JPH05276385A - Picture density signal generator - Google Patents

Abstract

PURPOSE:To reduce the occurrence of density jump or density variance on an output picture as much as possible and to easily design the output characteristic of pulse width modulation in advance. CONSTITUTION:This generator is provided with a bit number conversion means 1 which converts density gradation data DT to corrected density gradation data DT1 consisting of a larger number of bits, a D/A conversion means 2 which converts corrected density gradation data DT1 to an analog signal, a sine wave generating means 3 which generates a sine wave, and a comparing means 4 which compares the analog signal from the D/A conversion means 2 and the sine wave from the sine wave generatiang means 3 with each other to output a picture density signal SD. In order to maintain the linearity of the picture density signal SD to density gradation data DT, the bit number conversion means 1 is provided with a data correcting part 5 which imparts a characteristic corresponding to the sine wave on corrected density gradation data DT1 or a data correcting part 6 where not only the characteristic corresponding to the sine wave but also the development characteristic of a developing means are taken into consideration.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image density signal generator used in an image forming apparatus such as a copying machine or a printer which reproduces a halftone image, and more particularly, to an image density gradation data for each pixel. The present invention relates to an improvement in an image density signal generation device that generates a pulse width image density signal.

[0002]

2. Description of the Related Art Generally, in an image forming apparatus such as a laser printer which reproduces a halftone image, an image density signal having a pulse width corresponding to density gradation data for each pixel is generated,
A method of controlling the lighting time width of the laser according to the image density signal is adopted.

In such a type, a conventional image density signal generating device is, for example, Japanese Patent Laid-Open No. 62-18987.
The one described in Japanese Patent Publication No. 9 is known. For example, as shown in FIG.
It is taken into the 8-bit D / A converter 100 in synchronism with the 00 line gradation data clock CLK and converted into an analog signal having a pulse width corresponding to the gradation data, and this analog signal is the only low frequency component effective as image information. Through the low pass filter 101 through which the comparator 1 passes
02 and the 400-line gradation data clock is divided into ½ by the ½ divider 103, and the 200-line triangular wave generator 104 is synchronized with the divided clock. And a triangular wave is generated, the triangular wave is input to the other negative terminal of the comparator 102, and both inputs are compared in the comparator 102 to output an image density signal SD having a predetermined pulse width. ..

[0004]

However, in such a conventional image density signal generating apparatus, the low density area important as an image quality element is the tip of the triangular wave, which is ideal for the circuit design. It is difficult to generate such a triangular wave, and in particular, a large distortion due to a harmonic component is likely to occur at the tip of the triangular wave, which causes density jumps and uneven density on the output image. Such a waveform distortion cannot be estimated in advance as a circuit design, and it is extremely difficult to design the output characteristics of the pulse width modulation as much as that, and it is a great limitation to improve the accuracy of the pulse width modulation. Was becoming.

The present invention has been made in order to solve the above technical problems, and can minimize the occurrence of density jumps and density unevenness on an output image, and further, the output characteristics of pulse width modulation. An image density signal generation device that can be easily designed in advance.

That is, the first aspect of the invention is based on the image density signal generating apparatus for generating the image density signal SD having the pulse width corresponding to the density gradation data DT for each pixel as shown in FIG. A bit number converting means 1 for converting the density gradation data DT into the correction density gradation data DT1 having a larger number of bits, and a digital-analog conversion for converting the correction density gradation data DT1 from the bit number converting means 1 into an analog signal. Means 2, a sine wave generating means 3 for generating a sine wave having the same frequency as the cycle of the corrected density gradation data DT1 input to the digital-analog converting means 2, the analog signal from the digital-analog converting means 2 and the sine wave And a comparison means 4 for comparing the sine wave from the generation means 3 and outputting an image density signal SD having a pulse width corresponding to the analog signal. 1 is a data correction unit 5 in which the correction density gradation data DT1 is provided with a characteristic corresponding to a sine wave so that the linearity of the image density signal SD output from the comparison unit 4 with respect to the density gradation data DT is maintained. Is provided.

