JPH06164938A - Picture reader - Google Patents

Abstract

PURPOSE:To accurately and surely execute shading correction of an analog signal corresponding to a first photoelectric conversion element of a picture read sensor. CONSTITUTION:An analog signal picked up by a CCD 11A is read sequentially by an A/D converter 14 and shading correction is executed by comparison of it with each of plural division levels formed by dividing uniformly an upper limit level VRT and a lower limit level VRB set synchronously with the read. A digital signal obtained by picking up a reference white level is stored in a memory 15A and read corresponding to each picture element synchronously with the analog signal read from the CCD 11A at reading of an original image, the signal is D/A-converted by a D/A converter 16A and the result is set as an upper limit level VRT A digital signal at a low level corresponding to a final picture element of a non-image pickup area of the CCD 11A stored in the memory 15A is changed into a high level and the upper limit level VRT, corresponding to the first picture element of the image pickup area is set to a prescribed high level without a delay.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image reading device such as a digital copying machine or a facsimile.

[0002]

2. Description of the Related Art An image reading apparatus such as a digital copying machine or a facsimile is composed of an image reading sensor and an A / D converter, and an analog signal read by each photoelectric conversion element of the image reading sensor is converted into the A / D signal. It is designed to be converted into a digital signal and read by a converter.

As shown in FIG. 7, the image reading sensor 20 has a plurality of photoelectric conversion elements (hereinafter referred to as sensor elements).
A plurality of CCD linear image sensors 21 are arranged in a line, and are arranged in a line. The CCD linear image sensor 21 has a non-imaging region 211 at both ends, and the sensor elements in the non-imaging region 211 are shielded so that light does not enter from the outside.

Further, as shown in FIG. 8, the A / D converter 22 has a resistor circuit 221 for equally dividing the upper limit level V _RT and the lower limit level V _RB into multiple levels, and an input analog signal V _IN. Has a level correction circuit 222 for performing level correction on high and low signals by comparing each of the divided levels divided by the resistance circuit 221 and an encoder 223 for converting the level-corrected analog signal into a digital signal. , A / D conversion processing enables level correction of analog signals.

By the way, in this type of image reading apparatus, variations in characteristics such as light receiving sensitivity and dark voltage among the plurality of sensor elements, or substantial sensitivity between the sensor elements due to variations in light emission amount of the light source. Shading correction is performed to correct variations.

Conventionally, the shading correction is performed as follows. First, a digital signal (hereinafter referred to as black shading data) obtained by driving the image reading sensor 20 in a dark state in advance and a reflected light from a reference white light emitted from an exposure lamp having a predetermined light emission amount are received and obtained. The digital signals (hereinafter, referred to as white shading data) to be stored are stored in the black shading memory and the white shading memory respectively corresponding to each sensor element.
Next, when reading an original image in the image forming operation, the white reading memory and the black shading memory are used in synchronization with the reading of the analog signal of each sensor element from the image reading sensor 20 to the A / D converter. The white shading data and the black shading data of the sensor element to be read are read and set to the upper limit level and the lower limit level of the A / D converter.

The A / D converter 22 divides the white shading data (upper limit level) and the black shading data (lower limit level) into small levels of, for example, 256 steps, and divides the analog signal level of each sensor element. The above small level is corrected to a relative level having a sensitivity range between the upper and lower levels.

[0008]

In the conventional shading correction described above, the reference white color is read to obtain white shading data.
Since the non-imaging region that is shielded from light is formed at both ends of the linear image sensor 21, the white shading data for the sensor element in this non-imaging region is substantially the same as the black shading data. Therefore, the upper limit level set in the A / D converter 22 at the time of reading the original image becomes a low level during the A / D conversion of the analog signal in the non-imaging area, and the analog signal in the imaging area is A / D converted. The upper limit level is changed from the low level to the high level during a period in which the A / D conversion operation for the last sensor element in the non-imaging area shifts to the A / D conversion operation for the first sensor element in the imaging area. It will change rapidly to a high level.

