JPH06152956A - Image reader - Google Patents

Abstract

PURPOSE:To provide an image reader capable of suppressing quantization errors at minimum by effectively utilizing a conversion dynamic range for an A/D converter corresponding to the level fluctuation of line sensor output, accurately detecting, preserving and digitizing a zero level and even performing correction for each picture element. CONSTITUTION:In the image reader for obtaining image information by detecting electric signals obtained by converging light from an original surface by an image forming lens and making the light incident on an photoelectric converting element 1 as image signals, a sample-and-hold circuit 3 for AC coupling or DC coupling the output of the photoelectric converting element 1, respectively sample holding a light output period and a feed through period for each picture element and respectively outputting voltage differences, the DC amplifier 4 of variable gain DC coupled to the post stage of the sample-and-hold circuit 3 and a DC level shift circuit 6 DC coupled to the post stage of the DC amplifier 4 are provided and an A/D conversion circuit 7 is connected to the post stage of the DC level shift circuit 6.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image reading apparatus capable of A / D converting the output of a line sensor, in particular, faithfully to document information.

[0002]

2. Description of the Related Art As an example of conventional A / D conversion of an image reading apparatus, for example, Japanese Patent Laid-Open No. 1-298872.
There is one disclosed in the title of "Method of setting reference color data in image reading apparatus". This is because accurate reference color data can be set by subtracting the mask data of the light shield pixel signal from the A / D-converted reference color data, and level adjustment is not required so that mass production can be achieved. It is the one. Thus, setting the conversion reference value so as to include the maximum input signal to the A / D converter is a very important factor in obtaining a correct conversion value.

[0003]

However, in the case where the line sensor output of the light source is lowered due to deterioration over time, or the output level of each line sensor varies greatly, the maximum input to the A / D converter is increased. The levels will be different. Because of this, at any time,
At present, it is impossible to minimize the quantization error by effectively utilizing the conversion dynamic range of the A / D converter.

Even when the dark output level varies greatly from pixel to pixel, the correction is uniform regardless of the pixel, and it is difficult to obtain an output faithful to the document information.

[0005]

According to a first aspect of the present invention, light reflected or transmitted from a document surface is condensed by an imaging lens and is incident on a photoelectric conversion element, and the reflectance or transmission from the document surface is performed. In an image reading apparatus that obtains image information by detecting an electric signal according to the rate as an image signal,
The output of the photoelectric conversion element is AC-coupled or DC-coupled, and a sample hold circuit is provided for sampling and holding the feed-through period and the optical output period of each pixel, and outputting the voltage difference between them. Is provided with a DC amplifier of variable gain coupled to DC, and a DC level shift circuit coupled to DC is provided at a stage subsequent to the DC amplifier, and an A / D is provided at a stage subsequent to the DC level shift circuit.
The conversion circuit was connected.

According to a second aspect of the present invention, in the first aspect of the present invention, the direct current level shift circuit is provided with a variable direct current level shift means capable of setting a level shift amount according to a set gain of a variable gain direct current amplifier. I chose

According to a third aspect of the invention, in the first aspect of the invention, the DC level shift circuit is provided with an independent variable DC level shift means capable of independently setting the level shift amount.

[0008]

According to the first aspect of the invention, it is possible to effectively utilize the conversion dynamic range in the A / D converter in response to the level change of the line sensor output, and to minimize the quantization error. .

According to the second aspect of the present invention, the DC level that changes depending on the set gain of the DC amplifier is corrected by the DC level shift circuit according to the value, so that the optimum input DC level of the A / D conversion circuit is always maintained. It becomes possible to obtain the signal of.

According to the third aspect of the present invention, it is possible to always obtain the signal of the optimum input DC level of the A / D conversion circuit in response to all the DC level shift factors generated before the DC level shift circuit. It will be possible.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention according to claim 1 will be described with reference to FIGS. In this embodiment, the light reflected or transmitted from the document surface is condensed by the imaging lens and is incident on the photoelectric conversion element,
The present invention relates to an image reading apparatus that obtains image information by detecting an electric signal corresponding to the reflectance or the transmittance from a document surface as an image signal.

