JPH0537751A - Image forming device - Google Patents

Abstract

(57) [Summary] [Object] An object of the present invention is to provide an image forming apparatus capable of forming a high-quality image by selecting a pulse width modulation circuit and an intensity modulation circuit according to the characteristics of an image signal. In an image forming apparatus for forming a gradation image by irradiating a receptor with a beam modulated by an image signal input from the outside, an intensity modulation circuit for modulating the intensity of the beam based on the image signal. A pulse width modulation circuit that modulates the pulse width of the beam emitted based on the image signal and a selection circuit that selects one of the two modulation circuits.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus,
More particularly, it relates to an image forming apparatus capable of forming a gradation image by irradiating a receptor with a modulated beam.

[0002]

2. Description of the Related Art Conventionally, a dither method has been widely used to form an image having gradation, but in this method, if an attempt is made to improve gradation, the resolution will deteriorate.
Therefore, in recent years, it has been
Pulse width modulation to obtain gradation by changing the irradiation area in one pixel by modulating the beam irradiation time in each pixel, or to change the irradiation energy by modulating the beam intensity for each pixel. An image having gradation is formed by the intensity modulation for obtaining the gradation.

[0003]

However, in pulse width modulation, when an image is formed on the basis of an image signal obtained by reading an original document, such as a printed original document, whose gradation is formed by a halftone dot, by a scanner device, the halftone dot is formed. Moire due to the difference between the pulse width modulation period and the pulse width modulation period appears as fine stripes, which deteriorates the quality of the formed image. In order to prevent this, a space filter circuit must be inserted, resulting in an expensive and complicated structure.

Further, in the intensity modulation, since gradation is formed for each pixel, it is easy to obtain a high-definition image. On the other hand, since the irradiation energy of the beam directly affects the gradation of the image, it is possible to use a receptor such as a photosensitive drum. There is a problem in that the irradiation conditions change under the influence of unevenness in the moving speed and vibrations, and in some cases dark and light stripes occur in the output image.

The present invention has been made in order to solve the above-mentioned problems, and has a pulse width modulation circuit and an intensity modulation circuit, and by switching the two modulation circuits according to the nature of the image signal, the conventional method An object of the present invention is to provide an image forming apparatus capable of forming a higher quality image.

[0006]

To achieve this object, the image forming apparatus of the present invention forms a gradation image by irradiating a receptor with a beam modulated by an image signal input from the outside. In the image forming apparatus, the intensity modulation circuit that modulates the intensity of the beam based on the image signal, the pulse width modulation circuit that modulates the pulse width of the beam irradiated based on the image signal, and the two modulation circuits And a selection circuit for selecting either one.

[0007]

In the image forming apparatus of the present invention having the above-mentioned structure, when the image is formed by the image signal formed from the image having the halftone dot structure of the print original or the like, the selection circuit selects the intensity modulation circuit. An image is formed by performing the modulation by the intensity modulation method. In this case, since the modulation is performed continuously, moire does not occur, and an image of good quality can be obtained.

Further, when an image is formed by an image signal formed from an original such as a photograph, a painting or a CG (computer graphics), the selection circuit selects the pulse width modulation circuit, so the image is formed by the pulse width modulation method. Is formed. In this case, stripes due to uneven velocity of the receptor do not occur. Therefore, a high quality image can be obtained.

This relationship is shown in FIG. That is, when the modulation method is switched as in the present invention, neither stripes nor moire are generated in a non-dotted document as compared with the case of only intensity modulation.
In addition, compared to the case of using only pulse width modulation, moire does not occur in halftone dot originals, but stripes occur, but the appearance of stripes does not deteriorate the image quality as much as when moire occurs, so a single modulation method is used. Compared with the case, the image quality is improved as a whole.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

The configuration of the image forming apparatus of this embodiment will be described with reference to FIGS. 1 to 5.

As shown in FIG. 1, the image forming apparatus of this embodiment has a semiconductor laser element 1, a driving circuit 2, a deflecting device 3 and an f-
The θ lens 4, the photosensitive drum 5, the beam detection device 6, the reflection mirror 7, the pulse width modulation circuit 8, the intensity modulation circuit 9, and the selection circuit 10 are included.

The pulse width modulation circuit 8 is constructed as shown in FIG. 3, and comprises a D / A converter 81, a triangular wave generation circuit 82 and a comparison circuit 83, the operation of which is shown in FIG.
As shown in FIG. 3, a triangular wave synchronized with an image clock (not shown) and a waveform obtained by analog-converting the image signal D are compared by a comparison circuit, and a pulse is generated based on the result.

The intensity modulation circuit 9 is as shown in FIG. 5, and comprises a D / A converter 91, the operation of which is as follows.
The image signal D is D / A converted by the D / A converter 91 and output.

