JPH08160682A - Image forming device - Google Patents

Abstract

PURPOSE: To keep the stability of the gradation and color reproduction at a low-density section against the environment constant regardless of image revealing means even when multiple different kinds of image revealing means are used. CONSTITUTION: This image forming device is provided with a pulse width modulating means pulse-width-modulating the image density signal with the reference wave and an image forming means forming an image in response to the pulse- width-modulated signal outputted from the pulsewidth-modulating means. The image forming means is provided with two or more kinds of image revealing means forming a revelation image in response to the image density signal. The pulse-width-modulating means is provided with a reference wave generating means 36 generating multiple reference waves different in modulation cycle and a selecting means 37 selecting the image density signal to be outputted to the image forming means among the image density signals pulse-width- modulated by the reference wave generated by the reference wage generating means 36 according to the type of the image revealing means.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to an electronic device for forming a latent image on a photosensitive medium by scanning a light beam in accordance with a pulse width modulation signal of an image density signal, developing the latent image with toner, and forming an image. The present invention relates to a photographic image forming apparatus.

[0002]

2. Description of the Related Art In a printer or a copying machine, a digital electrophotographic system is widely adopted as a system capable of providing high speed and high image quality. In this method, pulse width modulation exposure is often performed using an analog screen generator or the like in order to perform optical scanning of a photosensitive medium using a light beam and reproduce the gradation of an image ( See, for example, Japanese Patent Laid-Open No. 1-280965).

In these systems, the image formation is carried out from the low density portion of the image density signal to the high density portion while keeping the light beam spot diameter and the number of lines constant. As a result, the exposure profile in the low-density area has a reduced contrast and becomes analog, and since the exposure amount itself is low, the reproducibility of dots and lines deteriorates, and the stability of the gradation and color reproduction environment is stable. There was a problem that the sex became worse.

For the above problems, stabilization of the light quantity of the light beam,
A method of stabilizing each element such as stabilization of toner concentration in the developing device, measuring temperature and humidity and toner concentration in the developing device, and controlling the developing bias and transfer current value to reproduce gradation and color for the environment. A method called process control for increasing stability has been proposed (for example, JP-A-4-3788).
(See Japanese Patent Laid-Open No. 2-367376). However, these methods require a highly accurate sensor and control mechanism, and have the drawback of being complicated and expensive.

Further, a method has been proposed in which the light beam spot diameter and the light emission intensity of the light beam are suppressed from lowering the contrast of the exposure profile to increase the reproducibility of dots and lines (for example, Japanese Patent Laid-Open No. 4-13163). Japanese Patent Laid-Open No. 4-97
374, and Japanese Patent Laid-Open No. 4-94261). However, these systems also require a control mechanism for varying the light beam spot diameter and the light emission intensity, which is disadvantageous in that it is complicated and expensive.

In order to solve such a drawback, the line reproducibility in the low density portion is reduced to improve the reproducibility of dots and lines in the low density portion, and the gradation and color reproducibility are stable against the environment. An invention for improving the property has been proposed in the previous patent application (Applicant Fuji Xerox Co., Ltd., Japanese Patent Application No. 6-234459, "Image forming apparatus").

On the other hand, in recent years, a copying machine for both black and white and full color has been developed. Such a copying machine is often used as a black-and-white copying machine rather than full-color, and one-component development with low running cost as a black-and-white visualization means is a two-component development with high image quality reliability used for color. May be used instead of. That is, one-component development is used for the black visualization means, and two-component development is used for the yellow, magenta, and cyan visualization means.

In the case of such a copying machine, the developing characteristic by the visualizing means, particularly the response characteristic to the spatial frequency of the input image, that is, MTF (modulation t).
The transfer function characteristics are different. Therefore, when the means for reducing the number of lines in the low density portion is used, there arises a problem that the stability of the gradation and color reproduction in the low density portion with respect to the environment differs depending on the visualizing means.

