JP2005123763A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus for reducing the labor and time required for sampling a test pattern for automatic gradation correction. <P>SOLUTION: The image forming apparatus samples the correction test pattern by high speed scanning in response to the number of samples, stores temporarily color data to an image memory and thereafter converts the data into color information so that the sampling is carried out at a higher speed than that of prior arts and also the labor of pattern replacement can be omitted by pattern replacement using an RDF. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a color image forming apparatus.

Conventionally, there is a full-color copying machine as a representative example of a color image forming apparatus. This is a laser beam type color electrophotographic printer that forms images in the order of frames for multiple output color components C (Cyan), M (Magenta), Y (Yellow), and K (Black). The halftone expression is realized by controlling the emission of the laser beam by the pulse width modulated signal. Further, in the image forming apparatus as described above, a predetermined pattern is formed on an image carrier or a recording medium, and the density of the predetermined pattern is read to perform density correction and gradation correction, and the quality of the output image There are known methods for stabilizing the above.
JP-A-1-61172

  However, in order to perform image density and gradation correction in image formation, a test pattern for density pattern and a test pattern printed with gradation patterns with the number of lines that can be output such as 200 lines, 400 lines, 800 lines, etc. Need to be read from scanning means such as a scanner, and if the number of test patterns increases, it will take time and effort to output test patterns many times, replace the document, and scan. It is.

  In addition, since it is necessary to read the gradation printed on the test pattern accurately at a predetermined interval in scanning, it is necessary to read the pattern little by little while moving the scanner forward little by little. There is a problem that it takes time to sample all the information printed on the pattern.

  In view of the above points, according to the present invention, the above-mentioned problems are solved by taking the following measures.

  That is, an image reading unit that scans an original and reads an image, a generation unit that generates an image signal from the image, a storage unit that stores the image, and an image formed on a recording medium based on the stored information And an image forming apparatus.

  An output means for outputting a test pattern for image correction from the image forming apparatus, a means for reading the output test pattern from the reading means, and an image read from the reading means are temporarily stored in the storage means. Means for storing, means for sampling color information data of the test pattern from the test pattern stored in the image storage means, and conversion table creating means for creating a conversion table for correcting an image from the sampled color information data .

  Correction means for correcting an image signal using the conversion table described above for an image signal generated by scanning a document, and means for forming an image on a storage medium based on the corrected image signal corrected by the correction means .

  The reading means for reading the above-described image correction test pattern image includes a reading means on the platen and a reading means using a document circulation device. In either case, a plurality of image correction test patterns are read. It can be read and stored in a plurality of image memories.

  The scan speed for reading the above-described image correction test pattern is variable, and is read at a scan speed corresponding to the sampling interval of the test pattern.

  As described above, according to the present invention, when density and gradation correction is performed, the density and gradation correction work that has been time-consuming and troublesome can be performed at high speed using normal copy control. Sampling is performed by taking the image pattern once into the memory by scanning, and by using RDF to exchange the test pattern, correct correction is achieved by reducing the time required for correction and reducing labor. It is possible to provide an image processing apparatus that can

  In the following, a full color copier will be described in detail as a preferred embodiment. The present invention is not limited to this embodiment.

[Overview of image forming apparatus]
FIG. 2 shows an overview of the image forming apparatus 103. As shown in FIG.

  First, in copying a document as a copying machine, 201 is a platen glass, on which a document 202 to be read is placed. The document 202 is irradiated with illumination 203, and an image is formed on the CCD 208 by the optical system 207 through mirrors 204, 205, and 206. Further, the first mirror unit 210 including the mirror 204 and the illumination 203 is mechanically driven by the motor 209 at the speed V, and the second mirror unit 211 including the mirrors 205 and 206 is driven at the speed 1 / 2V. The entire surface of the document 202 is scanned.

  This speed V can be set at any speed according to the variable magnification.

  An image processing circuit unit 212 processes the read image information as an electrical signal, temporarily holds it on the image memory 108, and outputs it as a print signal.

  The print signal output from the image processing circuit unit 212 is sent to a laser driver (not shown) and drives four semiconductor lasers (not shown).

  Reference numeral 213 denotes a polygon mirror, which receives four laser beams emitted from four semiconductor lasers (not shown).

