JP2000152001A - Electronic circuit for correcting nonlinear color slippage in raster output scanner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electronic solution which practically reduces color slippage to <20 μm. SOLUTION: An electronic circuit which corrects nonlinear color slippage is provided with a clock generator 62, calibrating means 42, and a correcting circuit 56. The calibrating means 42 is electrically connected to the clock generator 62 to calibrate the frequency of the clock to a prescribed frequency. The correcting means 56 is electrically connected to the clock generator 62 to give an individual correction curve to each scanning line, thus modulating the frequency of the clock generation means for each scanning line in accordance with its correction curve. The clock generation means 62, the calibrating means 42, and the correcting means 56 are so mutually connected that the clock frequency may be modulated during scanning of scanning lines and be calibrated to a prescribed frequency between two continuous scanning lines.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a registration method in a color xerographic printing system, and more particularly to the use of electronics to correct color misregistration (misregistration) between identical pixels of different colors. About the system. In the following, it is noted that for simplicity, the "color xerographic print system" is referred to as the "color print system".

[0002]

2. Description of the Related Art Typically, a color printing system includes a photoreceptor and four color stations. 1
Each color station that handles one color has a raster output scanner (ROS). Referring to FIG.
Prior art raster output scanner 10 for a printing system
(High speed scanning) diagram of FIG. The raster scanning system 10 includes a laser light source 12 as a scanning element,
A collimator 14, a front polygon optics 16, a rotating polygon mirror 18 having a plurality of facets, a rear polygon optics 20, and a photosensitive medium 22 are used.

A laser light source 12 sends a light beam 24 to a rotating polygon mirror 18 via a collimator 14 and a front polygon optics 16. The collimator 14 has a light beam 24
Are parallel, and the front polygon optics 16 converts the light beam 24 on the sagittal plane, that is, the transverse scanning plane, into the rotating polygon mirror 1.
Focus on 8. Facets 26 of rotating polygon mirror 18 reflect light beam 24 and cause the reflected light beam to rotate about an axis near the point of reflection of facet 26. The reflected light beam 24 is used via the rear polygon optics 20 to scan a photosensitive medium 22, such as a xerographic drum (photoreceptor). As the photoreceptor 22 moves, the light beam 24 scans all scan lines of the document on the photoreceptor to generate a latent image.

[0004] Typically, in a color printing system, a latent image is created for each base color, and each latent image is superimposed on a previous latent image.

[0005] After the first latent image has been created and developed, the modulated light beam from the next ROS begins to create a new latent image on that first latent image. Thus, each subsequent station creates and develops a latent image on the previous latent image. The process of creating and developing the latent image is repeated four times for four different colors (typically cyan, yellow, magenta, and black), each performed by one station. After the four different color toners are superimposed, the toner is transferred onto the paper.

Since each latent image is created on a previous latent image, it is very important that each pixel of each latent image be superimposed on the same pixel of the previous latent image / s. However, due to the movement of the photoreceptor belt and the tolerances of the optics of each ROS system, the location of the pixels in each latent image may be slightly different from the location of the pixels in the first latent image. This creates a problem known as color shift.

[0007] Color shift can occur in two states, linear or non-linear. Linear color shift occurs when the distance between adjacent pixels is equal, but the pixels are not exactly overlaid on the pixels of the previous latent image. Non-linear color shift occurs when the distance between adjacent pixels of each scan line of the latent image varies. The degree of variation in the distance between adjacent pixels may be different for each latent image. As a result, the problem of color misregistration is further increased.

[0008] It is difficult to correct non-linear color misregistration in high-speed scanning or scanning direction by an optical solution. Due to the intersection of the different elements, the four Rs of each printing system
The OS systems cannot be exactly the same. Thus, each ROS system introduces a different non-linear error. Correction of four different non-linear errors by optical solution is not feasible. Typically, optical solutions minimize nonlinear color shifts to about 100 μm, but do not reduce errors sufficiently.

[0009] The electronic solution provides a more accurate correction because each scan line of each ROS system is given an individual correction curve. In order to electronically correct nonlinear color shift in the high-speed scanning direction, it is necessary to change the timing between pixel clock pulses. Pixel clock pulses indicating the location of each pixel are used to modulate pixel information into the laser light beam.

[0010] Typically, the timing of clock pulses is at regular intervals, so that pixel information to the laser light beam is modulated at regular intervals. However, due to the non-linear color shift error, the pixels of the scan line are not arranged at equal intervals on the photoreceptor. Thus, the frequency of the clock is varied according to the same scan line pixel arrangement of the previous latent image in order to match the pixels of the subsequent latent image. This causes the pixel information to be modulated at different intervals in the laser light beam. Therefore, by changing the timing between the pulses (frequency modulation), the pixel can be arranged at a desired position.

To electronically correct nonlinear color shift,
There are some obstacles. For high resolutions, such as 600 pixels / inch, it is very difficult to change the frequency of the clock generator at such a high rate. Also, due to the tolerance of the clock generator in addition to frequency modulation,
Since the output of the pixel clock deviates from the reference frequency,
An error occurs in the arrangement of pixels.