[0009]

SUMMARY OF THE INVENTION The present invention eliminates the above-mentioned problems and drawbacks of the prior art, and realizes gradation and color reproduction in a low density portion even when a plurality of different visualization means are used. The purpose is to make the environmental stability the same regardless of the visualization means.

[0010]

In order to achieve the above-mentioned object, an image forming apparatus of the present invention (Claim 1) comprises a pulse width modulating means for pulse width modulating an image density signal using a reference wave, In an image forming apparatus having an image forming means for forming an image in accordance with a pulse width modulation signal output from the pulse width modulating means, two or more types of visualizing means for forming a visible image in accordance with an image density signal, and a modulating means. Reference wave generating means for generating a plurality of reference waves having different periods, and an image density signal output to the image forming means from the image density signal pulse-width modulated by the reference wave generated by the reference wave generating means. Selection means for selecting according to the type of imaging means is provided.

Further, the image forming apparatus of the present invention (Claim 2) is the one in which, in addition to the configuration of the above invention (Claim 1), means for reducing the number of lines in the low density portion is added. The means for reducing the number of lines has two or more periodically operating conversion means having different characteristics for converting the image density signal. At least one of the converting means has a characteristic of converting the output of the image density signal to the low density portion into an image density signal in the range of 0 or not visualized.

The selecting means in each of the above inventions can be configured to select according to the type of the visualizing means and the image density signal.

[0013]

2 (a), 2 (b) and 2 (c), in the present invention, the photosensitive medium is exposed using the beam scanning means, the image forming optical system and the pulse width modulating means in the image forming means. An example of the exposure energy profile on the photosensitive medium at the time is shown. The distance d _P (mm) between adjacent pixels is shown.
And the light beam spot diameter d _B is D, the results are shown when the values of D are 1/1, 1/2, and 1/3, respectively. D, the light beam spot diameter d _B (mm)
Is constant, and the number of lines is N (line / inch), it is determined to satisfy the following equation. d _P = 25.4 / N D = d _B / d _{P In} electrophotography, a bias potential is applied during development in order to prevent toner from adhering to the base. In FIG. 2, the boundary line corresponding to the bias potential is also shown by a broken line as the reversal development for developing the exposed portion. As is remarkable in FIG. 2A, as the pulse width (%) is reduced, the contrast of the exposure energy profile is reduced and becomes analog. The amount exceeding the boundary line corresponding to the bias potential is reduced, and dots and lines are not reproduced. As can be seen from FIG. 2, the values of D are 1/1, 1/2, 1/3
The smaller the value, the more the decrease in contrast is suppressed.
As a result, when the light beam spot diameter d _B is kept constant, the number of lines N is reduced and the value of D is reduced to improve the reproduction of dots and lines in the low-density portion, and the gradation to the environment. -It can be seen that the stability of color reproduction increases. On the other hand, in the medium and high density areas, it is preferable that the number of lines is high because it is difficult to see the structure of dots and lines as is conventionally known.

On the other hand, the above-mentioned optimum number of lines differs depending on the type of visualization means (for example, the visualization system and the type of developer). FIGS. 14A and 14B show the response characteristic with respect to the spatial frequency of the input image, that is, the MTF characteristic, for two different visualization means. From this figure, the visualizing means having the MTF characteristic shown in FIG. 14A and the visualizing means having the MTF characteristic shown in FIG. 14B have different response spatial frequency regions.
In the characteristic (a), the image spatial frequency range in which the contrast ratio of the output image to the input image is large is narrow, and the peak is at a place where the number of lines is low. The characteristics of FIG. 14B are characteristics in which the image spatial frequency range in which the contrast ratio of the output image with respect to the input image is large is relatively wide, and it is possible to respond up to a relatively high number of lines.

From the above, in electrophotography, when the light beam spot diameter d _B is constant, there is an optimum number of lines for reproducing each density, and an optimum number of lines is used for reproducing each density. It can be seen that by selecting it, a good image with excellent environmental stability can be obtained.

