an original image.

FIG. 7 is a diagram showing, in comparison, an image obtained when an ESP process is performed in a laser drive pulse forming circuit of a conventional electrophotographic apparatus and a waveform diagram showing a laser drive pulse forming process.

[Explanation of symbols]

1 ... D / A converter, 2 ... Sawtooth wave generator, 3, 5 ...
Comparing circuit section, 4 ... Laser diode, 6 ... Selection section.

Claims (1)

[Claims] 1. A sawtooth shape for forming a D / A conversion unit for converting image data composed of a plurality of bits into an analog image signal, and a laser drive pulse for turning on and off a laser diode which is a main cause element of image formation. A sawtooth wave generator that generates a wave, a comparison circuit unit that compares the voltage level of the sawtooth wave with the image signal output from the D / A converter, and the logic formed by this comparison are inverted from each other. An electrophotographic apparatus including a selection unit that selects two signals by a selection signal.