JP3434291B2 - Image forming apparatus and method - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an image forming apparatus and method applied to a digital copying machine or the like, and more specifically, it combines banding and image noise by combining multi-value writing by one-dot modulation with a minute matrix with less deterioration in resolution. The present invention relates to an image forming apparatus and method for realizing high-quality image formation by reducing the above.

[0002]

[Prior art]

For example, in writing processing in a digital copying machine, its resolution and gradation are important factors. Fine resolution and gradation that faithfully reproduce halftones are desired in copying processing for all originals including characters and photographs.

Conventionally, there is an area gradation method using a dither matrix as a method of expressing gradation (Japanese Patent Application Laid-Open No. 54-144126, Japanese Patent Application Laid-Open No. 56-17478, Japanese Patent Application Laid-Open No. 57-76977). It is disclosed in the bulletin etc.).

However, in the above-mentioned area gradation method, since a pixel is composed of a plurality of dots and density is expressed by the number of written dots, the resolution is lowered. In this case, assuming that the number of dots forming a pixel is N in the binary writing method, the number of gradations represents N gradations without including a white background, but the resolution is generally 1 / Drop to N.

On the other hand, a multi-gradation is realized without deteriorating the resolution 1
A dot multi-value writing method has been proposed. This is for modulating the density of one dot of writing in laser beam writing of an electrophotographic system, for example. The light modulation method of the laser diode for writing mainly includes a pulse width modulation method for modulating the exposure time and a power modulation method for modulating the exposure intensity. The above pulse width modulation method is disclosed in JP-A-62-49776 and the power modulation method is disclosed in JP-A-64-1547.

As one condition for improving the image quality of a digital copying machine, highly accurate halftone reproduction is required. In order to achieve both resolution and gradation, the 1-dot multi-value writing method is preferable.

[0007]

[Problems to be Solved by the Invention]

However, the 1-dot multi-value writing method has a drawback that banding easily occurs. In a digital copying machine, banding that occurs in a halftone region occurs due to driving unevenness and vibration of a photoconductor, scanning pitch unevenness of a writing optical system, and the like. The banding is
Appears as continuous band-shaped density unevenness in the main scanning direction. Particularly in the one-dot multi-value writing method, banding occurs due to overlapping of the skirts of the exposure beams in the intermediate exposure region due to uneven scanning pitch of the exposure laser diode in the sub-scanning direction.

[0008] Furthermore, due to the higher resolution, there is also a demand for accuracy with respect to banding. In addition, 1 at 400dpi, which is often used today
In the dot multi-value writing, in the current electrophotographic process, there is a problem that the image noise is generated due to the uneven density in the halftone solid portion regardless of the modulation method, and the halftone is not reproduced smoothly.

The present invention has been made in view of the above, and employs a method of combining a multi-valued writing by one-dot modulation with a minute matrix with less deterioration in resolution, thereby reducing banding and image noise. A first object of the present invention is to stabilize the image density and form a vertical line-based image by an inexpensive method to realize high-quality image formation. A second object is to form a high-quality image by forming continuous toner images in the main scanning direction.

[0010]

[Means for Solving the Problems]

In order to achieve the above object, the present invention provides an addition means for adding dot data of a first dot forming input image data and dot data of a second dot adjacent to the first dot. If the addition data added by the adding means is input and the signals indicating the dot period or the line period are sequentially input in a predetermined order, the addition data is smaller than a predetermined density value, the first Data obtained by converting the addition data as write data corresponding to either one of the dots and the second dot, and if the addition data is larger than the predetermined density value, the first dot or the second dot. The predetermined density value is output as write data corresponding to any one of the first dot and the write data corresponding to the other of the first dot and the second dot. Conversion means for outputting data obtained by converting the value obtained by subtracting the predetermined density value from the addition data as the embedded data; the write data corresponding to the first dot and the write data corresponding to the second dot In the case of writing by writing, the writing position of the writing data corresponding to the first dot and the writing position of the writing data corresponding to the second dot are exchanged with each other.

In addition, an addition step of adding the dot data of the first dot forming the input image data and the dot data of the second dot adjacent to the first dot, and the addition process by the addition means. When the addition data is input and the signals indicating the dot period or the line period are sequentially input in a predetermined order, and the addition data is smaller than the predetermined density value, the first dot or the second dot Data obtained by converting the addition data is output as write data corresponding to any one of the two, and if the addition data is larger than the predetermined density value, it corresponds to either the first dot or the second dot. The predetermined density value is output as the write data, and the write data corresponding to the other of the first dot and the second dot is output. And a conversion step of outputting data obtained by converting a value obtained by subtracting the predetermined density value from the addition data, and writing data corresponding to the first dot and writing data corresponding to the second dot are continuously performed. In the case of writing, a writing step of writing data corresponding to the first dot and a writing step of writing data corresponding to the second dot are exchanged.