Therefore, the present invention is based on the above consideration.
When forming a halftone image, the number of lines can be changed according to the type of visualization means. In the present invention (Claim 1), the pulse width modulation means is capable of performing pulse width modulation of a plurality of modulation periods, and the selection means uses the visualization means to obtain the optimum number of lines for the image density signal. Select the modulation cycle. For example, depending on the MTF characteristic of the visualization means,
When the modulation cycle is selected, the number of lines is reduced in the low density portion as described above with respect to the visualization means having the response characteristic as shown in FIG. Since the reproduction is good, select the output in which the pulse width modulation of the image density signal is performed by the reference wave with a relatively long modulation cycle, and in the middle and high density areas, the high line for obscuring the structure of dots and lines. In order to obtain the number, an output obtained by performing pulse width modulation of the image density signal with a reference wave having a relatively short modulation period is selected. Then, as shown in FIG. 14B, for the visualization means capable of responding to a higher number of lines than the visualization means of FIG.
14 so that the number of lines is slightly higher than that of the visualization means shown in FIG.
An output obtained by pulse-width-modulating the image density signal with a reference wave having a slightly shorter modulation wave than the reference wave used for the visualization means in (a) is selected.

Further, according to the present invention (claim 2), the modulation cycle is selected by adding the conversion to the image density signal and by the waveform selecting means, which is optimum for each visualization means. By doing so, the optimum number of lines was set according to the visualization means and the image density. The two or more image density signal converting means have different characteristics as shown in FIGS. 6 (a) and 6 (b), for example. Since one of the converting means has a characteristic that the output of the image density signal to the low density portion is 0 as shown in FIG. 6B, there is no output when the low density signal comes in FIG. 5C. . Since these converting means alternately operate in a time division manner as shown in FIGS. 5B and 5C, the input image signal is thinned out for the low density portion as shown in FIG. 5D. Thus, the pulse width modulated output has a lower number of lines for the low density portion and a higher number of lines for the middle and high density portions. After the image density signal is converted in this way, the image shown in FIG.
The visualization unit that responds to a higher number of lines than the visualization unit of (a) uses an image density signal with a reference wave that is slightly shorter than the reference wave of the modulation period used for the visualization unit of FIG. Pulse width modulation is performed.

In the present invention (claim 3), the selecting means is not limited to the type of the visualizing means, but depends on the type of the image density signal, for example, a halftone image or a character image. Since the selection is made, the appropriate number of lines can be determined more finely.

As described above, according to the present invention, even when a plurality of types of different visualizing means are used, the stability of the gradation and color reproduction in the low density portion with respect to the environment does not depend on the visualizing means. It can be constant.

[0020]

【Example】

(First Embodiment) FIG. 3 is a view showing the schematic arrangement of an embodiment of the image forming apparatus of the present invention. A charger 2, a rotary developing device 3, a transfer drum 4, a cleaner 5 and the like are arranged around the photoconductor 1 rotating in the direction of the arrow. Photoconductor 1
Are uniformly charged by the charger 2 in the dark part. In addition, a known contact charging device (charging brush,
If a charging roller, charging blade, charging belt) is used, ozone generation can be prevented.

The light beam scanning device 20 is composed of a semiconductor laser 21, a collimator lens 22, a polygon mirror 23, an image forming optical system 24, and the like.
Scan against. Further, the light beam is used as the original reading unit 1.
It is turned on and off by the light beam pulse width modulator 30 according to the density signal supplied from 0 or the like. The photoconductor 1 is exposed by the light beam pulse that is turned on and off, and an electrostatic latent image is formed. The spot diameter (1 / e ² ) of the light beam in the main scanning direction on the photoconductor 1 was set to 64 μm.

The rotary developing device 3 is composed of four developing devices respectively having yellow, cyan, magenta and black toners. The black developing device is a magnetic one-component developing device, and the other developing devices adopt a reversal developing method using a two-component magnetic brush developing device. An average toner particle size of 7 μm was used. The rotary developing device 3 is appropriately rotated to develop the electrostatic latent image with toner of a desired color. At this time, a bias voltage is applied to the developing roll to suppress toner adhesion to the white background portion.

