JP3828296B2 - Image reading device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image reading apparatus which is applied to a digital copying machine, a scanner device, and the like and reads image information using a line-shaped CCD image sensor.
[0002]
[Prior art]
In general, in an image reading apparatus using this type of linear CCD image sensor, a CCD output is sampled and held, and after adjusting its gain, A / D conversion is performed to obtain a digital signal corresponding to the image. ing.
[0003]
FIG. 5 shows a schematic configuration of an image reading apparatus in a conventional digital copying machine. The light emitted from the light source is applied to the original on the contact glass, and the reflected light from the original is imaged on a linear CCD (CCD image sensor) 1 through a mirror and a condenser lens, and is photoelectrically converted to an electrical signal. Is converted to The output of the CCD 1 is sampled and held in the analog processing circuit 3 on the same reading substrate 2, and after gain adjustment or the like, it is converted into a digital signal by the A / D converter 4. The digitized image data is transmitted to the digital processing circuit 6 on the image processing board 5 to be subjected to shading correction processing, filter processing, and the like, and finally transmitted to the writing unit for optical writing. The digital processing circuit 6 is controlled by the CPU 7. In addition to the memory 8 being connected to the CPU 7, the CPU 7 is responsible for controlling each part on both the boards 2 and 5.
[0004]
FIG. 6 shows a configuration example of the analog processing circuit 3 and the like mounted on the reading substrate 2. Here, it is assumed that the CCD 1 is a two-output type that outputs alternately in two systems of even pixels (EVEN) and odd pixels (ODD). For this reason, sample hold amplifiers 9 and 10 constituting line clamp circuits are provided for each system on the output side of the CCD 1. These sample and hold amplifiers 9 and 10 extract signal components. The extracted signal components are synthesized in time series by the multiplexer 11 into one system output, and input to the two-stage amplifiers 12 and 13 connected in series forming an amplifier circuit for gain adjustment. These sample hold amplifiers 9 and 10 perform white level correction (shading correction).
[0005]
Here, in the sample hold of the output of the CCD 1, a line clamp is applied to the CCD output. As shown in FIG. 7, the CCD output includes a black pixel output period and an effective pixel period for reading actual image data. Conventionally, the clamping operation is limited to the black pixel output period. Yes. In other words, clamping is performed during the black pixel output period (L level period of the clamp signal) to keep the potential between one line constant. During this black pixel output period, the photoelectric conversion element in the CCD 1 is shielded from light, and the level does not fluctuate even when light (document reflected light) enters. Accordingly, the output of the CCD 1 during this black pixel output period is referred to as “black level” with the absolute black reference of the image data. That is, there is no signal at a level lower than this.
[0006]
Here, when attention is paid to black level adjustment, in the two-output CCD 1 as shown in the figure, the black of the even-numbered pixels and the odd-numbered pixels are caused by the difference between the output levels of the two systems, the gain of the subsequent processing circuit, the error of the circuit constant, and the like. There is a difference in level, which is inconvenient for the obtained image. In other words, if the image data is processed through the multiplexer 11 as it is, the reference black level, which should be the same level as the original, is different. May occur.
[0007]
In adjusting such a black level difference, the offset level of one of the sample-and-hold amplifiers 9 and 10 is changed in the shaded black pixel output period, and the output after A / D conversion by the A / D converter 4 is performed. Can be said to be the same in the two systems.
[0008]
Here, as a countermeasure to match this black level difference with higher accuracy, conventionally, a background document tracking mechanism is used to read a dark document or the like (for example, the lower surface of the scale plate instead of the document) Naturally, the detected signal level of the background is small, and there is a signal level that is input to the reference terminal of the A / D converter to increase the apparent black level difference. Adjustment is performed in this state to achieve high accuracy. That is, when adjusting the black level difference, by feeding back the peak value when the black original is read as the reference voltage of the A / D converter, the difference is enlarged with a low reference voltage and highly accurate. To be adjusted.
[0009]
[Problems to be solved by the invention]
According to such a black level adjustment method, it is possible to adjust with higher accuracy than when no countermeasures are taken, but the detection variation of the background portion and the variation of the original when detecting (brightness / darkness) Because of this, the reference voltage of the A / D converter is not constant, so that there is a possibility that the adjustment accuracy varies.