[0012]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an electronic solution which substantially reduces the color shift problem to less than 20 μm. Another object of the present invention is to increase the high frequency (54M) by adding about 10kHz to the total change from peak to peak at about 3MHz.
(Hz) can be varied.

[0013]

According to the present invention, an electronic circuit for correcting non-linear color shift is disclosed. The electronic circuit includes a clock generator, a calibration unit, and a correction unit. The calibration means is electrically connected to the clock generator,
Calibrate the clock frequency to a predetermined frequency. The correction means is electrically connected to the clock generator to provide an individual correction curve for each scan line and modulates the frequency of the clock generation means for each scan line according to the respective correction curve. The clock generation means, the calibration means, and the correction means are connected together such that the clock frequency is modulated while the scan line is being scanned and is calibrated to a predetermined frequency between two successive scan lines. I have.

In accordance with another aspect of the present invention, a method for correcting a non-linear color shift of a scan line of a raster output scanner is disclosed. The method includes the steps of generating a clock frequency, modulating the clock frequency for each scan line according to a respective correction curve, stopping modulation at the end of each scan line, and prior to the start of the next scan line. The method comprises the steps of calibrating the clock frequency to a predetermined frequency, and repeating the modulation and calibration for all the scan lines of one page.

[0015]

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG.
Block diagram of S interface module board 40
It is shown. In FIG. 2, a voltage controlled oscillator (VC
O) 42 and a phase locked loop 44 are used to generate the correct main clock (Mclk).

Within phase locked loop 44, phase detector 46 receives an end of scan (EOS) signal and an end of count (EOC) signal. Based on the timing shift of the received signal, the phase detector 46 generates a pulse and sends it to the charge pump 48. Based on the received pulse, the charge pump 48 is the voltage V _d, sends to the voltage controlled oscillator (VCO) 42 via a low-pass filter 50. Low pass filter 50, before sending the voltage V _d to the VCO 42,
Decrease its noise level. _Vd voltage level is V
Controls the frequency of CO42. Based on the voltage V _d, V
CO42 (clock generator) is a main clock (Mcl)
k).

[0017] Mclk is sent to divider 52 and divided by M to generate a count end (EOC) signal.
M is the total number of pixels per scan line. In this case, 14.4 inch paper of 600 pixels / inch is used, and M is 8640. The EOC signal indicating the end of the pixel information is sent to the phase detector 46 via the alignment block 54. The alignment block 54 may be used for any movement of the photoreceptor belt in the scanning direction, or for each different ROS scan start (SOS)
Adjust the EOC signal for any misalignment between the signals.

In operation, the phase detector 46 compares the EOC signal with the EOS signal. The EOS signal is a signal generated in the ROS. Typically, two sensors are located within a ROS system to detect a start of scan (SOS) signal and an end of scan (EOS) signal. Just before the pixels are located, each sensor generates a start-of-scan (SOS) signal as the scanning laser light beam passes a contract point on the scan line. Similarly, when the scanning laser light beam passes the contracted point on the scanning line immediately after the pixel is arranged, each sensor stops scanning (EO).
S) Generate a signal. The SOS signal and the EOS signal are generated for each scanning line. Since the speed of the scanning laser light beam is constant, the time between the SOS signal and the EOS signal is the same for each scan line.

The EOC signal must match the EOS signal. If not, it means that the frequency of the main clock (Mclk) needs to be decreased or increased depending on whether the EOC signal is before or after the EOS signal, respectively. Therefore,
Depending on whether the EOC signal is before or after the EOS signal, the phase detector 46 generates a positive pulse or a negative pulse, respectively. The width of the pulse indicates the time lag between the EOS signal and the EOC signal. The phase detector 46 sends the generated pulse to the charge pump 48. Typically, EO
The timings of the S signal and the EOC signal do not match. As a result, phase detector 46 typically produces a negative or positive pulse. However, if the timing of the EOS signal and the timing of the EOC signal match, the phase detector 46
Does not generate a pulse.

In the absence of a pulse from the phase detector 46, the charge pump 48 generates a basic voltage which is
Send to the VCO 42 as _d . If the phase detector 46 produces a pulse, depending on whether the pulse is negative or positive, the charge pump 48 will reduce some voltage from its base voltage or add to it in proportion to the width of the pulse. Te, and sends the resulting voltage as a voltage V _d to the VCO 42.

Initially, V _d is equal to the base voltage since there is no EOC signal to compare with the EOS signal. Therefore, the first V _d, VCO 42 is to calibrate the frequency of the Mclk to the reference frequency (54 MHz). Then, at the end of each scan line, a new or the same V, respectively, depending on whether the EOC signal is offset from or in line with the EOS signal.
_d is sent out by the charging pump 48.

If the same V _d is delivered, Mcl
The frequency of k does not change. However, if the new V _d is fed is the difference between the basic voltage V _d, VCO
42 recalibrates the frequency shifted by Mclk and returns it to the reference frequency. This recalibration of the Mclk frequency only occurs at the end of each scan line, and the timing of the EOC signal is
This is performed only when the timing of the signal does not match.
After calibration or recalibration, the charging pump 48 to supply a constant voltage V _d to the VCO 42, the frequency of Mclk next S
It remains constant until the OS signal.