The transfer drum 4 is rotated by mounting a sheet on the outer periphery thereof. The developed toner image on the photoconductor is transferred to the transfer device 4b.
Is transferred to paper by. An electrostatic latent image is formed, developed, and transferred for each of yellow, cyan, magenta, and black colors. The toner on the paper obtained by this operation is fixed by the fixing device 9 to form a multicolor image.

As shown in FIG. 1, a pulse width modulator 30 for turning on / off a light beam includes a D / A converter 31, a triangular wave oscillator 36, comparison circuits 32, 33, 34 and 35, and a waveform selection circuit 37. Composed.

The D / A converter 31 includes a document reading section 10
It converts a digital image density signal supplied from the above into an analog image density signal.

The triangular wave oscillator 36 generates four types of triangular wave pattern signals, that is, reference wave signals. The ratio of the periods of the reference wave signals is set to 3: 4: 6: 12, which correspond to 600 lines, 300 lines, 200 lines and 150 lines, respectively.

The comparison circuits 32, 33, 34 and 35 compare the respective reference wave signals with the magnitude of the analog image density signal to generate a pulse width modulation signal. The waveform selection circuit 37 selects a pulse width modulation signal from among the plurality of pulse width modulation signals created by each comparison circuit in accordance with the values of the color signal, the image density signal and the halftone image-character image discrimination signal. Choose one. When it is determined from the halftone image-character image discrimination signal that it is a character image, a pulse width modulation signal corresponding to 600 lines is selected regardless of the values of the color signal and the image density signal. On the other hand, if it is determined that the black signal is a halftone image, and if it is determined that it is a signal of a low density portion having a pulse width of 20% or less than the image density signal, a pulse width modulation signal corresponding to 200 lines is selected, When it is determined that the signal has a density portion larger than 20%, a pulse width modulation signal corresponding to 300 lines is selected. Further, when it is judged that the image is a halftone image by any one of the color signals of yellow, magenta, and cyan, and it is judged that it is a signal of a low density portion having a pulse width of 20% or less from the image density signal, it is made to correspond to 150 lines. When the pulse width modulation signal is selected and it is determined that the signal has a density portion larger than 20%, the pulse width modulation signal corresponding to 300 lines is selected.

FIG. 4 shows the gradation / color reproduction environment of an image in which the number of lines is variable according to the visualization means of this embodiment and an image in which the number of lines is variable regardless of the visualization means. It is the result of comprehensive evaluation of stability and image quality. According to the present embodiment, it is understood that the stability of the gradation / color reproduction in the low density portion with respect to the environment can be made constant regardless of the visualization means.

As described above in detail, according to the present embodiment, when the halftone image is formed, the pulse width modulator 30 is used to generate the color signal (and thus the corresponding visualization means).
The number of lines can be changed according to the type of the image density signal and the number of lines can be optimized for each visualization means, which improves the reproducibility of dots and lines in the low density area. The stability of toning and color reproduction in the environment is improved.

As a method of varying the number of lines, the pulse width modulator 30 has a function of performing pulse width modulation in accordance with an analog image density signal and a reference wave signal of a predetermined cycle by the triangular wave oscillator 36, and the reference wave signal is used. , Two or more types having different cycles are prepared, and one of the two or more types of pulse width modulation signals obtained from the two or more types of reference wave signals is selected according to the image density signal. Therefore, the above effect can be achieved without requiring a complicated and expensive process control or a device for varying the emission intensity.

(Second Embodiment) This embodiment is an embodiment in which a pulse width modulator for turning on / off a light beam is realized by a structure different from that of the first embodiment. The second embodiment has the same structure as the first embodiment except for the pulse width modulator, and therefore its description is omitted. As shown in FIG. 5, the pulse width modulator includes a triangular wave oscillator 51, a waveform selection circuit 52, a comparison circuit 53, a D / A converter selection circuit 54, a first D / A converter 55, and a second D / A converter. It is composed of an A converter 56 and an adding circuit 57.

