JP2012245772A - Image processor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the influence of periodicity of a modulation signal for use in the generation of a spread spectrum clock on an image when the image is drawn using a signal obtained by demodulating the spread spectrum clock using image data.SOLUTION: An image processor includes a BD signal generating unit for generating a BD signal that functions as a horizontal synchronization signal and a clock generating unit 301 for generating a spread spectrum clock CLK 2. The clock generating unit 301 has a structure in which a modulation signal generating unit 41 is added to a PLL circuit. The modulation signal generating unit 41 generates a modulation signal SG1 such that a period T1 of the modulation signal SG1 and a period T2 of the BD signal satisfy the following formula: T2=T1×(n+0.5), wherein (n is an integer number).

Description

  The present invention relates to an image processing apparatus using a spread spectrum clock generator as a clock generation apparatus.

  As a countermeasure against electromagnetic interference (EMI: Electro-Magnetic Interference) due to electromagnetic waves radiated from electronic equipment, a spread spectrum clock generator (SSCG) is known. In electronic equipment, a clock is used to take various timings, and the clock is a main cause of EMI. The SSCG spreads the frequency spectrum of the clock by generating a clock whose frequency slightly varies. As a result, the peak value of the frequency spectrum of the clock is lowered to reduce electromagnetic noise.

  An image forming apparatus using SSCG as a clock generation apparatus has been proposed (see Patent Document 1). In this image forming apparatus, the frequency of the clock is changed in the same pattern with reference to a BD (Beam Detect) signal, and the clock is used to control the laser beam for drawing the electrostatic latent image on the photosensitive drum. The laser spot diameter and the laser spot interval are made uniform.

JP 2009-292083 A (FIG. 7)

  The spread spectrum clock is a clock whose frequency varies according to the voltage of the signal obtained by adding the voltage of the modulation signal and the voltage of the DC signal from the low-pass filter. Since the spread spectrum clock is generated using the modulation signal, the clock waveform is affected by the periodicity of the modulation signal. Therefore, when drawing an image with a signal obtained by modulating the spread spectrum clock with image data, the influence of the periodicity of the modulation signal appears as a shift in the drawing position of the pixel, which causes interference and causes the image to become regular. Shading occurs.

  FIG. 11 is a diagram showing the relationship between the regular gradation generated under the influence of the periodicity of the modulation signal SG1 and the modulation signal SG1. The horizontal pixel line PL is a line of pixels arranged in the main scanning direction D1 among the pixels constituting the image. By drawing one scanning line, one horizontal pixel line PL is formed. Reference numeral P1 represents a regular thin portion with light and shaded portions, and reference numeral P2 represents a dark portion with regular light and shade. In the horizontal pixel line PL, the light and dark portions P1 and the dark portions P2 are alternately arranged so that regular shading is generated.

  If the period of the horizontal synchronizing signal is an integral multiple of the period of the modulation signal SG1, regular shading appears as interference fringe noise in the image. That is, when the period of the horizontal synchronization signal is an integral multiple of the period of the modulation signal SG1, as shown in FIG. 11, the positions of regular shading in the main scanning direction D1 are the same, and as a result, extend in the sub-scanning direction D2. A plurality of line-shaped noises (interference fringe-shaped noises) N1 and N2 appear in the image. Noise N1 is a thin line, noise N2 is a dark line, and noise N1 and noise N2 appear alternately. If only one horizontal pixel line PL is viewed, regular shading is difficult to be visually recognized and does not cause any particular problem. However, a level that can be visually recognized if there are a plurality of line-shaped noises N1 and N2 extending in the sub-scanning direction D2. It becomes a problem.

  More specifically, when the frequency of the modulation signal SG1 is a typical 25 kHz, the period of the modulation signal is 40 μsec. If the period of the horizontal synchronization signal is 400 μsec, for example, 400 μsec = 40 μsec × 10, ten noises N1 of thin lines and ten noises N2 of dark lines appear alternately in the sub-scanning direction D2.

  An object of the present invention is to provide an image processing apparatus capable of reducing the influence of periodicity of a modulation signal used for generating a spread spectrum clock on an image when an image is drawn using the spread spectrum clock. To do.