  One of them scans the photosensitive drum 217 through the mirrors 214, 215 and 216, the next one scans the photosensitive drum 221 through the mirrors 218, 219 and 220, and the next one scans the mirror 222. , 223, and 224 are scanned through the photosensitive drum 225, and the next one is scanned through the mirrors 226, 227, and 228 and the photosensitive drum 229 is scanned.

  On the other hand, 230 is a developing device that supplies yellow (Y) toner, forms a yellow toner image on the photosensitive drum 217 in accordance with the laser beam, and 231 is a developing device that supplies magenta (M) toner. According to the laser light, a magenta toner image is formed on the photosensitive drum 221, and a developing device 232 supplies cyan (C) toner. According to the laser light, a cyan toner image is formed on the photosensitive drum 225. , 233 is a developing device for supplying black (Bk) toner, and forms a magenta toner image on the photosensitive drum 229 in accordance with the laser beam.

  The toner images of the four colors (Y, M, C, Bk) are transferred onto the paper, and a full color output image can be obtained.

  The paper fed from any of the paper cassettes 234 and 235 and the manual feed tray 236 is attracted onto the transfer belt 238 via the registration roller 237 and conveyed. In synchronism with the timing of paper feeding, toner of each color is developed in advance on the photosensitive drums 217, 221, 225, and 229, and the toner is transferred onto the paper as the paper is transported.

  The paper on which the toner of each color has been transferred is separated and transported by the transport belt 239, and the toner is fixed on the paper by the fixing device 240 and discharged onto the paper discharge tray 241.

  In the case of the double-sided operation, the paper fed from any of the paper cassettes 234 and 235 and the manual feed tray 236 passes through the registration roller 237 and is sucked onto the transfer belt 238 and conveyed. In synchronism with the paper feed timing, each color toner is developed in advance on the photosensitive drums 217, 221, 225, and 229. As the paper is transported, image formation on the first surface is performed, and the toner is fed to the paper. Is transferred to.

  The sheet on which the toner of each color has been transferred is separated, conveyed by a conveyor belt 239, fixed to the sheet by a fixing device 240, and passed through a sheet discharge vertical path 246 by a sheet discharge deflecting plate. It is conveyed to 245. Then, after a prescribed time after passing through the paper, the double-side reversing unit entrance roller reversely rotates, and the paper is reversed and conveyed to the double-sided path conveying unit 247 and conveyed to the double-sided path 244. At this time, the upper side of the sheet on the duplex path 244 is an image of the first side. When the paper is conveyed to the double-sided path, it is fed again immediately after the paper is aligned, image formation on the second side is performed, and the paper is discharged to the paper discharge tray 241 after passing through the fixing device 240. When the duplex operation is continuously performed on a plurality of sheets, refeeding from the duplex path and feeding from the sheet tray are alternately performed.

  The four photosensitive drums 217, 221, 225, and 229 are arranged at equal intervals with a distance d, and the sheet is conveyed at a constant speed v by the conveyance belt 239, and this timing synchronization is performed. The four semiconductor lasers are driven.

[Image signal flow]
Fig. 1 shows the flow of the image signal.

  A CCD sensor 208 outputs a read image as a digital signal for each of the three color components of red (R), green (G), and blue (B).

  A masking circuit 112 converts an input (R0, G0, B0) signal into a standard (R, G, B) signal by an operation according to the following equation.

|--| |--| |--|
| R | | c11 c12 c13 | | R0 |
G | = | c21 c22 c23 | | G0 |
| B31 | c31 c32 c33 | | B0 |
|--| |--| |--| ……… (1)
However, cij (i = 1, 2, 3 j = 1, 2, 3) is a constant unique to the apparatus considering various characteristics such as sensitivity characteristics of the CCD sensor / spectrum characteristics of the illumination lamp.

  Reference numeral 104 denotes a luminance / density conversion unit, which is constituted by a RAM or ROM look-up table, and performs calculations as shown in the following equation.