The D / A converter selection circuit 54 is composed of a counter, a flip-flop circuit, etc., counts the reference clock signal 58, and reads the original reading section 10.
The digital image density signal supplied from (FIG. 3) or the like is periodically distributed to the first and second D / A converters 55 and 56 having different characteristics and output.

The distributed digital image density signals are the first and second D / A converters 55, 5 having different characteristics.
After being converted into an analog image density signal by 6, it is again synthesized by the adding circuit 57 and input to the comparing circuit 53. The waveform Sig (1) in FIG. 5 (b) shows the output of the first D / A converter 55, and the waveform Sig (2) in FIG. 5 (b).
Indicates the output of the second D / A converter 56, and the waveform Sig (0) in FIG. 7C indicates the output of the adding circuit 57.

The triangular wave oscillator 51 has two types of triangular wave pattern signals (reference triangular waves Ref (1) and Ref (2)).
Occurs. The period of each pattern signal was made to correspond to 600 lines, 400 lines and 300 lines, respectively.

The waveform selection circuit 52 outputs two reference triangular waves Ref (1) and Ref (2) output from the triangular wave oscillator 51.
Among them, one triangular wave is selected based on the halftone image-character image discrimination signal and is output to the comparison circuit 52.

The comparison circuit 53, as shown in FIGS. 5E and 5F, selects the selected reference triangular waves Ref (1), Ref.
(2) is compared with the magnitude of the analog image density signal Sig (0) to create the pulse width modulation signal PWM (0).

FIG. 6 shows the first and second D / A converters 5 for converting 8-bit input digital data into voltage.
The D / A conversion characteristics of Nos. 5 and 56 are shown. Here the output voltage is 1
Have been standardized. The first D / A converter 55 is shown in FIG.
As shown in (a), the characteristic is such that an analog output corresponding to the enlarged value of the image density is obtained in the area where the input digital image density is less than 20%, and in the area where the image density signal is 20% or more and less than 50%. , The image density is such that an analog output corresponding to a slightly enlarged value is obtained.
In the region of% or more, the amplitude is not expanded and the analog output value corresponding to the digital value is obtained. Second D
As shown in FIG. 6B, the A / A converter 56 has a characteristic that the output value becomes zero in the area where the input digital data value is less than 20%, and in the area where the image density signal is 20% or more and less than 50%. The characteristic is that an analog output corresponding to a value in which the image density is greatly enlarged is obtained, and in the region of 50% or more, the amplitude is not enlarged and the analog output value corresponding to a digital value is obtained.

When the halftone image-character image discrimination signal indicates that the image is a halftone image and the color signal is black, the waveform selection circuit 52 selects a triangular wave having a period corresponding to 400 lines. Output to the comparison circuit 53. When generating this halftone image, if the digital image density signal is in the halftone region of 50% or more, there is no difference from the pulse width modulation method that is usually used, and halftones are generated on a 400-line parallel line screen. It In the halftone region where the digital image density signal is less than 50% and 20% or more, the first D / A converter 5
5 and the portion D / A-converted by the second D / A converter 56 periodically constructs the second D / A signal in the vicinity of 20% of the digital image density signal. The portion D / A converted by the A converter hardly contributes to image formation. Further, in the halftone area where the digital image density signal is less than 20%, the first
Only the part D / A converted by the D / A converter 55 contributes to image formation, and as a result, the number of lines in the low-density portion is halved, and the 200-line screen has a half of 400 lines. A halftone image is formed.