  An image processing apparatus according to an aspect of the present invention that achieves the above object is an image processing apparatus that draws an image using a spread spectrum clock, and that generates a horizontal synchronization signal when drawing the image. A signal generation unit, and a clock generation unit that generates the spread spectrum clock, wherein the clock generation unit varies the frequency of the spread spectrum clock in a cycle T1, and the cycle T1 and the horizontal synchronization signal The clock generator is set so that the period T2 satisfies the following expression.

T2 = T1 × (n + 0.5)
n is a positive integer According to the present invention, the period T1 in which the frequency of the spread spectrum clock fluctuates and the period T2 of the horizontal synchronization signal have a relationship of T2 = T1 × (n + 0.5), where n is a positive integer. The clock generation unit is set so as to satisfy. Since the period T1 is the same as the period of the modulation signal, the phase of the modulation signal is shifted by 180 degrees for each period of the horizontal synchronization signal. For this reason, regular shading that occurs due to the periodicity of the modulation signal does not appear in the image as a plurality of line-shaped noises extending in the sub-scanning direction, but between adjacent horizontal pixel lines in the image. Since it is averaged, regular shading can be difficult to recognize visually. Therefore, according to the present invention, the influence of the periodicity of the modulation signal used for generating the spread spectrum clock on the image can be reduced. In the present invention, the image drawing using the spread spectrum clock may be either a latent image drawing or a visible image drawing. The spread spectrum clock is used not only for various timing signals for processing necessary for drawing an image but also for generating a signal obtained by modulating the spread spectrum clock with image data.

  In the above configuration, the clock generation unit generates a divided signal obtained by dividing the spread spectrum clock, and a phase comparison that generates a phase difference signal indicating a phase difference between a reference clock and the divided signal. A low-pass filter that converts the phase difference signal into a DC signal, a modulation signal generator that generates a modulation signal used to generate the spread spectrum clock, a voltage of the DC signal, and a voltage of the modulation signal And an adder that generates an addition signal obtained by adding together, and an oscillator that controls an oscillation frequency according to the voltage of the input signal, and that outputs the spread spectrum clock whose frequency varies according to the voltage of the addition signal The modulation signal has the same period as the period T1, and the modulation signal generation unit includes the period T1 and the period of the horizontal synchronization signal. 2 and generates the modulated signal satisfies the following expression.

T2 = T1 × (n + 0.5)
n is a positive integer. According to this configuration, the clock generation unit can be set so that the period T1 and the period T2 of the horizontal synchronizing signal satisfy the above formula.

  In the above configuration, the modulation signal generation unit can generate the modulation signal asynchronously with the horizontal synchronization signal.

  As described above, in the present invention, since the period T1 of the modulation signal and the period T2 of the horizontal synchronization signal satisfy the relationship T2 = T1 × (n + 0.5), the modulation signal and the horizontal synchronization signal may be synchronous or asynchronous. Instead, the phase of the modulation signal is shifted by 180 degrees for each period of the horizontal synchronizing signal, and the effect of the present invention is produced in which regular gray levels are averaged between adjacent horizontal pixel lines. According to the configuration in which the modulation signal and the horizontal synchronization signal are asynchronous, the synchronization circuit can be omitted as compared with the configuration in which the modulation signal and the horizontal synchronization signal are synchronized.

  In the above configuration, the image forming unit includes an image forming unit configured to form and output an image drawn using a signal obtained by modulating the spread spectrum clock with image data, and the image forming unit outputs the spread spectrum clock to the image data. An exposure unit that irradiates a light beam that is a signal modulated by the light beam, an image carrier on which an electrostatic latent image is drawn by the light beam, and a developing unit that develops the electrostatic latent image with toner. can do.

  According to this configuration, the influence of the periodicity of the modulation signal used for generating the spread spectrum clock on the image formed on the paper by the electrophotographic method can be reduced.

  In the above configuration, the image forming unit can form and output a single color image represented by the toner of one color on a sheet.

  This configuration applies the present invention to the formation of a single color image (for example, a monochrome image or a yellow single color image) represented by a single color toner image. In a color image formed by superimposing a plurality of color toner images, the above-described plurality of line-shaped noises extending in the sub-scanning direction may become inconspicuous by superimposing the toner images. This is not the case because the toner images are not superimposed. Therefore, the present invention is preferably applied to the formation of a single color image represented by a single color toner image.

  According to the present invention, when an image is drawn using a spread spectrum clock, the influence of the periodicity of the modulation signal used for generating the spread spectrum clock on the image can be reduced.