The image forming apparatus according to the present invention operates as follows. The image forming apparatus and method according to the present invention adds the dot data of the first dot forming the input image data and the dot data of the second dot adjacent to the first dot, and the added data is When an even signal or an odd signal indicating the dot period or line period is input, write data corresponding to the even signal or the odd signal is generated, and the generated write data is used as the first dot or the second dot. When writing to the position corresponding to the dot of, the writing phase of the writing data corresponding to the even number signal or the odd number signal is controlled.
Lines are concentrated while lowering lines from 0 lines to 100 lines, and banding and image noise can be reduced by this line concentration.

Further, in the image forming apparatus and method according to the present invention, when a plurality of write data are continuously generated, a write data write position corresponding to an even signal and a write data write position corresponding to an odd signal are generated. Since and are switched alternately, it is possible to efficiently concentrate the lines with a simple configuration in which the writing positions are switched and controlled.

[0014]

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention will be described below in the order of the configuration of a digital copying machine, a modulation system of a writing laser diode, image reading signal processing, image processing, and a 2-dot multi-valued circuit with reference to the accompanying drawings.

Configuration of Digital Copying Machine FIG. 1 shows a digital copying machine including a laser printer to which general laser writing means is applied and an original reading device. In the figure, a contact glass 111 on which a read document is placed is illuminated by a light source 112, and reflected light from the image surface of the read document is reflected by mirrors 113, 114 and 115 and a lens 1.
An image is formed on the light receiving surface of the CCD image sensor 117 via 16. The light source 112 and the mirror 113 are contact glass.
The lower surface of 111 is mounted on a traveling body 118 that moves in parallel with contact glass 111.

Main scanning is performed by solid-state scanning of the CCD image sensor 117. Original image is 1 by CCD image sensor 117
The original is scanned dimensionally and the entire surface of the original is scanned by moving the optical system (sub scanning). In this example, the reading processing density is set to 400 dpi for both main scanning and sub-scanning, and a document of A3 size (297 mm × 420 mm) can be read.

Next, a laser printer constituting the above digital copying machine will be described. There are cases where the document reading device and the laser printer are integrally configured (in this embodiment), and cases where the configurations are separate and only electrically connected.

Each system such as a laser writing system, an image reproducing system, and a paper feeding system is integrally configured in the laser printer. The laser writing system includes a laser output unit 219, an image forming lens group 120, and a mirror 121, as shown in FIGS. 1, 2, and 3. A laser diode LD, which is a laser light source, is provided inside the laser output unit 219, and a writing unit is provided with a polygon mirror (polygon mirror) 219a that is rotated at a constant speed by a motor. The laser light output from the laser writing system is applied to the photosensitive drum 122 equipped in the image reproducing system.

As shown in FIG. 1, around the photosensitive drum 122,
Charging charger 123 for uniformly charging the photoconductor drum 122
An eraser 124, a developing unit 125 that visualizes the formed electrostatic latent image, a transfer charger 126 that transfers the image on the photoconductor drum 122 to the transferred transfer paper, and a photoconductor drum 122. A separation charger 127 and a separation claw 128 for separating the transfer paper, a cleaning unit 129 for cleaning the surface of the photosensitive drum 122 after the transfer processing, and the like are provided.

A beam sensor 330 that generates a main scanning synchronization signal (PMSYNC) is arranged at a position near the photosensitive drum 122 where the laser light is emitted (see FIG. 3). 131 is a conveyor belt, 132 is a fixing unit, 133 and 134 are paper feed cassettes, 135 and 136 are paper feed rollers, and 137 is a registration roller.

The operation of the above configuration will be described. The surface of the photoconductor drum 122 is uniformly charged to a high potential by the charging charger 123. When the surface of the photoconductor drum 122 is irradiated with laser light, the potential of the irradiated portion is lowered. Since the laser light is ON / OFF controlled according to the black / white of the recording pixel, the potential distribution corresponding to the recorded image, that is, the electrostatic latent image is formed on the surface of the photosensitive drum 122 by the irradiation of the laser light.

When the portion on which the electrostatic latent image is formed passes through the developing unit 125, toner adheres according to the level of the potential, and a toner image is formed by visualizing the electrostatic latent image. The recording paper is conveyed to the portion where the toner image is formed at a predetermined timing and overlaps the toner image.

After this toner image is transferred to the recording paper by the transfer charger 126, the recording paper is separated from the photoconductor drum 122 by the separation charger 127 and the separation claw 128. The separated recording paper is conveyed by a conveyor belt 131, and is thermally fixed by a fixing unit 132 containing a heater.
The paper is discharged to a paper discharge tray (not shown).

In the digital copying machine shown in FIG. 1, the paper feeding system is composed of two systems. One sheet feeding system is equipped with a sheet feeding cassette 133, and the other sheet feeding system is equipped with a sheet feeding cassette 134. The recording paper in the paper feed cassette 133 is fed by the paper feed roller 135. Also, the recording paper in the paper feed cassette 134 is the paper feed roller 1
Paper fed by 36.