Further, the second invention is used in an image recording apparatus for visualizing a latent image carried on a latent image carrier by a developing means and corresponds to density gradation data DT for each pixel. Based on the image density signal generator for generating the image density signal SD having the pulse width and used for forming the latent image on the latent image carrier, the same bit number conversion means 1 as in the first invention, digital analog Conversion means 2, sine wave generation means 3 and comparison means 4
The bit number converting means 1 is configured to maintain the linearity of the latent image corresponding to the image density signal SD output from the comparing means 4 with respect to the density gradation data DT of the visible image density by the developing means. In addition, a data correction unit 6 is provided in which the correction density gradation data DT1 is provided with a sine wave corresponding characteristic and a developing characteristic corresponding characteristic in advance.

With such a technical means, the difference in the number of converted bits of the number-of-bits conversion means 1 can be converted into the corrected density gradation data DT1 in a range in which density jumps and density unevenness on the output image do not occur. If there is any, it can be selected appropriately. Further, in order to improve the output accuracy from the comparison means 4, the comparison means 4 is controlled by gain control, for example.
It is desirable to control the purity of the reference waveform input to the input signal, and further it is desirable to design so that the noise of the output from the digital-analog conversion means 2 is removed by a filter. Furthermore, the correction mode of the data correction unit 5 added to the bit number conversion means 1 is set appropriately in consideration of the actual sine wave characteristic generated by the sine wave generation means 3. Regarding the correction mode of the means 6,
Further, it is appropriately set in consideration of the developing characteristics of the developing means. In the first invention, in order to obtain the same operation and effect as the second invention, the developing characteristics of the developing means may be designed to be corrected by different gradation correcting means. ..

[0009]

According to the technical means as described above, the reference waveform input to the comparing means 4 at the time of pulse width modulation is a sine wave containing no harmonic component. The analog signal from the digital-analog conversion means 2 is compared and the image density signal SD corresponding to the pulse width of the analog signal is output. At this time, by using the sine wave, on the low output side and the high output side of the digital-analog converting means 2, the output pulse width from the comparing means 4 corresponding thereto becomes a curve corresponding to the sine waveform. However, in the first aspect of the invention, the bit number converting means 1 installed on the front side of the digital-analog converting means 2 has the corrected density gradation data DT1 having a larger number of bits than the original density gradation data DT.
The conversion pattern is subjected to data interpolation by the function of the data correction section 5 so that the pulse width of the image density signal SD from the comparison means 4 becomes linear with respect to the original density gradation data DT. Since it is performed, the output from the digital-analog conversion means 2 also corresponds to the sine waveform, and the pulse width of the image density signal SD from the comparison means 4 becomes linear corresponding to the density gradation data DT. Is set. In the second aspect of the invention, the conversion pattern of the bit number converting means 1 can be changed by the developing means of the latent image corresponding to the image density signal SD output from the comparing means 4 by the function of the data correcting section 6. Since the data interpolation is performed so that the linearity of the visual image density with respect to the density gradation data DT is maintained, the output from the digital-analog conversion means 2 corresponds to a sine waveform considering the developing characteristics, The visible image density based on the image density signal SD from the comparison means 4 is set to be linear with respect to the density gradation data DT.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on the embodiments shown in the accompanying drawings. (Embodiment 1) FIG. 2 shows an embodiment 1 of an image density signal generating apparatus to which the present invention is applied, which is used, for example, in an electrophotographic laser printer. In the figure, reference numeral 10 is, for example, a bit number converter that takes in 8-bit 400-line gradation data DT in synchronization with 400-line gradation data clock CLK and converts it into 10-bit corrected gradation data DT1, and 11 is a correction step. Adjusting the adjustment data DT1 to the 400-line gradation data clock CLK
Of the analog signal from the 10-bit D / A converter 11 which is effective as image information in the analog signal from the 10-bit D / A converter 11 and is converted into an analog signal according to the pulse width of the corrected gradation data DT1. A low-pass filter that passes only the frequency component, 13 is a 1/2 frequency divider that divides the 400-line gradation data clock LK by 1/2, and 14 is a 200-line gradation data clock that is divided by 1/2. A 200-line synchronous sine wave generator that generates a sine wave in synchronization, 15 is a sine wave generator 14 for controlling the purity of the sine wave generated from the 200-line synchronous sine wave generator 14.
An automatic gain controller (AGC) 16 for automatically controlling the gain of the signal is compared with an analog signal from the low-pass filter 12 and a sine wave from the sine wave generator 14, and an image density signal having a pulse width corresponding to the analog signal is compared. S
It is a comparator that generates D (a voltage comparator in this embodiment).