1 is a diagram illustrating an outline of an internal structure of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a block diagram illustrating an electrical configuration of the image forming apparatus illustrated in FIG. 1. It is a schematic diagram which shows the state which irradiated the light beam to the light deflection surface of the polygon mirror in this embodiment. It is a block diagram which shows the irradiation control system of the light beam concerning this embodiment. It is a block diagram which shows the structure of the clock generation part which concerns on this embodiment. It is a figure which shows the waveform of a spread spectrum clock and a modulation signal. It is the figure which showed an example of the code | symbol of a modulation signal with the graph. It is a flowchart explaining drawing of the image using the spread spectrum clock produced | generated by the clock generation part which concerns on this embodiment as a clock signal. It is a figure which shows the waveform of a modulation signal and a BD signal in case the period of a BD signal (horizontal synchronizing signal) is 8.5 times the period of a modulation signal. In the case of FIG. 9, it is a figure which shows the relationship between regular shading and a modulation signal. It is a figure which shows the relationship between regular shading and a modulation signal, when the period of a horizontal synchronizing signal is an integral multiple of the period of a modulation signal.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram showing an outline of the internal structure of an image forming apparatus 1 according to an embodiment of the present invention. The image forming apparatus 1 can be applied to, for example, a digital multifunction machine having a color copy, a color printer, and a color facsimile function. The image forming apparatus 1 is a tandem system, and includes an apparatus main body 100 and an image reading unit 200 disposed on the apparatus main body 100.

  The image reading unit 200 reads an image (character, figure, photograph, etc.) with a CCD (Charge Coupled Device) or the like and outputs it as image data. The image reading unit 200 has a function of reading a color image. As a result, color copy and color facsimile can be transmitted.

  The apparatus main body 100 includes a sheet storage unit 110, an image forming unit 130, and a fixing unit 160.

  The paper storage unit 110 is disposed at the lowermost part of the apparatus main body 100 and includes a paper tray 111 that can store a bundle of paper P. The paper tray 111 is inserted into the apparatus main body 100 and attached. When the paper P is replenished, the paper tray 111 is pulled out from the apparatus main body 100. In the bundle of sheets P stored in the sheet tray 111, the uppermost sheet P is fed out toward the sheet conveyance path 115 by driving the pickup roller 113. The paper P is transported to the image forming unit 130 through the paper transport path 115.

  The image forming unit 130 forms a toner image on the conveyed paper P. The image forming unit 130 includes a magenta unit 133M, a cyan unit 133C, a yellow unit 133Y, and a black unit 133K, which are arranged in the order in which the toner images are transferred to the transfer belt 131. These units have the same configuration, and a magenta unit 133M will be described as an example.

  The magenta unit 133M includes a photosensitive drum 135 and an exposure device 137. Around the photosensitive drum 135, a charger 139, a developing device 141, and a cleaner 143 are arranged. The photosensitive drum 135 is an example of an image carrier. The image carrier may be a belt-like carrier.

  The charger 139 uniformly charges the peripheral surface of the photosensitive drum 135. The exposure device 137 generates light corresponding to magenta data among image data (image data output from the image reading unit 200, image data transmitted from a personal computer, image data received by facsimile, etc.), and uniformly. Irradiate the circumferential surface of the charged photosensitive drum 135. As a result, an electrostatic latent image having a magenta pattern is formed on the peripheral surface of the photosensitive drum 135. In this state, by supplying magenta toner from the developing device 141 to the peripheral surface of the photosensitive drum 135, a toner image having a magenta pattern is formed on the peripheral surface.

  The transfer belt 131 can move clockwise while being sandwiched between the photosensitive drum 135 and the primary transfer roller 145. A toner image having a magenta pattern is transferred from the photosensitive drum 135 to the transfer belt 131. The magenta toner remaining on the peripheral surface of the photosensitive drum 135 is removed by the cleaner 143. The above is the description of the magenta unit 133M.

  Above the magenta unit 133M, the cyan unit 133C, the yellow unit 133Y, and the black unit 133K, containers that store toner of corresponding colors, that is, a magenta toner container 147M, a cyan toner container 147C, and a yellow toner. A container for toner 147Y and a container for black toner 147K are arranged. Each color developing device 141 is supplied with toner from a corresponding container.