[0010]
The black level is preferably “0” from the viewpoint of securing the dynamic range of the image data. However, since it cannot take a value lower than “0” during A / D conversion, the true data is the reference. Even when the voltage is lower than “0”, “0” is output. For this reason, in reality, the black level needs to be as small as possible with a value larger than “0”.
[0011]
Conventionally, as a method of obtaining black pixel data, a method is used in which data of a black pixel output period of a CCD is read at the time of reading a document, and an average value of the data is subtracted from the document data. In other words, at the time of adjustment, this average value is temporarily stored in a register, and thereafter, the control CPU reads it in accordance with the adjustment and performs processing so as to obtain an appropriate value. It is only an average value of data in one line, and noise with a fast cycle that changes in units of pixels can remove some influence, but it is weak against low frequency noise such as a line cycle. There is.
[0012]
SUMMARY OF THE INVENTION An object of the present invention is to provide an image reading apparatus capable of stably and highly accurately adjusting a black level difference between a plurality of CCD image sensor outputs.
[0013]
It is another object of the present invention to provide an image reading apparatus that can automatically correct a black level output even if it falls below a reference voltage of an A / D converter and can always ensure an appropriate dynamic range. And
[0014]
Furthermore, an object of the present invention is to provide an image reading apparatus that has high black level detection accuracy and enables high-precision black level adjustment even in an apparatus that does not perform black shading.
[0015]
[Means for Solving the Problems]
According to a first aspect of the present invention, a line-shaped CCD image sensor that receives an optical signal corresponding to an image, photoelectrically converts it, and outputs it in a plurality of systems, and a line clamp circuit that performs line clamping on the output of the CCD image sensor And an analog processing circuit having an amplifier circuit, and an A / D converter for converting an analog signal output from the analog processing circuit into a digital signal, a reference terminal of the A / D converter a D / a converter, and scalp following circuit, and a fixed voltage circuit, selectively connectable to provided, the D / a converter in a black level adjustment between the analog signal output is divided into a plurality of systems Is connected to a reference terminal of the A / D converter, and a plurality of analog signals are transmitted using a predetermined reference voltage set in the D / A converter. A black level detecting means for detecting a level difference between the signals, and a correcting means for correcting the level difference detected by the black level detecting means .
[0016]
Therefore, when the black level is adjusted, the output of the D / A converter connected to the reference terminal of the A / D converter is held at a predetermined reference voltage, so that adjustment with stable accuracy is possible. In addition, the black level adjustment is prevented from being affected by variations caused by inputting the detection result of the background tracking circuit having variations due to variations in the document itself to the reference level of the A / D converter. be able to.
[0017]
The invention according to claim 2 is an output level detection means for detecting whether or not a minimum value output from the A / D converter is “0” at the time of scanning operation or standby.
When the output level detecting means detects that the minimum value is “0”, the output level detecting means includes a black level processing control means for performing black level readjustment by the black level adjusting circuit.
[0018]
Accordingly, when the read data subjected to A / D conversion falls below the reference voltage and the minimum value becomes “0”, the black level is automatically readjusted, so that an appropriate dynamic range can always be maintained. .
[0021]
DETAILED DESCRIPTION OF THE INVENTION
A first embodiment of the present invention will be described with reference to FIGS. The same parts as those shown in FIGS. 5 to 7 are denoted by the same reference numerals, and description thereof is omitted (the same applies to the following embodiments). In particular, the schematic configuration shown in FIG. 5 is used as it is. First, correction of the black level in the present embodiment is performed by changing the offset levels of the sample hold amplifier 9 (or 10) and the amplifier 13 (although the clamp is applied to each of the sample hold amplifiers 9 and 10). A black level correction can be similarly performed by adding a circuit and changing the clamp level, and a plurality of means can be considered as the black level correction means. It is assumed that the configuration is variable).