The fed recording paper is temporarily stopped while being in contact with the registration roller 137, and is conveyed to the photosensitive drum 122 at a timing synchronized with the progress of the recording process. Although not shown, each paper feed system is equipped with a size detection sensor for detecting the size of the recording paper in the cassette.

Modulation Method of Writing Laser Diode FIG. 4 is a block diagram of a pulse width modulation circuit used for pulse width modulation writing of a laser diode. Particularly, in order to obtain a pulse width signal by a method using a delay line. A plurality of delay elements 450 to 453 and a logic circuit (AND circuit 454,
450, OR circuit 456, 457) and selector 458. The select signal from the selector is the LD drive circuit 459.
To enter.

Generation of a pulse width signal in the pulse width modulation circuit in FIG. 4 will be described with reference to FIG. The write clock of about 50% duty is delayed by an arbitrary set time. When obtaining a minute pulse width of 50% or less,
The write clock and its delay signal are ANDed to obtain a pulse width inversely proportional to the delay amount (FIG. 5 (a)). To obtain a pulse width of 50% or more, OR the write clock and its delayed signal
And a pulse width proportional to the delay amount is obtained (FIG. 5 (b)). These pulse widths are selected by the selector 458 based on the written pixel data, and pulse width signals continuous in the main scanning direction are obtained. As mentioned above, multiple delay elements
By setting the output signal to an appropriate value using 450 to 453, the multi-gradation pulse width modulation writing is executed.

In this embodiment, the pulse width modulation is executed by the method using the delay line, but the pulse width modulation writing by the multi-gradation laser diode may be executed by other methods.

FIG. 6 shows a circuit configuration of the LD drive circuit 459 shown in FIG. The LD drive circuit 459 includes a D / A converter 655 as shown.
, Constant current source 652, transistor 656, current / voltage conversion circuit 653, A / D converter 654 and laser diode L
It consists of D and.

In the above configuration, the amount of light emitted from the laser diode is determined in advance by the constant current source 652 of the forward current of the laser diode, and the pulse width modulation writing is executed by switching with the pulse width signal.

Image Read Signal Processing FIG. 7 shows a detailed block diagram of the image read signal processing. A CCD (charge coupled device) 117 is capable of reading approximately 5000 pixels and 400 dpi, and simultaneously reads the reflected light in the main scanning direction of the document. The optical data accumulated in CCD 117 is converted into an electric signal (photoelectric conversion), waveform correction such as clamping, amplification and A / D conversion are executed and output to IPU (image processing unit) 800 as a 6-bit digital signal. .

More specifically, the analog data output of the CCD 117 is divided into two systems, EVEN and ODD, for high-speed transfer and output, and amplified by amplifiers 702 and 703 (signal amplification),
Input to the switching IC 703 composed of analog switches. Here, it is combined with a serial analog signal (signal combination). The analog signal synthesized by the switching IC 703 is amplified (variably amplified) by the amplifier 704 and input to the A / D converter 705. The image transfer rate of one pixel after composition is about 10MHz, and the A / D converter is synchronized with this.
At 705, it is converted into a digital signal of 6-bit 64 gradations (signal digitization).

Further, in the (variable) amplifier 704, in order to correct the light quantity fluctuation of the exposure fluorescent lamp, the reference white plate is read before scanning the original and the amplification degree thereof is controlled to an appropriate value.

Image Processing A digital signal for each pixel indicating the document density is input to an IPU (image processing device) 800 and subjected to image processing. IPU800
FIG. 8 shows the flow of image processing by the. IPU is multiple LSI
In addition to image processing, the following control based on the image processing is executed.

I. Shading Correction Since the linear light source of the fluorescent lamp is used and the light is condensed by the lens, the light amount becomes maximum at the central portion of the CCD 117 and decreases at the end portions. Further, the CCD 117 has variations in sensitivity among individual devices. In both of the above, the original read data is corrected based on the reference white plate read data for each pixel.

Ii. MTF Correction In an optical system using a lens or the like, the read output by the CCD 117 is read as if it was blunted by peripheral pixel information affected by the performance of the lens or the like. Therefore, when one pixel data is obtained, correction is performed based on the peripheral pixel level to obtain an image with high reproducibility.

Iii. Main scanning direction scaling In the present embodiment, the image reading and writing resolutions are the same 400 dpi, but the reading pixel frequency is about 10 MHz and the writing pixel frequency is about 12 MHz, so Performing frequency conversion. The clock conversion is realized by reading and writing in the two-line memory, and the main scanning magnification is calculated by the operation based on the peripheral pixel data in the main scanning direction.

Iv. Γ correction In addition to the above, the IPU (image processing device) also executes control of AGC and the like, image conversion such as masking, trimming, mirroring, black and white inversion, document size and density detection, image detection of markers and the like. is doing.

This embodiment is a combination of 256-tone output of 1 dot by pulse width modulation with a matrix of 2 dots in the main scanning and sub-scanning directions.

FIG. 9A shows a 1 × 2 matrix optical writing system, and FIG. 9B shows a 2 × 1 matrix optical writing system. In the low density portion, the exposure time of one dot is increased, and when the maximum exposure time is reached, the exposure time of the next dot is increased.