  As described above, a toner image having a magenta pattern is transferred to the transfer belt 131, and a toner image having a cyan pattern is transferred on the toner image. Similarly, a toner image having a yellow pattern and a toner having a black pattern are transferred. The image is transferred in layers. As a result, a color toner image is formed on the transfer belt 131. In this way, a color toner image is formed on the transfer belt 131 by transferring the toner image of each color pattern superimposed on the transfer belt 131. The color toner image is transferred by the secondary transfer roller 149 onto the paper P conveyed from the paper storage unit 110 described above.

  The sheet P on which the color toner image is transferred is sent to the fixing unit 160. The fixing unit 160 has a structure in which a fixing belt 165 is hung on a heating roller 161 and a fixing roller 163. The fixing belt 165 is sandwiched between the fixing roller 163 and the pressure roller 167. The paper P onto which the color toner image is transferred is sandwiched between these rollers. Accordingly, heat and pressure are applied to the color toner image and the paper P, and the color toner image is fixed to the paper P. The paper P is discharged to a paper discharge tray 169.

  FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus 1 shown in FIG. The image forming apparatus 1 is an example of an image processing apparatus that draws an image using a signal obtained by modulating a spread spectrum clock with image data. The image forming apparatus 1 has a configuration in which an apparatus main body 100, an image reading unit 200, a control unit 300, an operation display unit 400, and a communication unit 500 are connected to each other by a bus. The apparatus main body 100 and the image reading unit 200 have been described with reference to FIG.

  The control unit 300 includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), an image memory, and the like. The CPU executes control necessary for operating the image forming apparatus 1 on the hardware constituting the image forming apparatus 1. The ROM stores software necessary for controlling the operation of the image forming apparatus 1. The RAM is used for temporary storage of data generated during execution of software, storage of application software, and the like. The image memory temporarily stores image data (image data output from the image reading unit 200, image data transmitted from a personal computer, image data received by facsimile, etc.).

  The control unit 300 includes a clock generation unit 301 that generates a spread spectrum clock. The clock generation unit 301 will be described in detail later.

  The operation display unit 400 is provided with operation keys including hard keys. Specifically, a function key for switching between a start key, a numeric keypad, a stop key, a reset key, a copy, a printer, a scanner, and a facsimile are provided. The operation display unit 400 is provided with a touch panel. On the screen of the touch panel, various operations and details of operations are displayed and operation keys including soft keys are displayed.

  The communication unit 500 includes a facsimile communication unit 501 and a network I / F unit 503. The facsimile communication unit 501 includes an NCU (Network Control Unit) that controls connection of a telephone line with a destination facsimile and a modulation / demodulation circuit that modulates / demodulates a signal for facsimile communication. The facsimile communication unit 501 is connected to the telephone line 505.

  A network I / F unit 503 is connected to a LAN (Local Area Network) 507. A network I / F unit 503 is a communication interface circuit for executing communication with a terminal device such as a personal computer connected to the LAN 507.

  Next, a light beam irradiation control system that executes drawing of an electrostatic latent image will be described with reference to FIGS. FIG. 3 is a schematic diagram showing a state in which the light deflection surface 13 of the polygon mirror 11 is irradiated with the light beam LB in the present embodiment. FIG. 4 is a block diagram showing a light beam irradiation control system 15 according to the present embodiment. The irradiation control system 15 is provided corresponding to each color constituting the color image. In this embodiment, an irradiation control system 15 is provided for magenta, cyan, yellow, and black. The optical components such as the collimator lens and the fθ lens are not shown. The irradiation control system 15 includes a polygon mirror 11, a clock modulation unit 17, a light beam generation unit 19, a BD signal generation unit 21, and the like.

  The clock modulation unit 17 generates a signal obtained by modulating the spread spectrum clock CLK2 with image data. An ASIC dedicated to image processing that is one of the elements constituting the control unit 300 has the function of the clock modulation unit 17. The clock modulation unit 17 includes a buffer 23 and a modulation circuit 25.

  When drawing an electrostatic latent image, image data to be drawn is sent from the RAM (main memory) of the control unit 300 to the buffer 23. The buffer 23 temporarily stores data used for drawing a predetermined number of scanning lines SL. One scanning line SL corresponds to one horizontal pixel line PL of an image formed on a sheet. The BD signal SG2 functions as a horizontal synchronization signal. Every time the BD signal SG2 is input to the clock modulation unit 17, the clock modulation unit 17 sends the data stored in the buffer 23 to the modulation circuit 25 in synchronization therewith.