The selection circuit 10 is a switch circuit that selects and outputs one of the two modulation circuits according to a selection signal S described later.

Next, the operation of the image forming apparatus of the present invention will be described.

The laser beam emitted from the semiconductor laser device 1 is deflected by the deflecting device 3, and the f-θ lens 4
Thus, the photosensitive drum 5 is exposed at a constant speed. Photosensitive drum 5
Is previously charged to a predetermined potential, and the laser beam is pulse-width modulated or intensity-modulated by the image signal D as described later in detail, so that a latent image is formed on the photosensitive drum 5 according to the image signal D. Is done. Since the deflecting device 3 is rotating in the direction of arrow A in FIG.
Prior to the exposure, the laser beam scans the beam detection device 6 via the reflection mirror 7, and by this operation the beam detection device 6 generates a BD signal representing the scanning timing of the laser beam and an image signal generation for generating the image signal D. The signal is output to a device (not shown), and the image signal generator generates the image signal D in accordance with the timing of the BD signal. The image signal generator outputs the selection signal S depending on whether the generated image signal D has a halftone dot structure.

The image signal D input to the image forming apparatus is input to the pulse width modulation circuit 8 or the intensity modulation circuit 9 by the selection signal S. Further, either of the modulation circuits is selected according to the selection signal S, and the selected modulation signal is output. The output modulation signal is supplied to the drive circuit 2.

The drive circuit 2 operates as follows. As is well known, the semiconductor laser device 1 has a characteristic that laser emission is started when the drive current exceeds a threshold current as shown in FIG. It is known that the magnitude of the threshold current varies depending on the individual semiconductor laser device or changes depending on the temperature. Therefore, the drive circuit 2 superimposes the threshold current on the input modulation signal to drive the semiconductor laser element 1 with current. The measurement of the threshold current is performed intermittently between laser beam scans and does not affect the original image signal modulation operation. By the operation of the drive circuit 2, a laser beam having a light intensity proportional to the modulation signal can be obtained.
Therefore, regardless of whether the selection circuit 10 selects the pulse width modulation circuit 8 or the intensity modulation circuit 9, the photosensitive drum 5 can be exposed with a laser beam having a light intensity corresponding to the modulation signal.

A latent image is formed on the exposed photosensitive drum 5 and is developed by a well-known developing device (not shown and described) to obtain a visible image.

When the image signal D formed from an image having a halftone dot structure such as a print original is input to form an image, the selection circuit 10 selects the intensity modulation circuit 9 to perform modulation by the intensity modulation method. And an image is formed. In this case, since the modulation is performed continuously, moire does not occur,
An image of good quality can be obtained.

When the image signal D formed from a document such as a photograph, a painting or a CG (computer graphics) is input to form an image, the selection circuit 10 selects the pulse width modulation circuit 8. An image is formed by the pulse width modulation method. In this case, the photosensitive drum 5
Stripes due to uneven speed are not generated. Therefore, a high quality image can be obtained.

When the modulation method is switched as in the present embodiment, neither stripes nor moire is generated in the non-halftone dot document as compared with the case of only intensity modulation. Further, although the moire does not occur in the halftone dot original as compared with the case of only the pulse width modulation, the stripes are generated, but the occurrence of the stripes is caused by the speed unevenness of the photosensitive drum 5, and the degree thereof is slight, and the moire is small. Since the image quality does not deteriorate as much as in the case of occurrence of, the image quality is improved as a whole as compared with the case of a single modulation method.

The present invention is not limited to the embodiments described in detail above, but various modifications can be made. For example, in the present embodiment, the selection circuit selects the modulation circuit depending on whether or not the image signal is a halftone dot original, but the selection circuit may be selected by an operator's instruction.

[0025]

As is apparent from the above description, the image forming apparatus of the present invention irradiates the beam by switching between the pulse width modulation and the intensity modulation according to the property of the image signal. It is possible to obtain high quality images without

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an image forming apparatus of the present invention.

FIG. 2 is a diagram showing a relationship between a property of an image signal and an image obtained by a modulation method.

FIG. 3 is a diagram showing a configuration of a pulse width modulation circuit 8.

FIG. 4 is a diagram showing an operation of a pulse width modulation circuit 8.

FIG. 5 is a diagram showing a configuration of an intensity modulation circuit 9.

FIG. 6 is a diagram showing the characteristics of the semiconductor laser device 1.

[Explanation of symbols]

 8 pulse width modulation circuit 9 intensity modulation circuit 10 selection circuit

Claims (1)

Claim: What is claimed is: 1. An image forming apparatus for forming a gradation image by irradiating a receptor with a beam modulated by an image signal input from the outside, An intensity modulation circuit that modulates the intensity; a pulse width modulation circuit that modulates the pulse width with which the beam is irradiated based on the image signal; and a selection circuit that selects one of the two modulation circuits. A characteristic image forming apparatus.