In this embodiment, the object of which the pulse width is modulated by the image density signal from the image density signal generator is a laser diode. This laser diode generally does not respond to very small pulse inputs,
When a laser diode is used in an electrophotographic laser printer, the drive current amplitude of the laser diode is relatively large, so that the laser diode responds to an input pulse having a width of 3 to 5 ns or more. Based on such output characteristics of the laser diode, the output range of the 10-bit D / A converter 11 according to this embodiment has a sine wave analog output when the input is minimum (0), as shown in FIG. Is set to a value slightly higher than the bottom of the image density, and is adjusted so that the image density signal SD having a pulse width of about 5 ns appears at the output of the comparator 16, while the value when the input is maximum (1023). The analog output is set to a value slightly lower than the apex of the sine wave, and is adjusted so that the image density signal SD having a pulse width of, for example, about 95 ns appears at the output of the comparator.

Furthermore, this embodiment is characterized by the conversion rule of the bit number converter 10. Specifically, now, the 400-line gradation data clock CLK is 20
In the case of MHz, assuming that the conversion rule of the bit number converter 10 is linear as shown by the dotted line in FIG. 4, 200-line sine wave (10 MHz) and 10-bit D / A converter 1
When compared with the output of No. 1, the output pulse width of the comparator 16 is 5 ns along the pattern in which the upper and lower parts corresponding to a half cycle from the bottom to the apex of the sine wave are missing by 5 ns, as shown by the solid line in FIG. It varies in the range of 95 ns. Therefore, it is considered that if the conversion rule of the bit number converter 10 is appropriately changed, the change pattern of the output pulse width of the comparator 16 can be changed accordingly.
In this embodiment, the conversion rule of the bit number converter 10 is set as a sine wave shape as shown by the dotted line in FIG. 5, and the change pattern of the output pulse width of the comparator 16 is linear as shown by the solid line in FIG. That is, in other words, the 8-bit density gradation data DT is adjusted to obtain good linearity.

Therefore, according to this embodiment, the pulse width modulation accuracy of the image density signal SD with respect to the 8-bit density gradation data DT is extremely higher than that of the conventional triangular wave method (the method shown in FIG. 8), and the density is high. It was confirmed that a good output image without jumps and uneven density was obtained. In the electrophotographic laser printer of this embodiment,
The output image density is usually affected by the developing characteristics of the developing device (gamma characteristics of laser xerography), but an output image can be obtained in a form in which the developing characteristics are taken into consideration by a correction circuit (not shown). ..

In the above embodiment, 10-bit D / A
The output range of the converter 11 is made slightly larger than the sine wave amplitude so that a pulse of about 5 ns is output when the 10-bit corrected density gradation data DT1 is minimum (0). However, the present invention is not limited to this. Instead, for example, the output range of the 10-bit D / A converter 11 is made to coincide with the sine wave amplitude, and an offset is added to the data when the bit number converter 10 converts data from 8 bits to 10 bits. May be.

Example 2 The basic structure of this example is substantially the same as that of Example 1, but
The conversion rule of the bit number converter 10 is different from that of the first embodiment. That is, in the electrophotographic laser printer, the output image density is usually affected by the developing characteristic of the developing device (gamma characteristic of laser xerography), and a correction circuit for exclusive use is usually provided to take this developing characteristic into consideration. However, in this embodiment, in addition to the sinusoidal wave corresponding characteristic of the first embodiment, a correction function for considering the developing characteristic is added to the conversion rule of the bit number converter 10.

More specifically, assuming that the bit number converter 10 of the first embodiment is used and the image density signal SD from the comparator 16 is sent to the laser diode as it is, as shown in FIG. The development density based on SD is given as being influenced by the development characteristics of the developing device (gamma characteristics of laser xerography). Therefore, in this embodiment, as a conversion rule of the bit number converter 10, a change pattern of the output pulse width of the comparator 16 with respect to the 8-bit density gradation data DT is shown by a solid line in FIG. The position of the image density signal S varies depending on the device, but is a waveform corresponding to the mirror image relationship of the developing characteristics of the developing device), and the image density signal S
The development density for D is adjusted to be linear as shown by the solid line in FIG. Therefore, according to this embodiment, in consideration of the developing characteristics of the developing device, it was confirmed that the gradation reproducibility of the output image is kept good without providing a dedicated correction circuit.