  The modulation circuit 25 is a PWM (Pulse Width Modulation) circuit, for example, and receives the spread spectrum clock CLK2 and data sent from the buffer 23, and generates a signal obtained by modulating the spread spectrum clock CLK2 with image data. This signal is sent to the light beam generator 19 and converted into an optical signal.

  The light beam generation unit 19 includes a light source 27 that generates the light beam LB and emits the light beam LB, a driver that drives the light source 27, and the like. As the light source 27, for example, a semiconductor laser is used. The driver controls light emission of the light source according to a signal obtained by modulating the spread spectrum clock CLK2 with image data. Thereby, the light beam LB obtained by converting the signal obtained by modulating the spread spectrum clock CLK2 with the image data into the optical signal is irradiated.

  The polygon mirror 11 reflects the light beam LB generated by the light beam generator 19. The polygon mirror 11 is rotated by the motor 29 in the rotation direction R1. The polygon mirror 11 has a regular hexagonal side surface. This side surface is composed of six light deflection surfaces 13.

  The light beam LB emitted from the light source 27 is applied to the rotating polygon mirror 11, deflected by the light deflection surface 13 (reflected by the polygon mirror 11), and moves the scanning line SL onto the photosensitive drum 135 in the main scanning direction. Draw at D1. One scanning line SL is drawn by one light deflection surface 13. By repeating the drawing of one scanning line SL on the photosensitive drum 135 rotating in the rotation direction R2, an electrostatic latent image is drawn along the sub-scanning direction D2. The sub-scanning direction D2 corresponds to the rotation direction R2 of the photosensitive drum 135. In the developed surface of the photosensitive drum 135, the horizontal direction is the main scanning direction D1, and the vertical direction is the sub-scanning direction D2.

  The BD signal generation unit 21 is provided corresponding to each color, and generates a BD (Beam Detect) signal SG2 that becomes a horizontal synchronization signal when an electrostatic latent image is drawn on the photosensitive drum 135. The BD signal SG2 is a signal serving as a reference for aligning the position in the main scanning direction D1 when an electrostatic latent image is drawn on the photosensitive drum 135 with the light beam LB. After the BD signal SG2 is output from the BD signal generation unit 21 and a predetermined period elapses, drawing of an electrostatic latent image on the photosensitive drum 135 is started by the light beam LB modulated by the image data.

  The configuration of the BD signal generation unit 21 will be described. The BD signal generation unit 21 includes a BD sensor 31 and a BD signal conversion unit 33. The light beam LB is repeatedly scanned in the main scanning direction D1 within a scanning range longer than the dimension of the photosensitive drum 135 in the main scanning direction D1. The BD sensor 31 is installed at a position in the scanning range where the light beam LB receives the light beam LB before the photosensitive drum 135 starts scanning.

  The BD sensor 31 is a photo sensor. When the BD sensor 31 receives the light beam LB generated by the light beam generator 19 and reflected by the light deflection surface 13 of the rotating polygon mirror 11, the BD sensor 31 outputs the received light signal to the BD signal converter 33. The BD signal converter 33 shapes the received light signal into a rectangular wave BD signal SG 2 and outputs the BD signal SG 2 to the clock modulator 17. As described above, the image forming apparatus 1 functions as an image processing apparatus that draws an image using a signal obtained by modulating the spread spectrum clock CLK2 with image data.

  Next, the clock generation unit 301 that generates the spread spectrum clock will be described. FIG. 5 is a block diagram illustrating a configuration of the clock generation unit 301. The clock generation unit 301 has a configuration in which a modulation signal generation unit 41 is added to a PLL (Phase Locked Loop) circuit.

  The clock generation unit 301 includes a frequency divider 43, a phase comparator 45, a low-pass filter 47, and a voltage controlled oscillator (VCO) 49 as a PLL (Phase Locked Loop) circuit, and further includes an adder 51 and a digital / analog converter. And a modulation signal generator 41.

  The frequency divider 43 generates a frequency-divided signal obtained by frequency-dividing the spread spectrum clock CLK2 generated by the clock generator 301.

  The phase comparator 45 receives the reference clock CLK1 and the frequency-divided signal generated by the frequency divider 43, and generates a phase difference signal that is a pulse signal indicating the phase difference between the reference clock CLK1 and the frequency-divided signal.

  The low-pass filter 47 is also called a loop filter, and smoothes the phase difference signal output from the phase comparator 45 and converts it into a DC signal.