Generally, since the stray capacitance C is formed in the resistance circuit 221 in the A / D converter 22 as shown by the broken line in FIG. 8, the upper limit when shifting from the non-imaging area to the imaging area is set. The rise time is delayed by the stray capacitance C, and as shown in FIG. 9, the level cannot be completely raised to a high level before A / D conversion is performed on the first sensor element in the imaging area. The white shading correction of the analog signal for the first sensor element becomes insufficient. As a result, in the image after image formation, white streak is generated at the position corresponding to the first sensor element, and the image quality is significantly deteriorated.

In order to increase the speed of the image reading apparatus,
Since the A / D conversion processing period for each sensor element is shortened, the rising speed of the upper limit level at the time of shifting from the non-imaging region to the imaging region is relatively delayed, and the same problem as described above occurs.

The present invention has been made in view of the above problems, and provides an image reading apparatus capable of surely performing white shading correction for a light receiving signal of the first sensor element of a linear image sensor and enabling suitable image formation. The purpose is to

[0012]

According to the present invention, an image pickup means in which a plurality of photoelectric conversion elements are regularly arranged, a level setting means for setting an upper limit level and a lower limit level, and each read out from the image pickup means. The level correction means for performing level correction by comparing the analog signal level received by the photoelectric conversion element with each division level obtained by dividing the upper limit level and the lower limit level into a plurality of intermediate levels, and the correction means. A / D that converts the corrected analog signal to digital signal
In the image reading apparatus including the conversion means, the upper limit level is set to a high level within a predetermined period before the reading of the analog signal from the image pickup means to the level correction means, and after the reading of the analog signal is started. Is a level at which the analog signal obtained by imaging a predetermined reference white in synchronization with the analog signal received by each photoelectric conversion element read from the image pickup means is set to the upper limit level corresponding to each photoelectric conversion element. It is provided with a setting control means (Claim 1).

Further, according to the present invention, in the image pickup means, a non-image pickup area in which a received light signal is not used as an image signal is formed at the beginning, and at least the last photoelectric conversion element in the non-image pickup area is made to be able to receive light, and the level is set. The setting control means synchronizes the analog signal received by each photoelectric conversion element read from the image capturing means with the analog signal obtained by imaging the reference white to the upper limit level corresponding to each photoelectric conversion element. It is set (claim 2).

Further, according to the present invention, the image pickup means has a non-image pickup area formed by arranging light-shielded photoelectric conversion elements at the beginning.
A storage unit that stores a digital signal obtained by imaging the reference white color corresponding to each photoelectric conversion element and a digital signal that corresponds to the last photoelectric conversion element of the non-imaging area stored in the storage unit The level setting control means includes a storage signal changing means for changing to a level digital signal and a D / A converting means for converting a digital signal read from the storage means into an analog signal. The digital signal corresponding to each photoelectric conversion element is read from the storage means in synchronization with the analog signal received by each photoelectric conversion element, and the analog signal converted by the D / A conversion means is set to the upper limit level. (Claim 3).

[0015]

According to the image reading apparatus of the present invention, the analog signals picked up by the photoelectric conversion elements of the image pickup means are sequentially read by the correction means and set at the upper limit level corresponding to each photoelectric conversion element. And the lower limit level are divided, and level correction is performed by comparing each divided level generated, and the corrected analog signal is converted into a digital signal.

In the level correction, the upper limit level is set to a high level within a predetermined period before the reading of the analog signal from the image pickup means to the level correction means is started,
When the reading of the analog signal from the image pickup unit to the level correction unit is started, a predetermined reference white image is obtained in synchronization with the analog signal received by each photoelectric conversion element read from the image pickup unit. A high level analog signal is set corresponding to each photoelectric conversion element. As a result, when the A / D conversion operation of the analog signal received by the first photoelectric conversion element read from the image pickup means is performed, the upper limit level is set to the high level immediately before the start of the A / D conversion operation. Therefore, the level is set to a desired high level without delay, and the level correction of the analog signal corresponding to the photoelectric conversion element is reliably performed.

According to the second aspect of the present invention, the image pickup means has a non-imaging region in which the received light signal is not used as an image signal at the beginning, and at least the last photoelectric conversion element in the non-imaging region can receive light. Therefore, a high level analog signal corresponding to the photoelectric conversion element can be obtained by imaging the reference white color.

Therefore, in the level correction, the analog signal obtained by imaging the reference white in synchronism with the analog signal of each photoelectric conversion element read from the image pickup means corresponds to each photoelectric conversion element and has the upper limit. By setting the level, the upper limit level is set to a high level within a predetermined period before the start of reading the analog signal from the image pickup unit to the level correction unit.