When density reproduction is executed with two continuous dots in the main scanning direction, the writing of each dot may be performed with a pulse width that grows from its center, but by generating from one side, for example, the left side, 50% or more is generated. When the density, that is, the density of one dot is saturated, the two pulse widths become continuous, the effect of dot concentration is further increased, and the gradation becomes continuous. However, as will be described later, when further line concentration is performed, it is preferable that the pulse widths are formed alternately every two dots so that the generated pulse widths are continuous.

On the other hand, when performing density reproduction with 2 dots in the sub-scanning direction,
The writing of each dot is preferably performed with a pulse width that grows from the center so that there is no deviation in the main scanning direction of the dot due to the pulse width at each density.

Density reproduction is performed using two dots in the main scanning or sub-scanning direction as the target pixel. The read density of the CCD 117 is proportional to the amount of received light. Therefore, the amount of light received by the CCD 117 is linear with respect to the document reflection density, and the density data of 2 dots is added as a digital value. After that, γ conversion is applied to the added value and converted into writing density data by the above method. As a result, 512 dots with 2 dots in the main scanning and sub scanning directions
The gradation is realized.

2-Dot Multivalued Circuit FIG. 10 is a block diagram of a 2-dot multivalued circuit, in which line memories 1001 and 1002 connected in series for inputting a 6-bit signal input from a scanner and a latch 1003 are connected. ,
1004, the line memories 1001 and 1002, and the latches 1003 and 10
The adder 1005 is connected to 04 via the switches SW1 to SW4, and the ROM 1006 is connected to the adder 1005. The output of the ROM is output to the printer as an 8-bit data signal.

Hereinafter, i.1 × 2 matrix, ii.2 × 1 matrix, ii
i. It will be explained in detail by dividing it into concentrated dots. i.1 × 2 matrix When performing area gradation with 2 dots in the sub-scanning direction (1
The × 2 matrix) uses two line memories 1001 and 1002 to delay read data for two main scanning lines.
After that, the two 6-bit data are added by the adder 1005, and the 7-bit data is input to the ROM 1006 for γ conversion. In ROM1006, one table consists of 265 bytes, the first half 128 bytes is EVEN and the latter half 128 bytes is OD.
It is D data.

The first addition data is input to the address bus of the ROM 1006, and the EVEN data indicated by that address is output as write data. The same data is added on the next line, and the ODD data is output as write data from the data bus. EVEN,
ODD switching is performed in synchronization with the line cycle (PMSYNC).
After that, the process moves to the next two dots and the processing is repeated sequentially.

In the block diagram of the 2-dot multi-valued circuit shown in FIG. 10, the switch SW1 and EVEN / ODD are switched for each main scanning line, and the switches SW3 and SW4 select the data from the line memories 1001 and 1002. So that it is set to the upper side.

Ii. 2 × 1 matrix When area gradation is executed with 2 dots in the main scanning direction (2
The x1 matrix) uses two latches 1003 and 1004 to delay the read data for two dots in the main scanning direction.
Hereinafter, as in the case of the 1 × 2 matrix, addition processing, γ
The conversion process is executed and the write data is output. EVEN, ODD
Is switched in synchronization with the write dot cycle (WRITECLK). After that, the process moves to the next two dots and the processing is repeated sequentially.

In the block diagram of the 2-dot multi-valued circuit shown in FIG. 10, the switch SW2 and EVEN / ODD are switched every write clock, and the switches SW3 and SW4 select the data from the latches 1003 and 1004. Set it to the lower side.

Iii. Concentration of dots Converting the phase in writing to concentrate dots 10
When forming an image of 0 line, it is executed by dividing the switching cycle of EVEN and ODD by two. As described above, the loss of gradation information does not occur in all modes.

FIG. 11 shows an example of the γ conversion table used in this apparatus. The γ conversion table shown in FIG. 11 is one that outputs so that the copy density is almost equal to the original density (A).
There is a printer (B) that linearly outputs the input data for executing the tone check of the printer. When one of the EVEN dots reaches the maximum value up to the intermediate density, the exposure intensity of the ODD dot is increased. Thereby, the dots are concentrated while maintaining the density information of 2 dots.

Further, γ can be freely controlled by this γ conversion table, and the way of increasing 2 dots can also be changed. Further, since the density output characteristic also changes depending on the combination method with the area gradation, the γ conversion data is selected or the RAM is used for the conversion table and rewritten.

In general, the γ characteristic of the printer alone can be made linear by making the table value the inverse conversion of the γ characteristic of the printer represented by the print density with respect to the writing exposure light amount.

The 2-dot multi-valued circuit of FIG. 10 is configured in the IPU800,
Image data for each dot from the scanner is converted and output to the writing system. As a result of the above, the writing process of 512 gradations is realized by setting a unit of two dots in the main scanning direction and the sub scanning direction as one pixel.