  The adder 51 generates an addition signal obtained by adding the voltage of the DC signal output from the low-pass filter 47 and the voltage of the modulation signal SG1 generated by the modulation signal generation unit 41.

  The voltage controlled oscillator 49 is an oscillator that controls the oscillation frequency according to the voltage of the input signal, and outputs a spread spectrum clock CLK2 whose frequency varies according to the voltage of the addition signal output from the adder 51. The spread spectrum clock CLK2 is sent to the clock modulation unit 17 shown in FIG.

  The modulation signal generation unit 41 generates a modulation signal SG1 used for generating the spread spectrum clock CLK2. The modulation signal generation unit 41 includes a modulation signal code storage unit 55 and a modulation signal code output unit 57.

  FIG. 6 is a diagram showing waveforms of the spread spectrum clock CLK2 and the modulation signal SG1. The clock generator 301 varies the frequency of the spread spectrum clock CLK2 with a period T1. This period is the same as the period of the modulation signal SG1. Hereinafter, the cycle of the modulation signal SG1 is also referred to as a cycle T1.

  The modulation signal code storage unit 55 stores a code of the modulation signal SG1 having a frequency of 21.25 kHz in advance. The code is a digital value corresponding to a half-cycle waveform indicating the voltage of the analog modulation signal SG1. FIG. 7 is a graph showing an example of the code of the modulation signal SG1. The vertical axis represents voltage, and the horizontal axis represents time.

  The code may be stored in the modulation signal code storage unit 55 when the image forming apparatus 1 is shipped from the factory, or may be stored and changed later using the operation display unit 400 shown in FIG. Further, instead of the modulation signal generation unit 41 and the digital-analog converter 53, an oscillation circuit that outputs an analog signal having a cycle of T1, for example, 21.25 kHz may be used.

  The reason why the frequency of the modulation signal SG1 is 21.25 kHz will be described. The modulation signal generation unit 41 generates a modulation signal SG1 in which the period T1 of the modulation signal SG1 and the period T2 of the BD signal (horizontal synchronization signal) SG2 satisfy the following expression.

T2 = T1 × (n + 0.5)
n is a positive integer. This means that the phase of the modulation signal SG1 is shifted by 180 degrees every period T2 of the BD signal SG2. In this embodiment, the cycle T2 of the BD signal SG2 is 400 μsec, and the cycle T2 and the cycle T1 of the modulation signal SG1 are set to have a relationship of T2 = 8.5T1. From this relationship, the frequency f of the modulation signal SG1 is calculated as follows.

T1 = 400 μsec ÷ 8.5 = 47.058823529411764705882352941176
f = 1 / 47.058823529411764705882352941176
= 0.021250000000000000000000000000003
≒ 0.02125 = 21.25kHz
The modulation signal code output unit 57 extracts the code of the modulation signal SG1 stored in the modulation signal code storage unit 55, and repeatedly outputs the code of the modulation signal SG1. As a result, the modulation signal SG1 is output as a digital signal. The output modulation signal SG1 is converted into an analog signal (analog voltage signal) by the digital-analog converter 53 and sent to the adder 51.

  Next, image copy using the spread spectrum clock CLK2 according to the present embodiment as a clock signal will be described by taking a copy as an example. FIG. 8 is a flowchart for explaining this. The user sets a document on the image reading unit 200 shown in FIG. 2, and operates a start key provided on the operation display unit 400 (step S1). The control unit 300 determines whether the image forming apparatus 1 has already started up (step S3). If the image forming apparatus 1 is up, the spread spectrum clock CLK2 has been generated. If the control unit 300 determines that the image forming apparatus 1 has not yet started up due to the sleep mode or the like (No in step S3), the control unit 300 starts up the image forming apparatus 1 (step S5). This operation includes the start of generation of the spread spectrum clock CLK2.

  The spread spectrum clock CLK2 is generated as follows. As shown in FIG. 5, the reference clock CLK1 and the frequency-divided signal output from the frequency divider 43 are input to the phase comparator 45, and the phase comparator 45 calculates the phase difference between the reference clock CLK1 and the frequency-divided signal. The phase difference signal shown is output to the low-pass filter 47. The low-pass filter 47 converts the phase difference signal into a DC signal and outputs it to the adder 51.

  On the other hand, the modulation signal code output unit 57 reads the code of the modulation signal SG1 having a frequency of 21.25 kHz from the modulation signal code storage unit 55, and repeatedly outputs the code. As a result, the digital modulation signal SG1 is output from the modulation signal generation unit 41.