When the halftone image-character image discrimination signal indicates that the image is a halftone image and the color signal is any one of yellow, magenta and cyan, the waveform selection circuit 52 is selected.
Selects a triangular wave having a period corresponding to 300 lines and outputs it to the comparison circuit 53. Similar to the case of the black signal, when this halftone image is generated, if the digital image density signal is in the halftone region of 50% or more, the halftone is generated on the 300-line parallel screen. In the halftone region where the digital image density signal is less than 50% and 20% or more, the first D / A
The portion D / A converted by the converter 55 and the second D / A
A portion which is D / A converted by the A converter 56 is periodically formed, and when the digital image density signal is near 20%, most of the portion which is D / A converted by the second D / A converter forms an image. Will not contribute to. Further, in the halftone region where the digital image density signal is less than 20%, only the portion D / A converted by the first D / A converter 55 contributes to the image formation, so that the half of 300 lines is 150. A halftone image is formed on a linear line screen.

When the halftone image-character image discrimination signal indicates that it is a character image, the waveform selection circuit 5
2 selects a triangular wave having a period corresponding to 600 lines and outputs it to the comparison circuit 53. Since the character image is usually formed by the high-density portion, it is in the region of 50% or more of the conversion characteristics of the first and second D / A converters, and the character image is formed on the 600-line parallel line screen. , Reproduces character images very well.

FIG. 12 shows a modified digital color copying machine A-Color (trademark) manufactured by Fuji Xerox Co., Ltd.
For the environment of gradation / color reproduction of an image formed by using the pulse width modulation device described in the second embodiment of the present invention and an image formed by changing the number of lines constant without depending on the visualization means. It is the result of comprehensive evaluation of stability and image quality.
According to the present embodiment, it can be seen that the stability of the gradation / color reproduction in the low density portion with respect to the environment can be made constant regardless of the visualization means.

(Third Embodiment) This embodiment is an embodiment in which a pulse width modulator for turning on / off a light beam is realized by a structure different from that of the first or second embodiment. The configuration other than the pulse width modulator is the same as that described in the first embodiment, and therefore the description thereof is omitted.
As shown in FIG. 7, the pulse width modulation device of the present embodiment has a triangular wave oscillator 71, a waveform selection circuit 72, a comparison circuit 73, a lookup table selection circuit 74, and a first lookup table (LUT (1)) 75. , Second lookup table (LUT (2)) 76, and D / A converter 77.
It consists of. That is, in the present embodiment, the first and second D / A converters in the second embodiment are replaced by a first look-up table 75 and a second look-up table 7.
6 and a common D / A converter 77. The data conversion characteristics of the first and second lookup tables are shown in FIGS. 8 (a) and 8 (b). That is, as shown in FIG. 8A, the conversion characteristic of the first look-up table is that the output is doubled with respect to the input when it is less than 25%, and the conversion ratio is gradually reduced thereafter, and is always the maximum when it is 75% or more. It outputs a value. The conversion characteristic of the second lookup table is shown in FIG.
As shown in (b), 0 is always output when less than 25%, and 2
When it is 5% or more, the conversion magnification is gradually increased, and when it is 75% or more, an output approximately twice that of the input is obtained.

When the halftone image-character image discrimination signal indicates that the image is a halftone image and the color signal is black, the waveform selection circuit 72 selects a triangular wave having a period corresponding to 400 lines and compares it. Output to 73. When generating this halftone image, if the digital image density signal is in the halftone region of 25% or more, there is no substantial change on average with the pulse width modulation method that is normally performed, and a 400-line parallel line screen is used. Halftones are produced. Digital image density signal is 2
In the halftone area of less than 5%, the portion converted by the first look-up table 75 and the portion converted by the second look-up table 76 are alternately digitally converted, but the second look-up table 76 is used.
The output of the converted output part of is 0. That is, in the area where the digital image density signal is less than 25%, only the portion converted by the first lookup table 75 contributes to image formation, and the portion converted by the second lookup table 76 contributes to image formation. Does not contribute. As a result, the number of lines in the area of less than 25% is halved, and a halftone image is formed on the 200-line parallel screen which is half the 400-line.