An image at each density was output using the 2-dot multi-value writing method according to the present invention, and the effect of occurrence of banding (density unevenness in the band-shaped clothes scanning direction) was confirmed. The banding was significantly reduced in the 2-dot multi-gradation image as compared with the tonality image.

Next, a second embodiment of the present invention will be shown. Descriptions that overlap with those of the first embodiment will be omitted for simplification. FIG. 12 is a block diagram of a power modulation system of a laser diode (LD) according to a second embodiment of the present invention, in which a light emission level command signal includes a first current conversion means 1200 and a second current conversion means. It is input to 1201.

In the first current converting means 1200, the emission level command signal is
Light emission level command signal current (output current) according to its strength
Converted to I _S. Output current I _{S of} first current converting means 1200
Is the output P generated at the light receiving element 1202 of the laser diode LD1.
The input current I _S −I _L , which is the difference from the photocurrent I _L proportional to ₀ , is input to the current amplifier 1203.

The current amplifier 1203 outputs an output current A (I _S −I _L ) obtained by multiplying the input current I _S −I _L by A. On the other hand, the second current converting means 1201 converts the light emission level command signal into the output current I ₁ which causes the set light amount P _S to emit light. The sum of the output current I ₁ and the output current A (I _S −I _L ) of the current amplifier 1203, I ₁ + A (I _S −I _L ) becomes the forward current of the laser diode LD 1.

In this way, the laser diode LD1 has a forward current I
Obtain the optical output P ₀ determined by ₁ + A (I _S −I _L ). That is, the following relational expression holds. P ₀ = P {I ₁ + A (I _S −I _L )} P: optical output of laser diode LD 1 −function representing forward current characteristics

Since I ₁ is set so that I _S ≈I _L , it can be approximated as follows. P ₀ = P (I ₁ ) + [δp / δI] _{I = I 1} · A (I _S −I _L ) = P _S + η · A · (I _S −I _L ) Radiation sensitivity S of the light receiving element, laser diode LD 1 If the coupling efficiency with is δ, it is expressed as P ₀ = P _S + η ・ A ・ (I _S −P ₀・ S ・ δ), and P ₀ = {P _S / (1 + η ・ δ ・ S ・ A) } + I _S {η · A / (1 + η · δ · S · A)}.

Assuming that the crossover frequency of the photoelectric negative period loop is f ₀ , the step response of the optical output P ₀ can be approximately expressed as follows. P ₀ = I _S / δ ・ S + {P _S −I _S / δ ・ S} ・ exp (-2π ・ f ₀ t)

P _S set by the second conversion means 1201 is I _S / δS
However, if P _S fluctuates by 5% due to the droop characteristic, the time required for the error of P ₀ to be 0.4% or less even if f ₀ = 40 MHz is set. It will be about 10 ns.

Further, the total light amount (integral value of optical output ∫P _OUT ) error from immediately after changing the optical output P ₀ to the set time τ ₀ is 0.4.
When the crossover frequency f ₀ is τ ₀ = 50 ns, f ₀ ≧ 40 MHz is required, and a crossover frequency of this level can be easily realized. As explained above, this method enables high speed, high accuracy,
A high resolution laser diode control system can be realized.

By power-modulating the laser diode LD1 using this method, 256 analog signals are input to the emission level command signal, and one dot 25
Image output with 6 gradations is realized.

Next, a laser diode (LD) power modulation method according to the second embodiment using a plurality of constant current power supplies will be described. The laser diode drive control method in this embodiment is based on the forward current (I) of the laser diode shown in FIG.
And the emission intensity (L) (IL characteristic) are used.

The IL characteristic of this laser diode has a threshold current (It
It is almost linear in the forward current above h) and the differential quantum efficiency (n) at that time is treated as constant. As shown in FIG. 14, the control method is that the forward current is driven by the total current of a plurality of constant current sources 1441, 1442, 1443, 1444, and it is switched by write data to switches 1445, 1446, 1447.
Switch with. A bias current larger than the threshold current is supplied by the constant current source 1441, and the laser diode drive current is 3 bits 8 by the constant current sources 1442, 1443, 1444 weighted so that the current value becomes 1: 2: 4. To control the value. The current values at that time are I ₁ , I ₂ , and I ₃ , respectively, and the minimum bias current at which the switches 1445, 1446, and 1447 are not driven is I ₀ . Therefore, the emission intensity (light amount) by each current I _{0 to} I ₃ is as shown in FIG. 13, and the light amount by all combinations of the currents I _{0 to} I ₃ is 8 from L _{0 to} L ₇ and the difference in light amount is equal. can get.

The setting procedure at that time is executed as follows. (A) Laser diode emission intensity range is set to P _{0 to} P _max (however, P ₀ ≈0). (B) Laser diode minimum emission intensity P ₀ ← Determine the laser diode forward current I ₀ . (C) Laser diode maximum emission intensity P _max ← Laser diode forward current I ₀ + I _max determines I _max . (D) = I ₁ (1/7) · I _max , I ₂ = (2/7) · I _max , I ₃ =
(4/7) ・ I _max . From the above, if the number of constant current sources is n, a light emission intensity of 2 ⁿ can be obtained. For example, if eight constant current sources are used and switching is performed by 8-bit light emission data, 256 laser diode exposure outputs can be obtained. Is obtained.