  The digital modulation signal SG1 (voltage) is converted into an analog modulation signal SG1 (voltage) by the digital-analog converter 53 and sent to the adder 51. In the adder 51, the analog modulation signal SG1 (voltage) and the DC voltage output from the loop filter 47 are added to generate an addition signal. The addition signal is sent to the voltage controlled oscillator 49, whereby a spread spectrum clock CLK2 based on the modulation signal SG1 of 21.25 kHz is generated.

  If the control unit 300 determines that the image forming apparatus 1 is up (Yes in step S3), the control unit 300 starts a copy operation. First, the image reading unit 200 reads an original using the spread spectrum clock CLK2 as a clock signal (step S7). Image data generated by reading the document by the image reading unit 200 is sent to the clock modulation unit 17 shown in FIG. Further, the controller 300 rotates the polygon mirror 11 shown in FIG. 4 and irradiates the light beam LB from the light source 27 in order to generate the BD signal SG2. The light beam LB is reflected by the polygon mirror 11 and received by the BD sensor 31, and the BD signal generator 21 generates the BD signal SG2 (step S9). Here, a BD signal (hereinafter referred to as a first BD signal) SG2 for drawing the first scan line SL is generated, and the first BD signal SG2 is sent to the clock modulation unit 17.

  When the BD signal SG <b> 2 is input to the clock modulation unit 17, the clock modulation unit 17 sends data used for drawing the current scanning line SL stored in the buffer 23 among the image data to the modulation circuit 25. On the other hand, the spread spectrum clock CLK2 is sent to the modulation circuit 25 of the clock modulation unit 17, modulated by the data used for drawing the current scan line SL, and scanned by the light beam LB generated by the modulated signal. A line SL is drawn on the photosensitive drum 135 (step S11).

  Here, when the first BD signal SG <b> 2 is input to the clock modulation unit 17, the clock modulation unit 17 sends data used for drawing the first scan line SL stored in the buffer 23 to the modulation circuit 25. The spread spectrum clock CLK2 is modulated by data used to draw the first scanning line SL, and the first scanning line SL is drawn on the photosensitive drum 135 by the light beam LB generated by the modulated signal. Thus, the exposure apparatus (exposure unit) 137 shown in FIG. 1 irradiates the light beam LB, which is a signal obtained by modulating the spread spectrum clock CLK2 with image data, in accordance with the timing of the BD signal (horizontal synchronization signal) SG2. Yes.

  If the controller 300 determines that the last scanning line SL has been drawn (Yes in step S13), drawing of the electrostatic latent image is completed. On the other hand, if the control unit 300 determines that the last scanning line SL is not drawn (No in step S13), the process returns to step S9. Here, a BD signal (hereinafter, second BD signal) SG2 for drawing the second scanning line SL is generated in the BD signal generation unit 21 (step S9), and the second BD signal SG2 is the first. The BD signal SG2 is sent to the clock modulator 17 in the same manner. Then, the second scanning line SL is drawn on the photosensitive drum 135 (step S11). Steps S9 to S13 are repeated until the last scanning line SL is drawn.

  When the exposure process in the exposure apparatus 137 is completed by drawing the last scanning line SL, the control unit 300 controls each process of development, transfer, and fixing (step S15), thereby printing an image. The discharged paper is discharged to a paper discharge tray 169.

  The main effects of this embodiment will be described. FIG. 9 is a waveform diagram of the modulation signal SG1 and the BD signal SG2 (horizontal synchronization signal). FIG. 10 is a diagram showing the relationship between regular shading and modulation signal SG1 in the case of modulation signal SG1 and BD signal SG2 shown in FIG. 9, and corresponds to FIG. The horizontal pixel line PL, the thin portion P1, and the dark portion P2 are described in the “Summary of Invention” section.