According to the third aspect of the present invention, the digital signal obtained by picking up the image of the reference white color is stored corresponding to each photoelectric conversion element. The non-imaging area of the imaging means is shielded from light, and the digital signal corresponding to the last photoelectric conversion element of the non-imaging area obtained by imaging the reference white is stored at a low level. Further, the digital signal corresponding to the last photoelectric conversion element in the non-imaging area stored at the low level is changed to the high level.

Then, a digital signal corresponding to each photoelectric conversion element is read from the storage means in synchronization with the analog signal of each photoelectric conversion element read from the image pickup means and converted into an analog signal by the D / A conversion means. After that, by setting the upper limit level, the upper limit level is set to the high level within a predetermined period before the reading of the analog signal from the image pickup unit to the level correction unit is started.

[0021]

FIG. 4 is a schematic configuration diagram of an optical system of a digital copying machine (image forming apparatus) to which an image reading apparatus according to the present invention is applied.

The digital copying machine 1 has a platen glass 2 and a document retainer 3 on its upper surface, and an optical system L is provided under the platen glass 2. Further, a reference white portion 10 is provided at an appropriate position on the upstream side of the platen glass 2. The reference white portion 10 is a white reflector for taking in white shading data for white shading correction, which will be described later, and has a predetermined whiteness.

The optical system L is a light source 4 including a halogen lamp 41 and a reflector 42 for irradiating the original placed on the platen glass 2 with light, and a light image (original image) reflected from the original as follows. Reflecting mirror 51 leading to the image reading sensor 7,
52 and 53, a lens unit 6 for forming the original image on the image sensor 7 described below, an image reading sensor 7 for reading the original image, and an image reading unit 9 including a reading control unit 8,
At the time of image formation (copying), the document image is read by scanning in the R direction. As will be described later, the optical system L reads the image of the reference white portion 10 and collects the shading correction data during copying.

FIG. 3 is a schematic diagram showing the structure of the image reading section 49. The image reading sensor 7 includes three CCD linear image sensors (hereinafter referred to as CCDs) 11A and 11
B and 11C are arranged on the same line, and the original image is read line by line. Each CC
Each of D11A, 11B, and 11C is a plurality of light-receiving elements (hereinafter, referred to as sensor elements) such as photodiodes arranged one-dimensionally, and an area (hereinafter, referred to as an effective area) used for image reading in a central portion. Areas), and areas 111 that are not applied for image reading at both ends.
(A shaded area in the drawing; hereinafter referred to as an invalid area) is formed. The sensor element in the invalid area 111 is shielded from light so that no light is incident from the outside. Therefore, the CCDs 11A, 11B and 11C are
The connection portions are overlapped and connected so that the effective areas A, 11B, and 11C are continuous, and an image reading area for one line is formed in the entire effective area.

In the reading control section 8, the CCDs 11 mentioned above are provided.
Three A / D converters 12 corresponding to A, 11B and 11C
A, 12B, 12C and one image composition unit 13 are provided. Each of the A / D converters 12A to 12C converts an analog signal output from the corresponding CCD 11A, 11B, 11C into a digital signal and inputs the digital signal to the image synthesizer 13. The image synthesizing unit 13 synthesizes the input digital signals to generate digital image data for one line, and outputs the digital image data to an exposure control unit (not shown).

FIG. 1 is a block diagram showing the configuration of the A / D converter 12A. Further, FIG. 2 shows the A / D converter 1
4 is a schematic diagram showing the internal configuration of FIG.

The A / D converter 12A converts an analog signal input from the CCD 11 into a digital signal.
/ D converter 14, a pair of memories 15A, 15B for storing the digital signal for shading correction, and the digital signals read from the memories 15A, 15B are converted into analog signals, and the A / D converter 14 is respectively provided. V
A pair of D / A converters 16 input to the _RT input and the V _RB input
Each circuit in the A / D converter 12A is controlled by a CPU 17 which controls an image reading operation of the optical system L.