When the halftone image-character image discrimination signal indicates that the image is a halftone image and the color signal is any one of yellow, magenta and cyan, the waveform selection circuit 72 corresponds to 300 lines. A triangular wave having a cycle is selected and output to the comparison circuit 73. Similar to the case of the black signal, when the halftone image is generated, the digital image density signal is 2
In the halftone area of 5% or more, halftones are generated on a 300-line parallel screen. Digital image density signal is 25
In the halftone area of less than 50%, which is half of 300 lines, 150
A halftone image is formed on a linear line screen.

When the halftone image-character image discrimination signal indicates that it is a character image, the waveform selection circuit 7
2 selects a triangular wave having a period corresponding to 600 lines and outputs it to the comparison circuit 72. Since the character image is usually formed by the high-density portion, it is in the region of 25% or more of the conversion characteristics of the first and second lookup tables, and 6
A character image is formed on a 00 line parallel line screen, and the character image is reproduced extremely well.

(Fourth Embodiment) This embodiment is an embodiment in which a pulse width modulator for turning on / off a light beam is realized by a structure different from those of the first to third embodiments. The configuration other than the pulse width modulator is the same as that described in the first embodiment, and therefore the description thereof is omitted. As shown in FIG. 9, the pulse width modulator of the present embodiment has a triangular wave oscillator 91, first and second comparison circuits 92 and 93, and an OR.
Circuit 94, first and second look-up tables (L
UT (1)) 95, (LUT (2)) 96, first and second D / A converters 97 and 98, and a D / A converter selection circuit (not shown). FIG. 11A shows conversion characteristics by the first look-up table 95 and the first D / A converter 97, and FIG. 11B shows the second look-up table 96 and the second D / A conversion. The conversion characteristic by the device 98 is shown.

A D / A converter selection circuit (not shown) periodically distributes the digital image density signal supplied from the document reading section 10 (FIG. 3) to the first and second look-up tables 95 and 96 and outputs it. To do. The respective distributed digital image density signals are the first and second digital image density signals.
Characteristics are converted by the look-up tables 95 and 96 of the first and second D / A converters 97 and 98, and the first and second comparison circuits 92 and 9 are converted.
3 is input as an analog image density signal.

The waveform selection circuit 99 has four reference triangular waves Ref (1), Ref output from the triangular wave oscillator 91.
Among (2), Ref (3), and Ref (4), the halftone image-character selects two triangular waves based on the image discrimination signal and the color signal and outputs them to the comparison circuits 92 and 93, respectively.

The first and second comparison circuits 92 and 93 are
A pulse width modulation signal is created by comparing the magnitudes of a reference triangular wave and an analog image signal having different frequencies. Figure 10
(A) and (b) show the input / output relationship of the comparison circuit 92, and the comparison circuit 92 includes a first look-up table 95 and
When the conversion output Sig (1) from the A converter 97 and the reference triangular wave Ref (1) are input, the pulse width modulation signal PWM (1) of FIG. 10B is output. Figure 10 (c)
(D) shows the input / output relationship of the comparison circuit 92.
3, the conversion output Sig (2) by the second lookup table 96 and the D / A converter 98 and the reference triangular wave Ref.
When (2) is input, the pulse width modulation signal PWM (2) of FIG. 10 (d) is output. The two pulse width modulation signals PWM (1) and PWM (2) are combined by the OR circuit 94, and the desired pulse width modulation signal P shown in FIG.
Create WM (O). Digital image density signal is 2
In the area of less than 0%, only the portion converted by the first lookup table 95 and the DA converter 97 contributes to image formation, and the portion converted by the second lookup table 96 contributes to image formation. , The number of lines in the area of less than 20% is halved. Since the reference triangular wave is selected so that the number of lines is appropriate for each visualization means, a stable image can be obtained in the low density portion regardless of the visualization means.

(Fifth Embodiment) FIG. 12 shows a fifth embodiment in which a shift register 129 is provided in the preceding stage of the triangular wave oscillator in place of the pulse width modulator of the second embodiment, and each scan line is provided. 2 is a block diagram of a pulse width modulator for outputting a halftone dot image having an image forming angle by delaying the phase of the pattern signal generated by the triangular wave oscillator 121. FIG. The rest of the configuration and operation are similar to those of the second embodiment, so description thereof will be omitted.

