JPS63293062A - Image forming device - Google Patents

Abstract

PURPOSE:To enable a density in a highlight part or a high-density part in a laser beam printer output to be stably formed, by using an error dispersing method for a laser beam printer, etc., utilizing an electrophotographic method. CONSTITUTION:An 8-bit input image signal A having a general 256-gradation is converted to an 8-bit output image signal H suitable for a PWM-method laser beam printer in an error dispersing circuit 200. Moreover, the image signal B is inputted to a PWM circuit 400 and sent to an image forming part 300 through a laser driver circuit 500 and a semiconductor laser element 501, thus being formed to be a high-gradation image by an electrophotographic method in the image forming part 300.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus that is capable of substantially stably forming a density in a highlight part or a high density part of a laser beam printer output. Regarding.

[Prior Art] There is a laser beam printer that uses an electrophotographic method as a high-speed and low-noise printer. Its typical use is binary recording of characters, figures, etc. In this respect, since halftones are not required for recording characters, figures, etc., the structure of the printer can be simplified. By the way, there are printers that can form halftones even with such a binary recording method. As such printers, printers employing a dither method, a density pattern method, etc. are well known. However, as is well known, high resolution cannot be obtained with printers that employ the dither method or the density pattern method. Therefore, in recent years, a printer has been developed that uses a binary recording method but forms halftones by pulse width modulating (PWM) a laser beam with an image signal. This PW
According to the M method, images with high resolution and high gradation can be formed, and these conditions are essential for color image formation.

However, various new problems arise in PWM type laser beam printers. One is related to the problem of unstable formation concentration inherent in electrophotographic technology itself, and the other is related to pulse width modulation of a laser beam.

This will be explained in detail below.

FIG. 7 is a diagram showing the electrophotographic printer mechanism adopted in the apparatus of this embodiment. This printer mechanism section has axis 3
A photosensitive drum 301 as an image carrier rotated in the direction of the arrow about 06, a charger 302 and a developing device 3 arranged around the photosensitive drum 301 in the rotational direction of the drum.
03, transfer discharge device 304, and cleaning device 305
and a laser beam scanner arranged above the photosensitive drum 301 in the drawing.

Further, this laser beam scanner has a semiconductor laser section 30.
6, a polygon mirror 307 that rotates at high speed, an f-theta lens 308, and a light shielding plate. Upon receiving the input of a series of digital pixel signals, a PWM-modulated laser beam is oscillated in accordance with the signal, and the drum surface passing between the charger 302 and the developer 303 is scanned in the direction of the generatrix of the drum. to expose the drum surface.

However, in general, the characteristics of the photosensitive material of the photosensitive drum 301, such as its exposure sensitivity and residual potential, change due to changes in the surrounding environment and the passage of time. Furthermore, the developer density of the developer in the developing device 303 changes greatly due to changes in the amount of charge, etc. of the developer. These problems are density instability problems inherent in the electrophotographic technology itself, but they also greatly affect the formation of minute density by PWM laser beam printers.

Furthermore, as shown in FIG. 9, for example, there is a problem that the relationship between the forward current of the semiconductor laser and the laser output depends on the ambient temperature.

For this reason, various methods have been proposed for obtaining stabilized gradation images by appropriately controlling these fluctuation factors.

For example, Japanese Patent Publication No. 43-16199, Japanese Patent Publication No. 53-93
There are many known examples such as No. 030 and JP-A-50-119639. A detailed explanation of these will be omitted, but no matter which proposal is made, the problem of delicate density formation in highlight areas or high density areas, which the present invention aims to solve, remains.

Figure 5 is a circuit diagram of the PWM circuit adopted in the device of this embodiment.
FIG. 6 is a circuit diagram of the laser driver circuit employed in the device of this embodiment, and FIG. 8 is a timing chart showing the operation of the PWM circuit. In FIG. 5, 401 is a TTL latch circuit that latches an 8-bit image signal, and 402 is a TTL latch circuit.
403 is an ECL D/A converter, 404 is an ECL comparator that generates a pwM signal, and 405 is an ECL comparator that converts the logic level to a high-speed ECL logic level.
A level converter converts a CL logic level to a TTL logic level, 406 is a clock oscillator that generates a clock signal 2f with twice the frequency of the pixel clock signal f, and 407 generates a substantially ideal triangular wave signal in synchronization with the clock signal 2f. The triangular wave generator 408 has ECL logic circuits arranged everywhere in order to operate the 0 circuit, which is a 1/2 frequency divider that divides the clock signal 2f by 1/2, at high speed.