As shown in FIG. 2, the A / D converter 14 has a plurality of resistors connected in series in multiple stages, and the upper limit level V _RT and the lower limit level V _RB are, for example, 256 (= 2 ⁸ ).
A resistance circuit 141 for evenly dividing into intermediate levels of a plurality of stages such as stages, and a level correction circuit 142 for comparing the analog signal V _IN input from the CCD 11 with the respective division levels to correct the level of the analog signal. An encoder 143 for converting the converted level of the analog signal into, for example, a 6-bit digital signal, a latch circuit 144 for latching the converted digital signal, and an output circuit 145 for outputting the digital signal.

The A / D converter 14 calculates the level difference ΔV between the externally input lower limit level V _RB and upper limit level V _RT by 2
The analog signal V _IN input from the CCD 11 is compared with each of the division levels, and the analog signal is converted into an 8-bit digital signal by encoding the comparison result. Output.

The memory 15A is a white shading memory, and a digital signal (hereinafter referred to as white shading data) obtained by reading the image of the reference white portion is stored corresponding to each sensor element of the CCD 11 and is stored in the memory. A black shading memory 15B records a digital signal (hereinafter referred to as black shading data) corresponding to each sensor element of the CCD 11 obtained by reading the image of the reference white portion in a dark state without causing the light source 4 to emit light. It is supposed to be done.

Next, the operation of the image reading apparatus according to the present invention will be described with reference to the flowchart of FIG.

When the copy button is pressed and a copy operation is instructed (step S1), first, the optical system L is moved from the home position to the reading position (shading position) of the reference white portion 10 (step S2). Then, A /
After setting the preset upper limit level and lower limit level to the V _RT input and the V _RB input of the D converter 14 (step S
3) Without reading the light source 4, the image of the reference white portion 10 is read in a dark state (step S4). CCD 11A
The analog signal of the black image read by each sensor element is sequentially converted into a digital signal by the A / D converter 14 and stored as black shading data in the memory 15B corresponding to the sensor element (step S5). ). Similar processing is performed on the analog signal of the black image read by each sensor element of the CCDs 11B and 11C.

Then, the light source 4 is caused to emit light (step S
6) Read the image of the reference white portion 10 in a bright state (step S7). The analog signal of the white image read by each sensor element of the CCD 11A is sequentially converted into a digital signal by the A / D converter 14, and stored as white shading data in the memory 15A corresponding to the sensor element (step). S8). In addition, CCD 11B, 11C
The same processing is performed on the analog signal of the white image read by each sensor element.

Then, the light source 4 is turned off (step S
9), the white shading data corresponding to the last sensor element of the invalid area formed on the head side of the CCD 11 of the memory 15A (where the number of sensor elements in the invalid area is n, the nth sensor element) is valid. The first white shading data of the area is rewritten (step S10). Note that instead of the first white shading data in the effective area, the white shading data corresponding to the n-th sensor element may be rewritten with dummy data near the preset white shading data.

Subsequently, the first white shading data in the effective area is read from the memory 15A to the D / A converter 16A, converted into an analog signal by the D / A converter 16A (step S11), and then A / D. The upper limit level is set by inputting it to the V _RT input of the converter 14 (step S12).
Subsequently, the first black shading data of the effective area is read from the memory 15B to the D / A converter 16B,
After being converted into an analog signal by the / A converter 16B (step S13), it is input to the V _RB input of the A / D converter 14 and the lower limit level is set (step S14).

Then, the light source 4 emits light (step S1).
5) Scan the optical system L (scan in the R direction in FIG. 1) to read the original image. In the reading operation of the original image, C
The white shading data of the sensor element corresponding to the analog signal input to the A / D converter 14 is processed in synchronization with the analog signal of each sensor element output from the CD 11A. And black shading data are set to the V _RT input and the V _RB input, respectively, and the above analog signals are converted into digital signals while being subjected to shading correction. CCD 11B, 1
Similar processing is performed by the A / D conversion units 12B and 12C for the analog data of each sensor element output from 1C.

FIG. 6 is a waveform diagram of the upper limit level set to the V _RT input of the A / D converter 14 in the image reading operation.

In the figure, CLK is the A / D converter 1
4 is a clock that takes the conversion timing, and V _RT is A
This is the upper limit level set for the V _RT input of the / D converter 14.