C1 = -α x log10 (R / 255)
M1 = -α x log10 (G / 255) (2)
Y1 = -α x log10 (B / 255) (α is a constant)
An output masking / UCR circuit unit 106 converts the M1, C1, and Y1 signals into Y, M, C, and Bk signals that are toner colors of the image forming apparatus, and performs the following calculation.

|--| |--| |--|
C11 | a11 a21 a31 a41 | | C1 |
M | = | a12 a22 a32 a42 | | M1 |
Y13 | a13 a23 a33 a43 | | Y1 |
| Bk | | a14 a24 a34 a44 | | Bk1 |
|--| |--| |--| ……… (3)
However, aij (i = 1, 2, 3, 4 j = 1, 2, 3, 4) is a device-specific constant considering the color characteristics of the toner.
Bk1 = min (C1, M1, Y1) (4)
As described above, based on the above equations (2), (3), and (4), the C1, M1, Y1, and Bk1 signals based on the R, G, and B signals read by the CCD sensor are based on the spectral distribution characteristics of the toner. Corrected to C, M, Y, Bk signals and output.

  On the other hand, reference numeral 105 denotes a character / line image detection circuit, which is a character / line image detection circuit that determines whether each pixel in the document image is a part of the character or line image and generates a determination signal TEXT. A compression / decompression circuit 107 compresses the image signal (R, G, B) and the character / line drawing determination signal TEXT, stores the data in the memory 108 after reducing the amount of information, and reads the data read from the memory 108 Thus, the image signal (R, G, B) and the character / line drawing determination signal TEXT are expanded. The image compression / decompression circuit is not particularly described.

  Reference numeral 312 denotes a gamma correction circuit, which performs gamma correction according to the printer characteristics by the LUT.

[Operation of copy machine alone]
The operation of the copying machine alone will be described. In the case of copying machine operation, the image signal read by the CCD 208 is written into the memory 108 via the masking circuit 112 and the luminance / density conversion unit 104. The character / line drawing determination signal TEXT determined by the character / line drawing determination circuit 105 is written into the memory 108 after being compressed by the compression / decompression circuit.

  Further, the data read from the memory 108 is sent according to the image forming timing of the copying machine, and sent to the laser driver through a PWM circuit (not shown). The timing chart is shown in FIG.

  In FIG. 3, the image read by the CCD 208 is written into the memory 108 at the timing indicated by 401. Further, the image data written on the memory 108 is read at the timings indicated by 402, 403, 404 and 405. The timing relationships shown in 402, 403, 404, and 405 are read out at intervals of time d / v, as shown. Here, as described above, d is the interval between four drums arranged at equal intervals, and v is the speed of the sheet conveyed by the conveyance belt.

[PWM circuit]
FIG. 4 is a block diagram showing a configuration example of the PWM circuit. However, the configuration of FIG. 4 is for one color, and the configuration of FIG. 4 is required for each color of YMCK.

  Reference numeral 601 denotes a D / A converter that converts an input digital image signal into an analog signal and sends the analog signal to the comparator 605. Reference numeral 602 denotes an image triangular wave generator that emphasizes gradation, and generates a triangular wave with a period of one pixel. Reference numeral 603 denotes an image triangular wave generator that places importance on resolution, and generates a triangular wave having a period of two pixels. A selector 604 selects one of two triangular waves according to the determination signal TEXT, and sends it to the comparator 605.

  With the above configuration, in the image area that forms the character or line drawing whose determination signal TEXT is “1”, the triangular wave for the image that emphasizes the resolution output from the triangular wave generator 603 and the analog signal are output from the comparator 605. Are compared. Further, in an image area other than a character or line drawing, the comparator 605 compares the triangular wave for image output from the triangular wave generator 602 with an emphasis on gradation and the analog signal. The output of the comparator 605 is input as a PWM signal to the laser driver 606 that drives the semiconductor laser element 607.

  FIG. 5 is a diagram showing the state of pulse width modulation, and the upper part of FIG. 5 shows the state of pulse width modulation in an image that emphasizes gradation. The output 801 of the D / A converter 601 is compared with a triangular wave 802 having a two-pixel cycle, and a PWM signal 803 is obtained. On the other hand, the lower part of FIG. 5 shows a state of pulse width modulation in an image in which resolution is important. In 804, the output 804 of the D / A converter 601 is compared with a triangular wave 805 having a period of one pixel, and a PWM signal 806 is obtained.

  Actually, since the PWM signals 803 and 806 are adaptively switched by the determination signal TEXT and output, preferable image formation corresponding to the image area characteristics of the image to be formed is performed.