The operation of this configuration will be explained with reference to FIG.

Signal (2) indicates a clock signal 2f, and signal (2) indicates a pixel clock signal f with a period twice that of the clock signal 2f, which is related to the pixel number as shown in the figure. Also, inside the triangular wave generator 40f, in order to keep the duty ratio of the triangular wave signal at 50%, the clock signal 2f is divided by -H1/2, and then the triangular wave signal
is occurring. Further, this triangular wave signal (2) is converted to an ECL level (0 to -IV) to become a triangular wave signal (2).

On the other hand, the image signal is 2 from OOH (white) to FFI ((black).
Changes at 5651 tone levels. Symbol H is a hexagonal representation. The image signal 0 indicates the ECL voltage level obtained by D/A converting several image signal values. For example, the first pixel is FFH at the black pixel level, the second pixel is 80H at the halftone level, and the 3 pixels are halftone level 4
The fourth pixel 0H1 shows each voltage of 20H at the halftone level. The comparator 404 compares the triangular wave signal (2) and the image signal (2) to generate a pwM signal having a pulse width T, t, , t, , t4, etc. according to the pixel density to be formed. This PWM signal is then converted to a TTL level of Ov or 5V, becomes a PWM signal ■, and is input to the laser driver circuit 500.

In FIG. 6, 500 is a constant current type laser driver circuit, and 501 is a semiconductor laser element.

This semiconductor laser element 501 emits a laser beam when the switching transistor 502 is ONL/, and stops the laser beam when the transistor 502 is 0FFL/. The switching transistor 502 forms a current switch circuit together with the transistor 504 paired with it, and switches a constant current to the semiconductor laser element 501 according to the input PWM signal 0 to ON10.
FF (commutation) control. This constant current is supplied from a constant current source transistor 505, and the constant current value is variable, and the input 8-bit laser power value is converted into an analog voltage by a D/A converter 503.
A constant current value is determined based on comparison with a reference voltage.

However, the response characteristics of this laser beam have the following problems. In FIG. 8, if the maximum light emitting time per pixel is T (sec), ideally when the PWM signal changes the pulse width between ONT (sec), the laser beam of the semiconductor laser element 501 also pulses Only the width section should emit light. However, in reality, due to the semiconductor laser element 501 and its drive circuit 500, a response delay in emitting/stopping the laser beam as shown in waveform (3) occurs between the PWM signal O and the pulse width T and Although it is fine in the case of t2, the laser beam cannot be completely turned on in the case of the pulse width t3, and the semiconductor laser element 501 does not actually operate in the case of the pulse width t4. Beam effect (2) shows the emission state of the laser beam two-dimensionally. Since the first pixel is black, the laser beam is 1
ONL during the entire period of the pixel. However, when the pulse width of the PWM signal is extremely short, such as 13 sslons, there is not only the problem of whether or not the laser beam is actually generated, but even if the beam is barely emitted, it cannot be used with electrophotography. We have entered an extremely unstable region in terms of image formation, and we can no longer hope for stable density formation.In this way, there is a limit to the minimum pulse width that can be used to create gradation using the PWM method. If the limit is set to 13 = 10 nS, all gradations with pulse widths below this (highlight areas) will be white, and conversely, with regard to gradations in high density areas, the pulse width will be , becomes sufficiently long, but the gap between it and the adjacent pixel becomes extremely short, so that all black above a certain density becomes black.

[Problems to be Solved by the Invention] The present invention eliminates the above-mentioned problems of the prior art, and its purpose is, for example, to reduce the density in highlight areas or high-density areas of a laser beam printer output. An object of the present invention is to provide an image forming apparatus that can form an image substantially stably.

[Means for Solving the Problems] In order to solve the above problems, the image forming apparatus of the present invention has the following features:
Storage means for storing error signal E and input image signal A
an addition means for adding the error signal E to a predetermined threshold C; a comparison means for comparing the addition output of the addition means with a predetermined threshold value C; and when the comparison by the comparison means satisfies the condition (D<C), the addition output is is stored in the storage means to set the content of the output image signal B to be less than or equal to the predetermined threshold value C (for example, 0);
Further, when the condition (D<C) is not satisfied, the added output is made into an output image signal B, and the control means sets the content of the storage means to a predetermined initial value (for example, O); The outline of the present invention is to include a pulse signal generating means for generating a PWM signal in accordance with B, and a light beam generating means for turning on/off a light beam according to the PWM signal generated by the pulse signal generating means.