The analog signals read by the respective sensor elements of the CCD 11A are sequentially input to the A / D converter 14 from the beginning, and in synchronization with this, the white shading data corresponding to each analog signal from the memory 15A reaches the upper limit level. _Is set as the V _RT input of the A / D converter.

Since the nth sensor elements from the head of the CCD 11A are shielded from light in the invalid area, the white shading data obtained by reading the image of the reference white area 10 in this invalid area is substantially the same as the black shading data. Has become. However, as described above, the white shading data corresponding to the last sensor element (nth sensor element) in the invalid area is changed to the white shading data corresponding to the first sensor element in the effective area. , The upper limit level set corresponding to the (n-1) th sensor element in the invalid area is a low level near 0v, but the upper limit level corresponding to the nth sensor element is the first in the effective area. Is the same as the upper limit level corresponding to the sensor element.

Therefore, even when the stray capacitance C (shown by the slanted line in FIG. 2) is formed in parallel with the V _RT input in the A / D converter 14, the stray capacitance C causes There is no delay in the timing of setting the upper limit level corresponding to the first sensor element to a desired level, and the first signal is generated when the analog signal corresponding to the first sensor element in the effective area is A / D converted. It is possible to reliably set the upper limit level corresponding to the sensor element of
White shading correction of the analog signal can be reliably performed on the first sensor element in the effective area. As a result, it is possible to prevent the occurrence of non-uniform image density such as white stripes in the portion corresponding to the first sensor element in the effective area of the copied image.

Further, as the speed of image reading operation is increased, the above A
Even if the processing speed of the / D converter 14 is increased, the setting speed of the upper limit level corresponding to the first sensor element in the effective area is not relatively delayed, and the first speed in the effective area is not increased. Analog signal corresponding to the sensor element of
The upper limit level corresponding to the first sensor element can be reliably set at the timing of D conversion.

In the above embodiment, the white shading data corresponding to the last sensor element (nth sensor element) in the invalid area in the memory 15A is changed to the white shading data corresponding to the first sensor element in the effective area. Although it was changed, the n-th sensor element in the invalid area of the CCD 11A, 11B, 11C can receive light without being shielded, and the high-level white shading data is read at the time of reading the white shading data. Good.

In this case, the CCDs 11A, 11B, 11C
An analog signal for white shading correction received by is stored in association with each sensor element, and the A / D converter is synchronized with the reading of the analog signal from the CCDs 11A, 11B, 11C when reading an original image. It is preferable to input the analog signal for white shading correction to the V _RT input of 14. By doing so, the process of step S10 in FIG. 5 becomes unnecessary, and the process can be speeded up and simplified.

[0045]

As described above, according to the present invention,
The level of the analog signal of each photoelectric conversion element read from the image pickup means is compared with each division level formed by dividing the set upper limit level and lower limit level into a plurality of intermediate levels, and after the correction, In the image reading apparatus for converting the analog signal into a digital signal and reading the analog signal, the upper limit level is set to a high level within a predetermined period before the reading of the analog signal from the image pickup unit to the level correction unit is started, and the analog signal is read. After the signal reading is started, the analog signal obtained by imaging a predetermined reference white in synchronization with the analog signal received by each photoelectric conversion element read from the image pickup means is set to the upper limit corresponding to each photoelectric conversion element. The level is set to the upper limit level before and after the start of the A / D conversion operation of the analog signal received by the first photoelectric conversion element read from the image pickup means. Since the fluctuation of the analog signal is reduced, the A / D of the analog signal corresponding to the first photoelectric conversion element of the image pickup means can be obtained even when the response speed of the upper limit level setting is slow or the processing speed of the A / D converter is increased. You can set the upper limit level to the desired level without delay at the start of conversion operation,
The level correction of the analog signal corresponding to the first photoelectric conversion element of the image pickup means can be accurately performed. As a result, deterioration in image quality due to inaccurate level correction of the analog signal corresponding to the first photoelectric conversion element is reduced.

Further, at the beginning of the image pickup means, a non-imaging region in which the received light signal is not used as an image signal is formed, and at least the last photoelectric conversion element in the non-imaging region can receive light, and in the level correction, a predetermined reference is set. Since the high level analog signal obtained by capturing the white image is set to the upper limit level corresponding to the last photoelectric conversion element in the non-image capturing area, the same effect as described above can be obtained with a simple structure.