Next, image processing in the second embodiment will be described. In the present embodiment, 256 dots of 1 dot output by laser diode power modulation are combined with a matrix of 2 dots in the main scanning and sub scanning directions.

FIG. 15 shows a 1 × 2 matrix optical writing system. In the low density area, increase the exposure power from the previous one dot,
The exposure power of the next subsequent dot having the maximum value is increased.

The density reproduction is executed by using the two dots in the sub-scanning direction as the target pixel. The read density of the CCD 117 is proportional to the amount of received light. Therefore, the amount of light received by the CCD 117 is linear with respect to the reflection density of the original, and the density data of 2 dots is added to the digital value, and the added value is subjected to γ conversion and converted into the writing density data by the above method. As a result, 512 gradations are realized with 2 dots in the main scanning direction.

The chart of the formed halftone density region occurs as shown in FIG. In the figure, the density is filled in from the EVEN dots. FIG. 16A and FIG. 16B for performing area gradation in the sub-scanning direction
The 1 × 2 matrix of is a horizontal line tone.

FIG. 16B is a diagram in which the writing phase of FIG. 16A is changed alternately, and two dot lines are formed in the main scanning direction to form an image of 100 lines. As a result, the number of gradations does not change, but the lines are concentrated and the apparent resolution is reduced to half.

Next, a 2-dot multi-valued circuit according to the second embodiment will be described. FIG. 17 is a block diagram of a 2-dot multi-valued circuit, in which line memories 1701 and 1702 connected in series for inputting a 6-bit signal input from a scanner and a switch SW1 are connected.
And an ROM 1704 connected to the adder 1703. The ROM17
The output of 04 is output to the printer as an 8-bit data signal.

The following is a detailed description, divided into i.1 × 2 matrix and ii. Dot concentration. i.1 × 2 matrix When performing area gradation with 2 dots in the sub-scanning direction (1
The × 2 matrix) uses two line memories 1701 and 1702 to delay read data for two main scanning lines.
After that, the two 6-bit data are added by the adder 1703, and the 7-bit data is input to the ROM 1704 for γ conversion. In ROM1704, one table consists of 265 bytes, the first half 128 bytes is EVEN, and the latter half 128 bytes is OD.
It is D data.

The first addition data is input to the address bus of the ROM 1704, and the EVEN data indicated by that address is output as write data. The same data is added on the next line, and the ODD data is output as write data from the data bus. EVEN,
ODD switching is performed in synchronization with the line cycle (PMSYNC).
After that, the process moves to the next two dots and the processing is repeated sequentially.

In the block diagram of the 2-dot multi-valued circuit shown in FIG. 17, the switches SW1 and EVEN / ODD are switched for each main scanning line.

Ii. Concentration of dots Converting the phase in writing to concentrate dots 10
When forming an image of 0 line, the switching cycle of EVEN and ODD is performed by dividing the line cycle (PMSYNC) by two. As described above, the loss of gradation information does not occur in all modes.

An image at each density was output by using the 2-dot multi-value writing method according to the present embodiment, and the effect of occurrence of banding (band-shaped density unevenness in the sub-scanning direction) was confirmed.

For image output, banding is forcibly generated by adding 1% rotational speed unevenness at a 2 mm pitch of the photosensitive drum.
In the sensory evaluation, as shown in FIG. 13, the banding was significantly reduced in the 2-dot multi-gradation image as compared with the 1-dot multi-gradation image by the power modulation of the laser diode.

Next, a third embodiment of the present invention will be described. For simplification, the same parts as those in the above-mentioned embodiment are
The description is omitted. In this embodiment, one dot 256 gradation output by pulse width modulation is combined with a matrix of 2 dots in the main scanning and sub scanning directions.

FIG. 9A shows a 1 × 2 matrix optical writing system, and FIG. 9B shows a 2 × 1 matrix optical writing system. In the low density area, the exposure time of one dot is increased, and when the maximum exposure time is reached, the exposure time of the next dot is increased.

When density reproduction is executed with two continuous dots in the main scanning direction, each dot may be written with a pulse width that grows from the center of the dot. When the density, that is, the density of one dot is saturated, the two pulse widths become continuous, the effect of dot concentration increases, and the gradation becomes continuous. However, when performing line concentration further, make sure that the generated pulse width is continuous.
It is preferable that the pulse width is formed alternately for each dot.

A write pulse generating circuit in this case is shown in FIG. In FIG. 18, a sawtooth wave generation circuit 1801 composed of a constant current source and a charger / discharger, a D / A converter 1802 for converting write multivalued data into analog data, and the sawtooth wave generation circuit 1801. From the D / A converter 1802 and a comparator 1803 for comparing the output from the D / A converter 1802.