[Outline of automatic gradation correction]
In the present embodiment, in order to obtain image density and gradation stability during formation of a full-color image, density and at least one gradation control (hereinafter referred to as “automatic gradation correction”) are performed.

  FIG. 7 is a flowchart illustrating an example of control in automatic gradation correction. 8A to 8E are display examples of the operation panel of the image forming apparatus 103. FIG.

  Control starts when an “automatic gradation correction” key (not shown) on the operation panel is pressed. The screen shown in FIG. 8A is displayed on the operation panel. When the “Test Print” key is pressed, in step S123 shown in FIG. 7, first, it is determined whether there is a recording sheet necessary for forming the test pattern, and a warning is displayed if it is determined that there is no sheet. Is done.

  First, in order to form the test pattern 1, the test pattern 1 is output using a standard contrast potential (described later) according to the environmental conditions of the image forming apparatus 103 as an initial value. As shown in FIG. 9, the test pattern 1 is a band pattern 901 (Y), 902 (M), 903 (C), 904 of intermediate gradation density from the maximum density patch (density signal level 255) of four colors of YMCK. (Bk).

  When test pattern 1 is output, test pattern 2 and test pattern 3 are output. Note that when the test pattern 2 and the test pattern 3 are output, the gamma correction function of the γ correction unit 312 is stopped.

  As shown in FIG. 12, the test pattern 2 includes the M patch group 1101 and 1105, the Bk patch group 1102 and 1106, the Y patch group 1103 and 1107, and the C patch group 1104 and 1108 in 64 rows of 4 rows and 16 columns. It consists of a group of gradation patches. By assigning these 64-gradation patches with a low density region out of 256 gradations in a concentrated manner, the gradation characteristics in the highlight portion can be adjusted favorably. The patch groups 1101, 1102, 1103, and 1104 are configured with patches having a resolution of 200 LPI (lines / inch), and the patch groups 1105, 1106, 1107, and 1108 are configured with 400 LPI patches. The test pattern 3 has a color scheme different from that of the test pattern 2 so as not to be mistaken for the test pattern 2, and further has a resolution of 400 LPI and 800 LPI.

  In each test pattern, patches of the same gradation pattern may be output at two resolutions, but if the gradation characteristics differ greatly due to differences in resolution, a gradation pattern corresponding to the resolution is set. Is preferred.

  When the output of the three types of test patterns is completed, the screen changes to the test pattern reading screen. The test pattern reading can be performed either by placing the test pattern directly on the platen glass 201 or by an unillustrated document circulation device (hereinafter referred to as RDF). Two methods are provided for reading from: Here, a case where a test pattern is read from the platen glass will be described first.

  When reading the test pattern from the platen glass, when the test pattern is placed on the platen glass 201, the operation panel displays the screen shown in FIG. 8B. When the “read” key is pressed, reading of the test pattern 1 is started in step S102. In normal copying, reading is performed with an accuracy of 400 DPI. However, since sampling at a test pattern does not require high-resolution sampling, reading is performed at the fastest scan corresponding to the number of data samplings. For example, in the case of the present embodiment, the number of samplings in the scanning direction is about 2 DPI when the number is the largest, so that even when the scanner is operated at a speed of 10 times, the calculation can be performed sufficiently.

  The read RGB data of the test pattern 1 is converted into an optical density by the LUT of the luminance density conversion unit 104. Note that the coefficient calculated using the equation (2) is set in advance in the LUT of the luminance density conversion unit 104. That is, the correction coefficient α in Expression (2) is adjusted so as to obtain the optical density. The converted data passes through the compression circuit 107 and is recorded as image data in the memory 108. After that, color information is sampled from the data recorded in the image memory, and it is determined whether the image is test pattern 1. If it is determined that the image is not test pattern 1, a warning message is displayed on the operation panel. Is displayed (S111), and the operator correctly resets the test pattern 1 on the document table, and then reads step s102 again.

  When the scan of test pattern 1 is completed normally, the screen shown in FIG. 8C is displayed on the operation panel. When the output test pattern 2 is placed on the platen glass 201 and the “read” key is pressed, reading of the test pattern 2 is started in step S104.

  When the “read” key is pressed in step s104, the test pattern 2 is scanned and recorded in the memory 108 in the same procedure as the test pattern 1, and then it is determined whether it is the test pattern 2, If test pattern 2 is not placed correctly, a warning message is displayed on the operation panel (s112), and after the operator has correctly placed test pattern 2 on the platen, it reads step s104 again. .