[0022]
First, a D / A converter 14 that exchanges data with the CPU 7 is provided. The D / A converter 14 has, as one function thereof, a function of correcting the black level by being connected to the sample hold amplifier 9 and the amplifier 13 and varying the offset level. Further, a background tracking circuit 16, a fixed voltage circuit 17, and a D / A converter 14 are provided to the reference terminal 15 of the A / D converter 4 so as to be selectively connectable via an analog switch 18. Yes. The background tracking circuit 16 is connected to the reference terminal 15 by switching the analog switch 18 when the background tracking mode is set, and outputs the peak value read from the black document to the reference terminal 15 of the A / D converter 4. The fixed voltage circuit 17 supplies a fixed voltage as a reference voltage to the reference terminal of the A / D converter 4 when selected by the analog switch 18. The D / A converter 14 functions as a variable voltage circuit in comparison with the fixed voltage circuit 17 and is connected to the reference terminal 15 by switching the analog switch 18 when the black level adjustment mode of the present embodiment is set. It becomes. The D / A converter 14 supplies a predetermined reference voltage set by the CPU 7 to the D / A converter 4. Here, in the present embodiment, functions such as black level detection means are newly added to the CPU 7 that controls each part with the digital processing circuit 6 as the center. This black level detection means switches the analog switch 18 so that the D / A converter 14 is connected to the reference terminal 15 of the A / D converter 4 when adjusting the black level between the analog signals output in two systems. A function of detecting a level difference between two analog signals using a predetermined reference voltage set by the lever D / A converter 14 is executed.
[0023]
With such a configuration, operation control during black level adjustment will be described with reference to the flowchart shown in FIG. First, the lamp is turned on in order to perform a black original reading operation with the reading carriage of the scanner in the home position standby state. Next, when the background tracking mode is set, the analog switch 18 is switched and the peak value by the background tracking circuit 16 is output to the reference terminal 15 of the A / D converter 4. In this state, the black pixel output period data in the CCD 1 is read, the black levels of the two systems of the even and odd pixels are recognized, the difference between them is detected, and the black level adjustment value is set so that the difference disappears. Is adjusted so that the offset voltages of the amplifiers 9 and 13 in the analog processing circuit 3 are adjusted. Such processing is processing that has been conventionally performed as black level adjustment in the background tracking mode.
[0024]
On the other hand, when the black level adjustment mode according to the present embodiment is set, the background tracking mode is not set, and the D / A converter 14 is switched to the reference terminal 15 by switching the analog switch 18 as reference switching processing. For the connection state. Then, a predetermined voltage for lowering the reference voltage is set to the reference terminal 15 of the A / D converter 4 through the D / A converter 14. In this state, the black pixel output period data in the CCD 1 is read, the black levels of the two systems of the even and odd pixels are recognized, the difference between them is detected, and the black level adjustment value is set so that the difference disappears. Is adjusted so that the offset voltages of the amplifiers 9 and 13 in the analog processing circuit 3 are adjusted. That is, the processing after setting the reference voltage is the same as in the conventional background tracking mode, but the reference voltage is the value set in the D / A converter 14 by the CPU 7, and the density of the original being read Since it is a stable and constant voltage that is not affected at all, it can be detected with high accuracy.
[0025]
A second embodiment of the present invention will be described with reference to FIG. In the present embodiment, it is detected whether the appropriately adjusted black level is always maintained at the correct level, and when there is an abnormality, the black level is readjusted and corrected to stabilize the image data. Is intended to be obtained. That is, as described above, the black level is originally desirably “0” from the viewpoint of securing the dynamic range of the image data. However, since it cannot take a value lower than “0” during A / D conversion, “0” is output even when the true data is lower than the reference voltage of the A / D converter 4. For this reason, it is necessary to take a small value as much as possible, which is larger than “0”. The minimum value of the read data after A / D conversion at the time of the reading operation or at the standby time is “0” (for example, in 8-bit image data, “0” is the black side, “255” is the white side, and the black level is (When adjusted to “5”), there is a possibility that a signal lower than the reference voltage of the A / D converter 4 is input, which is not preferable from the viewpoint of protection of the element. Further, even though the black level is adjusted to be a constant value, taking “0” may indicate that there is an abnormality in the black level adjustment circuit.
[0026]
Therefore, in the present embodiment, a comparator 19 that functions as output level detection means is connected to the output side of the A / D converter 4. The comparison reference value of P [7, 0] on the P terminal side of the comparator 19 is set to “0”, and the output of the A / D converter 4 is given as Q [7, 0] on the Q terminal side. . Here, the comparator 19 detects the CPU 7 or the like when the output (Q [7,0]) of the A / D converter 4 coincides with P [7,0] = 0, that is, when it becomes “0”. A detection signal is output to the image processing substrate 5 side including the detection signal. The CPU 7 constantly monitors the output signal from the comparator 19, and when this detection signal is output, the black level is readjusted by the black level adjusting circuit after the current job is completed. Here, when the comparator 19 detects that the minimum value of the output of the A / D converter 4 is “0”, the CPU 7 of the present embodiment readjusts the black level by the black level adjustment circuit. The function of black level processing control means for performing the above is added.