Generation of a pulse signal by the write pulse generating circuit shown in FIG. 18 is shown by signals at output points A, B and C in FIG. 18th
With the write pulse generating circuit shown in the figure, a pulse signal for each dot is generated from the left side of the dot writing position. That is, 2
With continuous output of dot processing, the pulse width signal becomes continuous,
Two pixels in the main scanning direction form one pixel. The state of dot formation is shown in FIG.

On the other hand, when the density reproduction is performed with two dots in the sub-scanning direction, the writing of each dot has a pulse width that grows from its center so that there is no deviation in the main scanning direction of the dot due to the pulse width at each density. Is better. The pulse width signal generating method in this case is executed using the delay line shown in FIG.

It can also be executed by the pulse generating circuit shown in FIG. The pulse generation circuit of FIG. 21 is configured by replacing the sawtooth wave generation circuit 1801 shown in FIG. 18 with a triangular wave generation circuit 2101. As a result, the writing dot grows from its center as shown in FIG.

Further, when the phase conversion is performed as described above, dots at both ends of the pixel, that is, dots for which density conversion within one dot is performed by pulse width modulation, are pulsed so that the pulse signal becomes continuous. Generate from the center side. The state of pixel formation is shown in FIG. The pulse generation circuit is provided by providing a sawtooth wave generation circuit 1801 shown in FIG. 18 and a sawtooth wave generation circuit having an opposite phase and alternately selecting them. As a result, the pulse width located on the right side of the pixel is formed on the left side of the dot and the pulse width located on the left side of the pixel is formed on the right side of the dot, and the pulse width signals are continuous.

The state of the pulse width signal is shown in FIG. The density is reproduced by using two dots in the main scanning or sub-scanning direction as the target pixel. The read density of the CCD 117 is proportional to the amount of received light. Therefore, the amount of light received by the CCD 117 is linear with respect to the document reflection density, and the density data of 2 dots is added as a digital value. After that, γ conversion is applied to the added value and converted into writing density data by the above method. As a result, 512 dots with 2 dots in the main scanning and sub scanning directions
The gradation is realized.

The chart of the formed halftone density region occurs as shown in FIG. In the figure, the density is filled in from the EVEN dots. FIGS. 24 (a) and 24 (c) which execute area gradation in the sub-scanning direction.
The 1 × 2 matrix of 2 is a horizontal line tone, and the 2 × 1 matrix of FIGS. 24 (b) and 24 (d), which performs area gradation in the main scanning direction, is a vertical tone.

FIGS. 24 (c) and 24 (d) are FIGS. 24 (a) and 24 (b), respectively.
Alternately, the writing phase is changed alternately, and two dot lines are formed in the main scanning and the sub-scanning to form an image of 100 lines. As a result, the number of gradations does not change, but the lines are concentrated,
Apparent resolution is halved.

The 2-dot multi-gradation image is smoother with less density unevenness in the halftone region than the 1-dot multi-gradation image. Further, the density decrease, the halftone trailing edge whiteout, etc., which occur at the boundary between gradations, are improved. In the 1-dot multi-gradation image, there is no regularity in how toner is attached, which causes the half-tone image to look noisy. Obviously reproduced, this looks visually smooth.

Further, in the processing of a plurality of dots, for example, 3 dots, banding is eliminated, the density is stable, and the halftone is reproduced smoothly. The multi-gradation writing method using a plurality of dots according to this method is an image processing method that can be used regardless of the laser diode modulation method such as power modulation or pulse width modulation other than this embodiment.

[0093]

【The invention's effect】

As described above, according to the present invention, the dot data of the first dot forming the input image data and the first dot data
Data is added to the dot data of the adjacent second dot, and the added data is input, and when the signals indicating the dot period or the line period are sequentially input in a predetermined order, the addition data Is smaller than the predetermined density value, the data obtained by converting the addition data as the write data corresponding to either the first dot or the second dot is output, and if the addition data is larger than the predetermined density value, A predetermined density value is output as write data corresponding to one of the first dot and the second dot, and a predetermined density is calculated from the addition data as write data corresponding to the other of the first dot and the second dot. The data obtained by converting the value obtained by subtracting the value is output, and the write data corresponding to the first dot and the write data corresponding to the second dot are output. When writing continuously, the writing position of the writing data corresponding to the first dot and the writing position of the writing data corresponding to the second dot are configured to be exchanged, so that, for example, 200 lines are changed to 100 lines. It is possible to reduce lines and concentrate lines, thereby reducing banding and image noise.

Further, according to the present invention, it is possible to efficiently concentrate lines with a simple configuration in which the writing positions are exchanged and controlled.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a configuration of a digital copying machine to which an image forming apparatus according to the present invention is applied.

FIG. 2 is an explanatory diagram showing a configuration of a laser writing system in the digital copying machine shown in FIG.

3 is an explanatory diagram showing a configuration of a laser writing system in the digital copying machine shown in FIG.

4 is a block diagram showing a configuration of a pulse width modulation circuit used in the digital copying machine shown in FIG.