Preferably, one aspect of the predetermined threshold value C is that the predetermined threshold value C corresponds to a minimum image signal value at which the light beam of the light beam generating means can achieve a substantial on or off state.

[Operation] In this configuration, the storage means stores the error signal E. The addition means adds the error signal E to the input image signal A. The comparison means compares the addition output from the addition means with a predetermined threshold value C. Preferably, the predetermined threshold value C corresponds to the minimum image signal value at which the light beam of the light beam generating means described below can achieve a substantial on or off state. Then, when the comparison by the comparison means satisfies the condition (D<C), the control means stores the addition output in the storage means to make the content of the output image signal B equal to or less than the predetermined threshold C, and ( D
When the condition <C) is not satisfied, the added output is made into the output image signal B, and the contents of the storage means are set to a predetermined initial value. Further, the pulse signal generating means generates a PWM signal according to the output image signal B. The light beam generating means turns on/off the light beam according to the PWM signal generated by the pulse signal generating means.

[Embodiments] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a block diagram of a laser beam printer according to an embodiment of the present invention. In FIG. PWM circuit, 500 is the 6th
The laser driver circuit in the figure, 501 is a semiconductor laser element,
300 is an image forming section shown in FIG. 7, and 301 is a photosensitive drum.

In this configuration, for example, an 8-bit input image signal A having a normal 256 gradation level is converted in the error diffusion circuit 200 into an 8-bit output image signal B suitable for a PWM laser beam printer. Furthermore, this image signal B is PW
Input to M circuit 400, laser driver circuit 500,
A high-gradation image is formed by electrophotography in the image forming section 300 via the semiconductor laser element 501.

3(A) and 3(B) are diagrams showing the relationship between the input image signal A and the output image signal B of the error diffusion circuit, and FIG. 3(A)
indicates the density of each pixel to the human image signal. This is, for example, an image signal having normal gradation characteristics. Further, FIG. 3(B) shows the density of each pixel of the output image signal B after error diffusion by the error diffusion circuit 200.
This is an image signal having gradation characteristics suitable for an M-scheme laser beam printer.

For convenience of explanation, the minimum pulse width of PWM with which the printer of this embodiment can form a stable density is, for example, t, = 1.
Assume that it is 0 ns, and that the density threshold value set correspondingly is (32). Here (32) is 32 in decimal
shows.

In FIG. 3(A), the density of the first human-powered pixel Gll is (10), so (10)<(32), and even if this pixel is PWM-converted, the density corresponding to pixel G11 will actually be A visible image cannot be formed. Therefore, in this case, the density of the output pixel Gll in FIG. In this embodiment, the pixel is postponed to the next pixel.
Note that the density of the above output pixel Gll is set to (32) other than (0).
It may be set to a smaller value, since it will not be printed substantially.

Next, the density of the human pixel Gljl is (10).

If the postponed error density (10) is added to this, the provisional density of the input pixel G12 becomes (20).

However, this still holds (20) < (32), which can be expressed as P
Even with WM conversion, an effective visible image cannot be formed. Therefore,
Printing is performed with the density of the output pixel GI2 set to (0), and the cumulative error density (20) generated in this printing is postponed.

Next, the density of the input pixel aSS is (18).

Adding the postponed cumulative error density (20) to this, the tentative density of input pixel GI3 becomes (38).At this point, (38)≧(32), so if this is PWM converted, the printer can A visible image of density (38) can be accurately formed. Therefore, in this case, the density of output pixel G13 is set to (3
8) and set the cumulative error density diffused by this print to (0).This means that the cumulative error density has been adjusted within the minute area of print pixels Gll to G13, and this can be seen macroscopically. From a physical perspective, this means that a substantially appropriate concentration has been formed. Moreover, each error density in the example is accumulated unless it is diffuse printed, so
After all, there is a high possibility that it will be adjusted within a small area.
The same applies to the input pixel G14°GI6.

Note that the cumulative error density may be set to a predetermined initial value other than (0). This allows other types of density correction to be performed.

Next, the density of input pixel G18 is (60).

Furthermore, since the postponed error density is (0) at this point, the density of the input pixel G16 becomes (60). Therefore, since (60)≧(32), a visible image of density (60) can be accurately formed on the spot by PWM conversion. Therefore, the density of the output pixel GI8 is printed at (60), and this printing does not generate an error density that should be diffused. Next input pixel Gl? The same applies to G-th 1.