Further, the digital signal obtained by picking up the image of the reference white color is stored in the storage means corresponding to each photoelectric conversion element, and is also stored in correspondence to the last photoelectric conversion element in the non-image pickup area of the image pickup means. The digital signal corresponding to each photoelectric conversion element is read from the storage means in synchronism with the analog signal of each photoelectric conversion element read from the image pickup means in the level correction, Since the analog signal is converted and the upper limit level is set, the same effect as described above can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an A / D conversion unit of an image reading apparatus according to the present invention.

FIG. 2 is a schematic diagram showing an internal configuration of an A / D converter.

FIG. 3 is a schematic diagram showing a configuration of an image reading unit of the image reading apparatus according to the present invention.

FIG. 4 is a schematic configuration diagram of an optical system of a digital copying machine (image forming apparatus) to which the image reading apparatus according to the present invention is applied.

FIG. 5 is a flowchart showing a reading operation of the image reading apparatus according to the present invention.

FIG. 6 is a waveform diagram of the upper limit level set for the V _RT input of the A / D converter in the image reading operation.

FIG. 7 is a schematic configuration diagram of an image reading sensor applied to a conventional image reading device.

FIG. 8 is a main part configuration diagram showing a configuration of an A / D converter applied to a conventional image reading apparatus.

FIG. 9 is a diagram showing a waveform of an upper limit level set in a conventional A / D converter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Digital copying machine (image forming apparatus) 2 Platen glass 3 Original presser 4 Light source 41 Halogen lamp 42 Reflector 51-53 Reflector 6 Lens part 7 Image reading sensor 8 Read control part 9 Image reading part 10 Reference white part 11A, 11B , 11C CCD linear image sensor 12A, 12B, 12C A / D converter 13 Image synthesizer 14 A / D converter 141 Resistance circuit 142 Level correction circuit 143 Encoder 144 Latch circuit 145 Output circuit 15A, 15B Memory 16A, 16B D / A converter 17 CPU L Optical system

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazuyoshi Ebe 2-6-80 Shimohanazawa, Yonezawa City, Yamagata Prefecture Yonezawa NEC Corporation

Claims (3)

[Claims] 1. An image pickup means in which a plurality of photoelectric conversion elements are regularly arranged, a level setting means for setting an upper limit level and a lower limit level, and an analog received by each photoelectric conversion element read from the image pickup means. Level correction means for performing level correction by comparing the signal level with each of the divided levels obtained by dividing the upper limit level and the lower limit level into a plurality of intermediate levels, and an analog signal corrected by the correction means as a digital signal. In an image reading apparatus including an A / D conversion unit for converting to, the upper limit level is set to a high level within a predetermined period before the start of reading an analog signal from the image pickup unit to the level correction unit, and After the reading of the analog signal is started, it is obtained by imaging a predetermined reference white in synchronization with the analog signal received by each photoelectric conversion element read from the imaging means. An image reading apparatus comprising level setting control means for setting the analog signal to the upper limit level corresponding to each photoelectric conversion element. 2. The image reading apparatus according to claim 1,
In the imaging means, a non-imaging region in which the received light signal is not used as an image signal is formed at the beginning, and at least the last photoelectric conversion element in the non-imaging region is made receivable.
The level setting control means synchronizes with an analog signal received by each photoelectric conversion element read out from the image capturing means and captures an analog signal obtained by imaging the reference white color in accordance with each photoelectric conversion element. An image reading device characterized by being set to a level. 3. The image reading device according to claim 1,
The image pickup means has a non-image pickup area formed by arranging light-shielded photoelectric conversion elements at the beginning, and a storage means for storing a digital signal obtained by picking up the reference white color corresponding to each photoelectric conversion element, Storage signal changing means for changing the digital signal corresponding to the last photoelectric conversion element of the non-imaging area stored in the storage means to a high level digital signal, and the digital signal read from the storage means to an analog signal. D / A conversion means for converting, and the level setting control means synchronizes with an analog signal received by each photoelectric conversion element read from the image pickup means and digitally corresponds to each photoelectric conversion element from the storage means. Read the signal,
An image reading apparatus, wherein the analog signal converted by the / A conversion means is set to the upper limit level.