  Next, the screen shown in FIG. 8D is displayed on the operation panel. When the output test pattern 3 is placed on the platen glass 201 and the “read” key is pressed, reading of the test pattern 3 is started in step S106. Is done.

  When the “read” key is pressed in step s106, the test pattern 3 is scanned and recorded in the memory 108 in the same procedure as the test pattern 2, and then it is determined whether it is the test pattern 3, If test pattern 3 is not placed correctly, a warning message is displayed on the operation panel (s113), and after the operator has correctly placed test pattern 3 on the platen, step s106 is read again. .

  Next, a case where a test pattern is read from RDF will be described.

  When test pattern 1, test pattern 2 and test pattern 3 are set in RDF (not shown), the screen shown in FIG. 8E is displayed. At this time, the order of the test patterns set in the RDF does not matter. When the reading key is pressed, reading is started in step S106. First, one test pattern is drawn to the platen, scanned, stored in the memory 108, and the read pattern is the test pattern 1, test pattern 2 from the color information of the image data stored in the image memory. Whether the test pattern 3 is selected is determined.

  When the scanning is completed, the second original is drawn into the document table, scanned, and stored in the image memory. After the pattern is determined, the last test pattern is drawn in and scanned and stored in the image memory. The scanning at this time is read at a scanning speed corresponding to the number of samplings, as in the case of scanning a test pattern from the document table.

  If the read pattern is not correct (S109), a warning message is displayed on the operation panel (s114), and after the operator correctly sets the test pattern to RDF, read step s108 again. Do.

  When all the test patterns have been read, the color information of each test pattern stored in the image memory is read. Which part of the image memory is to be read out is determined in advance because the number and position of the color patches of each pattern are determined, and color information is sampled according to that information.

  The surface of the photosensitive drum when the photosensitive drum primarily charged by the contrast potential used for forming the test pattern 1, that is, the developing bias potential, is scanned by the laser beam output from the semiconductor laser element driven at the maximum light emission level. When the potential difference is a and the maximum density at that time is Da, the density value relative to the surface potential of the photosensitive drum is almost linear as shown by the solid line L in FIG. It is. However, in the two-component development system, when the toner density in the developing device changes and decreases, the density value with respect to the surface potential of the photosensitive drum is non-linear in the region near the maximum density as shown by the broken line N in FIG. There is a case. Therefore, the control amount is determined by setting a maximum control target value of 1.7 with a margin of 0.1 in the final maximum density target value 1.6.

  The contrast potential b is obtained using the following equation. However, ka is a correction coefficient, and it is preferable to optimize Ka according to the type of development method in step S120.

b = (a + ka) × 1.7 / Da ……… (10)
Next, a method for obtaining the grid potential and the developing bias potential from the contrast potential b will be briefly described. FIG. 10B is a diagram showing an example of the relationship between the grid potential and the surface potential of the photosensitive drum.

  The grid potential Vg is set to -300V, the surface potential Vd when the photosensitive drum is scanned with a laser beam with the light emission level of the semiconductor laser element minimized, and the light emission level of the semiconductor laser element is maximized to laser the photosensitive drum. The surface potential Vl when scanned with a beam is measured with a surface potential meter 708 (see FIG. 6). Similarly, Vd and Vl are measured when the grid potential Vg is set to -700V. The relationship between the grid potential Vg and the surface potential of the photosensitive drum is obtained by interpolating and extrapolating between the obtained -300 V and -700 V data. Control for obtaining the potential data is referred to as “potential measurement control”.

  Then, a development potential Vdc is set by providing a predetermined potential difference Vback (for example, 150 V) so that the so-called covering toner that adheres to the image from the obtained Vd does not occur. The contrast potential Vb is a differential voltage between the development biases Vdc and Vl. The larger the Vb, the larger the maximum density. Further, it can be obtained from the grid potential Vg and the development bias potential Vdc for obtaining the obtained contrast potential Vb and FIG. 10B.

  In this embodiment, in step S121, as described above, the contrast potential Vb is obtained so that the target value 1.7 of the maximum density is obtained, and the grid potential Vg and the development bias potential Vdc are set so that the contrast potential Vb is obtained. .