[0051]

According to the present invention, the number of lines is made variable in accordance with the type of the visualizing means, or the type of the visualizing means and the type of the image density signal, and each visualizing is performed in the low density portion. By selecting the appropriate number of lines for the method, it is possible to visualize the stability of the gradation and color reproducibility in the low density area with respect to the environment, without the need for complicated and expensive process control and variable emission intensity devices. It can be constant regardless of the means.

[Brief description of drawings]

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

FIG. 2 is an explanatory view of the operation of the present invention.

FIG. 3 is a diagram showing a configuration of a pulse width modulator according to the first embodiment.

FIG. 4 is a diagram showing evaluation results of stability of gradation / color reproduction against environment and image quality.

FIG. 5 is a diagram showing a configuration of a pulse width modulation device according to a second embodiment.

6A is a D / A conversion characteristic of a first D / A converter, and FIG. 6B is a D / A conversion characteristic of a second D / A converter.

FIG. 7 is a diagram showing a configuration of a pulse width modulation device according to a third embodiment.

FIG. 8A is a conversion characteristic of the first LUT, and FIG. 8B is a conversion characteristic of the second LUT.

FIG. 9 is a diagram showing a configuration of a pulse width modulation device according to a fourth embodiment.

FIG. 10 is a waveform chart for explaining the operation of the fourth embodiment.

11A is a conversion characteristic of a first LUT and a D / A converter, and FIG. 11B is a conversion characteristic of a second LUT and a D / A converter.

FIG. 12 is a diagram showing a configuration of a pulse width modulation device according to a fifth embodiment.

FIG. 13 is a diagram showing evaluation results of stability of gradation / color reproduction with respect to environment and image quality.

14 (a) and 14 (b) are diagrams showing examples of response characteristics of the visualization means.

[Explanation of symbols]

31 ... D / A converter, 32-35 ... Comparator, 36 ... Triangular wave oscillator, 37 ... Waveform selection circuit.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical indication location H04N 1/405 1/46 (72) Inventor Masayo Higashi 2274 Hongo, Ebina-shi, Kanagawa Fuji Xerox stock Company (72) Inventor Masahiko Kubo 2274 Hongo, Ebina, Kanagawa Prefecture Fuji Xerox Co., Ltd. (72) Inventor Kazuhiro Iwaoka 2274, Hongo, Ebina City, Kanagawa Fuji Xerox Co., Ltd.

Claims (3)

[Claims] 1. An image forming apparatus having pulse width modulation means for pulse width modulating an image density signal using a reference wave, and image forming means for forming an image according to the pulse width modulation signal output from the pulse width modulation means. In two, two or more types of visualizing means for forming a visual image according to an image density signal, reference wave generating means for generating a plurality of reference waves with different modulation periods, and reference generated by the reference wave generating means An image forming apparatus, comprising: selecting means for selecting an image density signal output to the image forming means from an image density signal pulse-width modulated by a wave according to the type of the visualization means. 2. An image forming apparatus having pulse width modulation means for pulse width modulating an image density signal using a reference wave, and image forming means for forming an image according to the pulse width modulation signal output from the pulse width modulation means. In two, there are two or more types of visualizing means for forming a visual image according to an image density signal, and two or more cyclically operating conversion means having different characteristics for converting the image density signal. At least one of the means has a characteristic of converting the output of the image density signal to the low density portion into an image density signal of 0 to a non-visualized range, and a reference wave generation for generating a plurality of reference waves with different periods. Means and an image density signal output to the image forming means from the image density signal pulse-width modulated by the reference wave generated by the reference wave generating means, as the type of the visualization means. Flip the image forming apparatus characterized in that a selection means for selecting. 3. The image forming apparatus according to claim 1 or 2, wherein the selection unit selects according to the type of the visualization unit and the image density signal.