[0027]
Therefore, according to the present embodiment, the black level is automatically readjusted when the A / D converted read data falls below the reference voltage and the minimum value becomes “0”. Dynamic range can be maintained.
[0028]
A third embodiment of the present invention will be described with reference to FIG. In the present embodiment, in the black level adjustment, the influence of noise is reduced so that a more accurate black level can be obtained. Therefore, in the present embodiment, the CPU 7 is added with a function of arithmetic means for multiplying and averaging the average value of the data in the main scanning black pixel output period in the CCD 1 during the shading operation by the shading correction circuit (not shown). The adjustment circuit is configured to adjust the black level for the output of the CCD 1 using the average of the average values calculated by the calculation means.
[0029]
Generally, the shading processing by the shading correction circuit performed at the time of document reading is performed in order to correct variations in sensitivity of the CCD 1 and variations in optical systems such as lenses and mirrors. The document scanning period is set in the main scanning direction. In the period asserted by the signal “LGATE” indicating the reference white plate reading period and the signal “SHGATE” indicating the reference white plate reading period, the multiple addition averaging process is executed. Here, in the present embodiment, during black level adjustment, the signal “LGATE” is forcibly asserted for a certain period in the black pixel output period, regardless of the reading position, and the black pixel data obtained in that period is A calculation process for averaging the average values is performed, and a normal addition averaging process is performed for black level adjustment. Note that this arithmetic processing may be performed in a hardware configuration in addition to being performed in the CPU 7.
[0030]
Therefore, even in a device that does not perform black shading, the black level detection accuracy is high by using the average value of the black pixel data averaged during the black pixel output period using the shading correction circuit. Accurate black level adjustment is possible.
[0031]
【The invention's effect】
According to the first aspect of the present invention, when the black level is adjusted, the output of the D / A converter connected to the reference terminal of the A / D converter is held at the predetermined reference voltage, so that the black with stable accuracy can be obtained. Level adjustment is possible. In addition, the black level adjustment is prevented from being affected by variations caused by inputting the detection result of the background tracking circuit having variations due to variations in the document itself to the reference level of the A / D converter. be able to.
[0032]
According to the second aspect of the present invention, even when the A / D converted read data falls below the reference voltage and the minimum value becomes “0”, the black level is automatically readjusted. The dynamic range can be maintained.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a first embodiment of the present invention.
FIG. 2 is a flowchart showing black level adjustment processing.
FIG. 3 is a block diagram showing a second embodiment of the present invention.
FIG. 4 is a time chart showing a third embodiment of the present invention.
FIG. 5 is a block diagram showing a schematic configuration of an image reading apparatus used in a conventional example and in the present embodiment.
FIG. 6 is a block diagram showing a schematic configuration on the side of the reading substrate.
FIG. 7 is a time chart showing a clamping operation.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 CCD image sensor 3 Analog processing circuit 4 A / D converter 9, 10 Line clamp circuit 12, 13 Amplifier circuit 14 D / A converter 15 Reference terminal 19 Output level detection means

Claims (2)

A linear CCD image sensor that receives an optical signal corresponding to an image, photoelectrically converts it, and outputs it in a plurality of systems, a line clamp circuit that performs line clamping on the output of the CCD image sensor, and an analog processing circuit having an amplifier circuit And an A / D converter that converts an analog signal output from the analog processing circuit into a digital signal,
A reference terminal of the A / D converter is provided so that a D / A converter, a background tracking circuit, and a fixed voltage circuit can be selectively connected,
A predetermined reference set in the D / A converter by connecting the D / A converter to a reference terminal of the A / D converter when adjusting the black level between the analog signals divided into a plurality of systems. Black level detecting means for detecting a level difference between the analog signals of a plurality of systems using a voltage;
An image reading apparatus comprising: correction means for correcting a level difference detected by the black level detection means . Output level detection means for detecting whether or not the minimum value output from the A / D converter is “0” during scanning operation or standby;
Claims when it minimum value by the output level detecting means is "0" is detected, characterized in that it comprises a black level processing control means for causing the re-adjustment of the black level by said black level adjusting circuit Item 2. The image reading apparatus according to Item 1 .

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