FIG. 2 is a block diagram showing details of the error diffusion circuit 200 of the embodiment. In the figure, 201 is an adder (AD
D), 202 and 203 are registers, 204 is a comparator circuit (CMP), and 205 is an inverter circuit.Although not shown, ECL logic circuits are employed everywhere for high-speed processing here as well.

In this configuration, first, the density (10
) is input to the input terminal A of the adder circuit 201. The contents of the register 202 (error density) are initially reset and hold the density (0). As a result, a temporary density (10) is output to output terminal 0 of the adder circuit 201. This provisional density (10) is supplied to registers 202, 203 and CMP 204. In the CMP 204, the tentative density (10) at the input terminal C is compared with the predetermined threshold value (32) at the input terminal C. The output terminal (D<C) of the CMP 204 outputs a logic level when the condition D<C is satisfied.

At present, since the condition D<C is satisfied, a logic level is output to the output terminal (D<C). The logic level signal enables the data LOAD function to register 202. Further, the level of the logic level signal is inverted by the inverter circuit 205, and the RESET signal of the register 203 is inverted.
enable functionality. As a result, a certain clock signal CL
When K rises, the register 202 holds the temporary density (10) of the addition output as the error density, and the register 203 holds the output density (0) instead. Therefore, the human pixel Gll is printed with an output density (0).

Next, the density (10) of the input pixel G12 is determined by the addition circuit 201.
Enter. At this point, the register 202 has the error density (
10) is maintained. As a result, the provisional density (20) is output to the output terminal 0 of the adding circuit 201. Furthermore, this provisional density (20) is stored in registers 202, 203 and CMP2.
04. The CMP 204 compares the temporary density (20) with the threshold value (32). Even at this point, since the condition D<C is satisfied, the logic level signal at the output terminal (D<C) enables the data LOAD function to the register 202 and the RESET function of the register 203. As a result, when the next clock signal CLK rises, the register 202 returns the cumulative error concentration (20) of the addition output.
The register 203 holds the output density (0). Therefore, the input pixel GI2 is also printed with the output density (0).

Next, the density (18) of the human pixel GI3 is added to the addition circuit 201.
Enter. At this point, register 202 holds the cumulative error density (20). As a result, the adder circuit 20
The provisional density (38) is output to the output terminal 0 of 1. Furthermore, this temporary density (38) is applied to registers 202, 203 and CM.
Supplied to P204. CMP204 is provisional concentration (38)
and the threshold value (32) are compared. At this point, DE
Since the condition C is not satisfied, the logic O level signal at the output terminal (D<C) enables the RESET function of the register 202 and enables the data LOAD function to the register 203. As a result, when the next clock signal CLK rises, the register 202 holds the error density (0), and instead the register 203 holds the output density (3) of the addition output.
8) Retain. Therefore, the input pixel GI3 has an output density (
38) and the cumulative error was diffused. The same applies to the subsequent input pixel G 14+G Is.

Next, the density (60) of the input pixel G18 is determined by the addition circuit 201.
Enter. At this point, the register 202 has the error density (
0) is maintained. As a result, the provisional density (60) is output to the output terminal 0 of the adding circuit 201. Furthermore, this temporary density (60) is stored in registers 202, 203 and CMP 20.
4. The CMP 204 compares the temporary density (60) with the threshold value (32). At this point, the condition of D<C is not satisfied, so the logic of the output terminal (D<C) is
The level signal enables the RESET function of register 202 and the data LOAD function to register 203. As a result, when the next clock signal CLK rises, the register 202 holds the error density (0), and the register 203 holds the output density (60) of the addition output.

Therefore, the human pixel G18 is printed as is at the output density (60). Continued human power pixel GL? The same applies to the order of ~Gt.

[Other Examples] In the above-mentioned example, the minimum pulse width t, m1ons (!threshold value 32゛) of PWM that can form a visible image is fixed, but the invention is not limited to this, and even finer density control can be achieved by variably controlling this. can be done.

Furthermore, although the explanation has been given using a laser beam printer using electrophotography as an example, it goes without saying that the present invention is applicable not only to laser beam printers but also to thermal printers.

Further, in the above embodiment, a case has been described in which the error density is postponed, but the invention is not limited to this; in fact, when digital control is performed by a computer, error diffusion to a plurality of peripheral pixels can be easily performed.