  Also in this embodiment, regular shading occurs in the horizontal pixel line PL due to the influence of the periodicity of the modulation signal SG1, but 8.5 times the period T1 of the modulation signal SG1 is set to the period T2 of the BD signal SG2. Yes. Therefore, since the cycle T1 of the modulation signal SG1 and the cycle T2 of the BD signal SG2 (horizontal synchronization signal) satisfy the relationship of T2 = T1 × (n + 0.5), the modulation signal SG1 is generated every cycle of the BD signal SG2. Will be 180 degrees out of phase. Therefore, the regular shading affected by the periodicity of the modulation signal SG1 becomes a plurality of line-shaped noises N1 and N2 extending in the sub-scanning direction D2 shown in FIG. As shown in FIG. 6, since the average is performed between the adjacent horizontal pixel lines PL, regular shading can be made difficult to be visually recognized. Therefore, according to the present embodiment, the influence of the periodicity of the modulation signal SG1 used for generating the spread spectrum clock CLK2 on the image can be reduced.

  Further, according to the present embodiment, the modulation signal generator 41 shown in FIG. 5 generates the modulation signal SG1 asynchronously with the BD signal SG2. As described above, in the present embodiment, since the period T1 of the modulation signal SG1 and the period T2 of the BD signal SG2 satisfy the relationship T2 = T1 × (n + 0.5), the modulation signal SG1 and the BD signal SG2 are synchronized. Regardless of the asynchronization, the phase of the modulation signal SG1 is shifted by 180 degrees for each period of the BD signal SG2, so that regular shading is averaged between adjacent horizontal pixel lines PL in the above-described embodiment. An effect occurs. According to the configuration in which the modulation signal SG1 and the BD signal SG2 are asynchronous, the synchronization circuit can be omitted as compared with the configuration in which the modulation signal SG1 and the BD signal SG2 are synchronized.

  This embodiment is particularly effective when the image forming unit 130 forms and outputs a single color image (for example, a monochrome image or a yellow single color image) represented by a single color toner image. In a color image formed by superimposing toner images of a plurality of colors, the line-shaped noises N1 and N2 shown in FIG. 11 may become inconspicuous when the toner images are superimposed. This is not the case because they are not superimposed. Therefore, it is preferable to apply this embodiment to the formation of a single color image represented by a single color toner image.

1 Image forming device (an example of an image processing device)
21 BD signal generator (an example of a horizontal sync signal generator)
135 Photosensitive drum (an example of an image carrier)
137 Exposure device (an example of exposure unit)
141 Developing Device (Example of Developing Unit)
SG1 modulation signal SG2 BD signal (an example of horizontal synchronization signal)
PL Horizontal pixel line CLK2 Spread spectrum clock T1 Period of modulation signal, Period of fluctuation of frequency of spread spectrum clock T2 Period of BD signal

Claims (5)

An image processing apparatus that draws an image using a spread spectrum clock,
A horizontal synchronization signal generator for generating a horizontal synchronization signal when drawing the image;
A clock generation unit for generating the spread spectrum clock,
The clock generator varies the frequency of the spread spectrum clock at a cycle T1, and the clock generator is set so that the cycle T1 and the cycle T2 of the horizontal synchronization signal satisfy the following expression: Processing equipment.
T2 = T1 × (n + 0.5)
n is a positive integer The clock generator is
A frequency divider for generating a divided signal obtained by dividing the spread spectrum clock;
A phase comparator that generates a phase difference signal indicating a phase difference between a reference clock and the divided signal;
A low pass filter for converting the phase difference signal into a DC signal;
A modulation signal generator for generating a modulation signal used for generating the spread spectrum clock;
An adder for generating an addition signal obtained by adding the voltage of the DC signal and the voltage of the modulation signal;
An oscillator that controls an oscillation frequency according to a voltage of an input signal, and a voltage-controlled oscillator that outputs the spread spectrum clock whose frequency varies according to the voltage of the addition signal.
The period of the modulation signal is the same as the period T1;
The image processing apparatus according to claim 1, wherein the modulation signal generation unit generates the modulation signal in which the period T1 and the period T2 of the horizontal synchronization signal satisfy the following expression.
T2 = T1 × (n + 0.5)
n is a positive integer   The image processing apparatus according to claim 2, wherein the modulation signal generation unit generates the modulation signal asynchronously with the horizontal synchronization signal. An image forming unit that forms and outputs an image drawn using a signal obtained by modulating the spread spectrum clock with image data;
The image forming unit includes:
An exposure unit that emits a light beam that is a signal obtained by modulating the spread spectrum clock with image data;
An image carrier on which an electrostatic latent image is drawn by the light beam;
The image processing apparatus according to claim 1, further comprising: a developing unit that develops the electrostatic latent image with toner.   The image processing apparatus according to claim 4, wherein the image forming unit forms and outputs a single color image represented by the toner of one color on a sheet.