  Next, the role of the γ correction unit 312 and a method for correcting the gradation will be described. FIG. 11 is a characteristic conversion chart showing an example of density reproduction characteristics.

  A first area I shown in FIG. 11 shows an image reading characteristic for converting an original image into a density signal, a second area II shows a conversion characteristic of a γ correction unit 312 that performs gamma correction on the density signal, and a third area III. Indicates the gamma characteristic of the printer indicating the relationship between the laser output signal and the image density, and the fourth region IV indicates the relationship between the document density and the output image density. That is, the characteristics of the fourth region IV represent the overall gradation characteristics in the image forming apparatus 103. In this embodiment, since each color 8-bit digital signal is handled, the number of gradations of each color is 256.

  Further, the gamma characteristic of the printer in the third region III becomes as indicated by a solid line J by the maximum density control in which the target value of the maximum density is set higher. If the control for increasing the target value of the maximum density is not performed, the gamma characteristic of the printer may not reach the target density 1.6 as indicated by the solid line H. For printers that show the characteristics of the solid line H, no matter how the gamma correction unit 312 is set, the gamma correction unit 312 does not have the ability to increase the maximum density, so the density between DH and 1.6 is reproduced. It becomes impossible.

  In the image forming apparatus 103, in order to make the characteristics of the fourth region IV linear, correction is performed by the gamma conversion characteristics of the second region II by the amount of curvature of the print gamma properties of the third region III. The gamma conversion characteristics given to the γ correction unit 312 can be easily obtained by simply reversing the input / output relationship of the printer gamma characteristics of the third region III.

  Next, the density information of the test pattern 2 recorded in the image memory is obtained in the same manner as the test pattern 1 (S123), and gradation correction for the 200 and 400 glands is performed. At this stage, the gamma characteristic of the printer shown in the third region III of FIG. 11 can be obtained. In step S110, the input / output relationship of the obtained gamma characteristic is exchanged, whereby the gamma conversion characteristic of the γ correction unit 312 is obtained. Set. When obtaining the gamma conversion characteristics, only the number of gradation patterns of test pattern 2 is available, so the missing data is interpolated so that the laser output level corresponds to all levels from 0 to 255 in the density signal. compensate.

  Next, the density information of the test pattern 3 recorded in the image memory is obtained in the same manner as the test pattern 2 (S125), and the gradation of the 800 gland is corrected.

  As described above, in this embodiment, the reproducibility of the image density and gradation (200 gland, 400 gland, 800 gland) is controlled by the control according to the present invention, and the full color image is obtained by the stable image density and gradation reproducibility. Can be formed.

Diagram showing the flow of image signals Overview of image forming device Timing chart showing the operation of a copier alone Block diagram showing configuration example of PWM circuit Diagram showing how pulse width modulation works The figure which shows the detailed structural example of an image formation part. Flow chart showing an example of control in automatic gradation correction 8A to 8E are display examples of the operation panel of the image forming apparatus. Diagram showing an example of test pattern 1 10A and 10B are diagrams for explaining a method of correcting the maximum density from the density information. Characteristic conversion chart showing examples of density reproduction characteristics The figure which shows an example of the test pattern 2 and the test pattern 3

Claims (5)

  Image reading means for scanning an image to read an image, generating means for generating an image signal from the image, storage means for storing the image, and forming means for forming an image on a recording medium based on the stored information An image forming apparatus.   An output means for outputting a test pattern for image correction from the image forming apparatus, a means for reading the output test pattern from the reading means, and an image read from the reading means are temporarily stored in the storage means. Means for storing, means for sampling color information data of the test pattern from the test pattern stored in the image storage means, and conversion table creating means for creating a conversion table for correcting an image from the sampled color information data .   Correction means for correcting an image signal using the conversion table described above for an image signal generated by scanning a document, and means for forming an image on a storage medium based on the corrected image signal corrected by the correction means .   The reading means for reading the above-described image correction test pattern image includes a reading means on the platen and a reading means using a document circulation device. In either case, a plurality of image correction test patterns are read. It can be read and stored in a plurality of image memories. An image forming apparatus, wherein a scan speed for reading the test pattern for image correction described above is variable, and is read at a scan speed corresponding to a sampling interval of the test pattern.

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