As soon as scanning of the scanning lines starts and the SOS signal is generated, the frequency of the main clock (Mclk) must be changed to correct the nonlinear color shift of the pixels of each scanning line. The frequency of the main clock (Mclk) is varied (modulated) based on a predetermined correction curve for each scan line.

The correction curve is generated through a test. During testing, the placement of the pixels in each scan line is measured as the first image is being generated on the photoreceptor. For subsequent images, for each scan line, the pixel arrangement is compared to the pixel arrangement of the same scan line in the first image to generate an error signal. Based on this error signal,
A correction curve is generated and stored in look-up table 56.

Referring to FIG. 3, the main clock (Mc
correction curve 6 used to modulate the frequency
0 is shown. The vertical axis represents frequency, and the horizontal axis represents the number of increments or corrections. The correction curve 60 is 86
Shown along one scan line of a 14.4 inch paper with 40 pixels. The correction curve 60 has 86 increments (corrections) along the entire length of the scan line. This means that there is one error correction for every 100 pixels. The number of increments can be selected to be greater or less than 86. However, the increment of 86 reduces the color shift by less than 20 μm.

The range of the correction curve from peak to peak is designed to be within ± 3% of the frequency of the main clock (Mclk). This means that the frequency of the main clock (Mclk) is ± 1.62 (3
%). The frequency of the correction curve can be calculated as follows.

(Equation 1) Therefore, the correction curve shows the high frequency (54M) of the main clock (Mclk) in fine increments of about 10 kHz.
Hz).

Referring back to FIG. 2, Mclk is the SO
The synchronization (Sync) signal generated by the S signal is also sent to the pixel clock generator 62. The pixel clock generator 62 synchronizes Mclk with the Sync signal and sends it to the counter 64 as a pixel clock (Pclk). Therefore, Pclk is
Except that it is synchronized to start with the signal
This is a clock having the same frequency as Mclk.

The counter 64 counts the number of pixels and increments the count by receiving each Pclk. The count from counter 64 is sent to look-up table 56 to indicate which correction curve and which increment needs to be sent. While the scan line is being scanned, look-up table 56 converts each correction curve to a digital correction curve to an analog correction voltage.
The signal is sent to the VCO 42 via the / A converter 66. In this specification, “while a scanning line is being scanned” means “S
Note that the term "time between the OS signal and the EOS signal".

[0029] In the node 68, the correction voltage is added to the sent from the charging pump to the VCO 42 V _d. The correction voltage is applied to the main clock (M
modulate the frequency of clk). This process continues until an end-of-scan (EOS) signal is generated.

At the end of the scan, the reset (Rst) signal generated by the EOC signal resets the counter 64. Therefore, the look-up table 56 stops sending the correction curve and terminates the Mclk frequency modulation.
Also, at the end of the scan, if the frequency of Mclk deviates from the reference frequency, as described above, that frequency is recalibrated. This recalibration ensures that each scan line starts at the same frequency (reference frequency).

[0031] The disclosed embodiments of the present invention provide fast and asynchronous modulation of fast frequencies. Further, embodiments of the present invention reduce non-linear color shifts to less than 20 μm.

In the present invention, it should be noted that for more subtle corrections, the number of increments or corrections in the correction curve can be increased to more than 86. This requires a larger look-up table and a larger digital-to-analog converter (DAC).

[0033] Numerous changes in structural details and combinations and arrangements of elements may be made without departing from the true spirit and scope of the invention as set forth in the appended claims. Please be careful.

[Brief description of the drawings]

FIG. 1 is a perspective view (high-speed scanning) of a raster output scanner of a printing system according to the related art.

FIG. 2 is a block diagram of a ROS interface module board of the present invention.

FIG. 3 is a diagram showing a correction curve used to modulate the frequency of a main clock (Mclk).

[Explanation of symbols]

 Reference Signs List 10 raster output scanner 12 laser light source 14 collimator 16 front polygon optics 18 multi-facet rotating polygon mirror 20 rear polygon optics 22 photosensitive medium 24 light beam 26 facet 44 phase lock loop 60 correction curve 68 node

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Hamid Tea. Barramian United States 90501 Torrance West, California Two Twenty-Nine Place 2424 (72) Inventor E-mail Zodmorodi United States 91304 West Hills, California Carmenita Lane 7621

Claims (1)

[Claims] 1. A means for generating a clock signal having a frequency, and a calibrating means electrically connected to the clock generating means, wherein the clock generating means responds to the calibrating means with a clock frequency. Correction means electrically calibrated to a predetermined frequency, and providing an individual correction curve for each scan line to the clock generation means, the clock generation means comprising: Modulating a clock frequency for each scan line according to a respective correction curve in response to the means, wherein the clock generation means, the calibration means, and the correction means modulate the clock frequency only while the scan line is being scanned. And a raster output configured and arranged to calibrate the clock frequency to a predetermined frequency between two consecutive scan lines. An electronic circuit that corrects for nonlinear color shifts in force scanners.