5 is an explanatory diagram showing generation of a pulse width signal by the pulse width modulation circuit shown in FIG.

[Figure 6] It is a circuit diagram which shows the drive circuit of a laser diode.

FIG. 7 is a block diagram illustrating each unit that executes image reading signal processing.

FIG. 8 is a block diagram showing a flow of image processing by the image processing apparatus.

FIG. 9 is an explanatory diagram showing an optical writing method of a 1 × 2 matrix and a 2 × 1 matrix, respectively.

FIG. 10 is a block diagram of a 2-dot multi-valued circuit used in the image forming apparatus according to the present invention.

FIG. 11 It is a table showing 2-dot multi-valued γ conversion.

12 is a block diagram showing a power modulation method of a laser diode (LD) used in the digital copying machine shown in FIG.

FIG. 13: Forward current (I) and emission intensity (L) of laser diode
It is a graph which shows the relationship (IL characteristic) with.

FIG. 14 It is a circuit diagram which shows the control system of a laser diode.

FIG. 15 It is explanatory drawing which shows the optical writing system of 1x2 matrix.

FIG. 16 It is a chart figure which shows the halftone density area | region formed.

FIG. 17 is a block diagram of a 2-dot multi-valued circuit according to the second embodiment.

FIG. 18 It is a block diagram which shows the structure of a pulse signal generation circuit.

19 is an explanatory diagram showing signal waveforms at various parts of the pulse signal generation circuit shown in FIG. 18.

20 is an explanatory diagram showing a state of dot formation by the pulse signal generation circuit shown in FIG.

FIG. 21 is a block diagram showing the configuration of another pulse signal generation circuit.

22 is an explanatory diagram showing a state of dot formation by the pulse signal generation circuit shown in FIG. 21. FIG.

FIG. 23 It is explanatory drawing which shows the state of pixel formation.

FIG. 24 It is a chart figure which shows the halftone density area | region formed.

[Explanation of symbols]

117 …… CCD image sensor 122 …… Photosensitive drum 219 ... Laser output unit 330 …… Beam sensor 450〜453 …… Delay element 454,455 …… AND circuit 456,457 …… OR circuit 458 …… Selector 459 …… LD drive circuit 652 ... Constant current source 702,704 …… Amplifier 703 ...... Switching IC 705 …… A / D converter 800 …… IPU (Image processing unit) 1001,1002,1701,1702 ...... Line memory 1003,1004 …… Latch 1005,1703 …… Adder 1006,1704 …… ROM 1200 ...... first current conversion means 1201 …… Second current conversion means 1202 …… Light receiving element 1203 ... Current amplifier 1441-1444 …… Constant current source 1445 to 1447 …… Switch 1801 …… Sawtooth wave generator 1802 …… D / A converter 1803 …… Comparator 2101 ... Triangle wave generator

   ─────────────────────────────────────────────────── ─── Continued front page       (56) Reference JP-A-2-111573 (JP, A)                 JP-A-64-47546 (JP, A)                 JP-A-2-212172 (JP, A)                 JP-A-2-287469 (JP, A)                 JP-A-3-217168 (JP, A)                 JP 62-101175 (JP, A)                 JP-A-60-240277 (JP, A)                 JP 62-233981 (JP, A)

Claims (2)

(57) [Claims] 1. An addition unit for adding dot data of a first dot forming input image data and dot data of a second dot adjacent to the first dot; Added data is entered,
Further, when the signals indicating the dot period or the line period are sequentially input in a predetermined order, if the addition data is smaller than the predetermined density value, it corresponds to either the first dot or the second dot. Data obtained by converting the addition data is output as write data, and if the addition data is larger than the predetermined density value, the predetermined density is obtained as write data corresponding to either the first dot or the second dot. A conversion unit that outputs a value and outputs data obtained by converting a value obtained by subtracting the predetermined density value from the addition data as write data corresponding to the other of the first dot or the second dot; The write data corresponding to the first dot and the second data
Control means for switching the write position of the write data corresponding to the first dot and the write position of the write data corresponding to the second dot when the write data corresponding to the dot is continuously written. An image forming apparatus characterized by the above. 2. An adding step of adding dot data of a first dot forming input image data and dot data of a second dot adjacent to the first dot; and adding by the adding means. Added data is entered,
Further, when the signals indicating the dot period or the line period are sequentially input in a predetermined order, if the addition data is smaller than the predetermined density value, it corresponds to either the first dot or the second dot. Data obtained by converting the addition data is output as write data, and if the addition data is larger than the predetermined density value, the predetermined density is obtained as write data corresponding to either the first dot or the second dot. A conversion step of outputting a value and outputting data obtained by converting a value obtained by subtracting the predetermined density value from the addition data as write data corresponding to the other of the first dot or the second dot; The write data corresponding to the first dot and the second data
When the write data corresponding to the dot is continuously written, the write data corresponding to the first dot and the write data corresponding to the second dot are exchanged with each other. An image forming method characterized by the above.