[0017]

As described above, according to the first aspect of the present invention, the reference input wave to the comparing means for pulse width modulation is replaced by a triangular wave having a large number of harmonic components and a small number of harmonic components. By adopting a sine wave and increasing the number of bits of the density gradation data, data interpolation corresponding to the curved portion of the sine wave is performed, and the pulse width modulation of the image density signal from the comparison means for the density gradation data is performed. Since the linearity is maintained, the adverse effects caused by the waveform distortion that occurred in the method using a triangular wave with many harmonic components can be solved all at once, and density jumps and density unevenness on the output image can be minimized. Moreover, the output characteristics of pulse width modulation can be easily designed in advance.

Further, according to the second aspect of the invention, when converting the number of bits of the image density data, not only the above-mentioned characteristic corresponding to the sine wave is added, but also the developing characteristic of the developing means is taken into consideration. Since the linearity of the visible image density by the developing means with respect to the density gradation data is maintained, the density jump on the output image without using a dedicated correction circuit to consider the developing characteristics of the developing means. Generation of density unevenness can be minimized, and the output characteristics of pulse width modulation can be easily designed in advance.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a configuration of an image density signal generation device according to the present invention.

FIG. 2 is an explanatory diagram showing a first embodiment of an image density signal generation device to which the present invention is applied.

FIG. 3 is an explanatory diagram showing output ranges of a 10-bit D / A converter and a comparator used in the embodiment.

FIG. 4 is an explanatory diagram showing a relationship between a bit number converter and a comparator output used in the embodiment.

FIG. 5 is an explanatory diagram showing conversion rules of the bit number converter used in the embodiment.

FIG. 6 is an explanatory diagram showing the relationship between the bit number converter used in the image density signal generating apparatus according to the first embodiment and the developing characteristics of the developing unit.

FIG. 7 is an explanatory diagram showing a conversion rule of a bit number converter used in the image density signal generation device according to the second embodiment.

FIG. 8 is an explanatory diagram showing an example of a conventional image density signal generation device.

[Explanation of symbols]

DT ... Density gradation data, SD ... Image density signal, 1 ... Bit number conversion means, 2 ... Digital-analog conversion means, 3 ... Sine wave generation means, 4 ... Comparison means, 5, 6 ... Data correction section

Claims (2)

[Claims] 1. An image density signal generator for generating an image density signal (SD) having a pulse width corresponding to the density gradation data (DT) for each pixel.
T) is the corrected density gradation data (DT
1), and the correction density gradation data (DT1) from the bit number converting means (1)
To analog signal, and a sine wave generating means for generating a sine wave having the same frequency as the cycle of the corrected density gradation data (DT1) input to the digital to analog converting means (2). (3) is compared with the analog signal from the digital-analog conversion means (2) and the sine wave from the sine wave generation means (3), and an image density signal (SD) having a pulse width corresponding to the analog signal is output. A comparing means (4), and the bit number converting means (1) includes an image density signal (S) output from the comparing means (4).
In order to maintain the linearity of D) with respect to the density gradation data (DT), the correction density gradation data (DT1) is provided with a data correction unit (5) which is provided with a characteristic corresponding to a sine wave. Image density signal generating device. 2. A pulse width corresponding to density gradation data (DT) for each pixel, which is used in an image recording apparatus for visualizing a latent image carried on a latent image carrier by a developing means. In the image density signal generator for generating an image density signal (SD) and used for forming a latent image on the latent image carrier, the density gradation data (DT) is corrected density gradation data having a larger number of bits. A bit number converting means (1) for converting into (DT1),
Digital-analog conversion means (2) for converting the corrected density gradation data (DT1) from this bit number conversion means (1) into an analog signal, and corrected density gradation data input to this digital-analog conversion means (2) The sine wave generating means (3) for generating a sine wave having the same frequency as the cycle of (DT1) is compared with the analog signal from the digital-analog converting means (2) and the sine wave from the sine wave generating means (3). , An image density signal (S with a pulse width corresponding to the analog signal)
D) is output to the comparison means (4), and the bit number conversion means (1) is provided with a latent image developing means corresponding to the image density signal (SD) output from the comparison means (4). A data correction unit (6) in which correction density gradation data (DT1) is provided with a sine wave corresponding characteristic and a development characteristic corresponding characteristic in advance so that the linearity of the image density with respect to the density gradation data (DT) is maintained. An image density signal generation device, characterized by being provided with.