FIGS. 4(A) and 4(B) are diagrams for explaining the operating principle of the two-dimensional error diffusion method. FIG. 4(A) shows a case where excessive error density in printing of a certain pixel is distributed to multiple surrounding pixels.In the main scanning direction, column numbers 1°2, . . . , and row numbers 1゜2, .
) is fixed. Now, focusing on input pixel G22, when this pixel density is (192), (192)>(128)
Since this satisfies the following, this output pixel signal is turned ON, and printing is performed with black density (for example, 256 density). However, since the original density of pixel G22 is (192), if this is printed at 256 density, an over-density error of 256-192=(64) will occur for this pixel G22. Therefore, this density error can be calculated by, for example, the surrounding 4
Pixel G23. It is diffused into Gs+, Gsz, and Gss to form an appropriate density for the entire image. Specifically, for example, an error of 64/4=(16) is diffused into the pixel G23. That is, (16) is subtracted from the original density (for example, 198) of pixel G23. This is because the error density (16) has already been distributed to the pixel G23. The remaining three pixels Gsh, G32. The same applies to G33. Then, at the timing of printing pixel G23, (1
82)>(128), and since the condition is satisfied, this output pixel signal is turned ON, and pixel G2! is also printed with black density.

Then, a new error 25 occurred in printing pixel G2s.
6-182- (174) is divided and the surrounding 4 pixels G24. G sz, G ss, G s4
spread to.

In this way, by sequentially shifting the pixel of interest in the column direction and then shifting the pixel of interest in the row direction, errors for all pixels are made appropriate, and even a conventional binary printer can express halftones.

Moreover, it has the advantage that the resolution does not deteriorate.

FIG. 4(B) shows a case where a density error that is insufficient in a certain pixel is distributed to surrounding pixels. Similarly,
Focusing on input pixel G22, when this pixel density is (64), since (64)>(128) is not satisfied, this output pixel signal is turned OFF and printed with white density (for example, O density). . However, the original density of pixel G22 is (
64), so if this is printed with O density,
For this pixel G2□, an error of insufficient density of 0-64=(-64) has occurred. Therefore, this density error is changed in the same manner as above to calculate the density error of the surrounding four pixels G23. G
31. G32. Diffusion into G ss. In this case, (16) is added to the original density of pixel G230.

[Effects of the Invention] As described above, according to the present invention, by using the error diffusion method in a laser beam printer using electrophotography, the inherent instability of electrophotography or the printer's A stable laser beam printer can be obtained by eliminating problems such as instability in high-speed response of the laser and the laser driver circuit 500 that drives the laser due to increased speed.

[Brief explanation of drawings]

FIG. 1 is a block configuration diagram of a laser beam printer according to an embodiment of the present invention, FIG. 2 is a block diagram showing details of an error diffusion circuit 200 according to an embodiment, and FIGS. 3(A) and (B) are error diffusion A diagram showing the relationship between the human input image signal A and the output image signal B of the circuit, Figures 4 (A) and (B) are diagrams for explaining the operating principle of the two-dimensional error diffusion method, and Figure 5 is the present example. A circuit diagram of the PWM circuit adopted in the device, FIG. 6 is a circuit diagram of the laser driver circuit adopted in the device of this embodiment, and FIG. 7 is a diagram showing the electrophotographic printer mechanism adopted in the device of this embodiment. FIG. 8 is a timing chart showing the operation of the PWM circuit, and FIG. 9 is a graph showing the characteristics of an example of a semiconductor laser. In the figure, Zoo...error diffusion circuit, 400-...PWM
Circuit, 500... Laser driver circuit, 501...
Semiconductor laser element, 300... Image forming section, 301...
・It is a photosensitive drum. Upper world l -111→- Main review □

Claims (2)

[Claims] (1) Storage means for storing the error signal E; Addition means for adding the error signal E to the input image signal A; Comparison means for comparing the addition output D by the addition means with a predetermined threshold C; and the comparison means When the comparison by (D<C) satisfies the condition (D<C), the added output D is stored in the storage means to make the content of the output image signal B equal to or less than the predetermined threshold value C, and (D<C).
control means for converting the addition output D into an output image signal B and setting the contents of the storage means to a predetermined initial value when the condition C) is not satisfied; and generating a PWM signal in accordance with the output image signal B. An image forming apparatus comprising: a pulse signal generating means for generating a light beam; and a light beam generating means for turning on and off a light beam according to a PWM signal generated by the pulse signal generating means. (2) The predetermined threshold value C corresponds to the minimum image signal value at which the light beam of the light beam generating means can realize a substantial on or off state. Image forming device.