In such an image reading apparatus, a circuit as shown in FIG. 1 is constructed here. That is, in the subsequent stage of the CCD sensor 1 as a photoelectric conversion element, the output of the CCD sensor 1 is AC coupled (or, via the capacitor 2).
A correlated double sample-hold (S / H) circuit 3 as a sample-hold circuit that may be DC-coupled) is connected. The correlated double S / H circuit 3 samples and holds the field through period and the light output period for each pixel and outputs the voltage difference. A variable gain direct current amplifier 4 as a direct current coupled variable gain direct current amplifier 4 is connected to the subsequent stage of the correlated double S / H circuit 3. This variable gain DC amplifier 4 has a D
An A / A conversion circuit 5 is attached. A direct-current-coupled direct-current level shift circuit 6 is connected to the subsequent stage of the variable-gain direct-current amplifier 4. Further, an A / D conversion circuit 7 and a shading correction circuit 8 are sequentially connected to the subsequent stage of the DC level shift circuit 6. The shading correction circuit 8 includes a correction memory 9.

FIG. 2 shows the internal structure of the correlated double S / H circuit 3. This circuit includes three S / H circuits 10,
11 and 12, and a subtraction circuit 13.
In this case, the S / H circuit 10 causes the operation at point a in FIG. 3C, and the S / H circuits 11 and 12 cause the operation at point b in FIG. 3C.

In such a structure, the CCD sensor 1
The output from is a waveform A as shown in FIG.
This output is guided to the correlated double S / H circuit 3 via the capacitor 2 and has a waveform B as shown in FIG. 3B. As shown in FIG. 2, the correlated dual S / H circuit 3 has S
The / H circuit 10 samples and holds the feedthrough level, and the S / H circuits 11 and 12 sample and hold the optical output level. Output of S / H circuit 11 and S / H circuit 12
The subtraction circuit 13 subtracts the difference from the output of the above, and thereby the waveform B as shown in FIG. 3B can be obtained. This correlated double S / H circuit 3 enables signal output that is less affected by sensor noise. The waveform B in FIG. 3B is a waveform in which the relative level relationship between the pixels is maintained at DC corresponding to the read pixel, and in order to store this, the correlated dual S / H circuit From the latter stage of 3 to the A / D conversion circuit 7, all are DC-coupled. This makes it possible to omit the clamp circuit for zero level reproduction, which is lost by AC coupling, and incompleteness of the clamp operation can be removed in principle. The capacity 2 may be omitted.

In the variable gain DC amplifier 4, the dynamic range as the read output signal is adjusted to the input range width of the A / D conversion circuit 7. In this case, the gain setting is performed by applying gain setting data to the input terminal 14, converting the gain setting data into an analog value by the D / A conversion circuit 5, and using this voltage value. As a method for creating the gain setting data, for example, it is possible to find the optimum data by looking at the maximum and minimum values of the signal after A / D conversion in advance. Further, in the DC level shift circuit 6, the dynamic range of the signal is adjusted to the input range of the A / D conversion circuit 7. In this case, the level shift amount is adjusted by the semi-fixed resistor 15 so as to be within the input range of the A / D conversion circuit 7 regardless of the gain set in the variable gain DC amplifier 4. Further, the A / D conversion circuit 7 converts it into a digital signal, and the shading correction circuit 8 converts it into an illumination system,
Brightness variations due to the image forming system, sensitivity variations of the CCD sensor 1 and the like are corrected. In this, data stored in advance is read from the correction memory 9 and calculated for each pixel.

Therefore, the conversion dynamic range of the A / D converter 7 is effectively utilized in response to the level change of the line sensor output, and the quantization error is minimized.
Moreover, it is possible to accurately detect and store the zero level, and obtain a digitalized read output from the output terminal 16.

Next, an embodiment of the invention described in claim 2 will be described with reference to FIG. The description of the same parts as those in the first aspect of the present invention will be omitted, and the same reference numerals will be used for the same parts.

Here, the DC level shift circuit 6 is provided with a variable DC level shift means 17 capable of setting the level shift amount according to the set gain of the variable gain DC amplifier 4.

Therefore, by providing the variable DC level shift means 17 in this way, the variable gain DC amplifier 4 is provided.
The DC level that changes depending on the set gain can be corrected by the DC level shift circuit 6 according to the value, and thus the signal of the optimum input DC level of the A / D conversion circuit 7 can always be obtained.

Next, an embodiment of the invention described in claim 3 will be described with reference to FIG. The description of the same parts as those in the first aspect of the present invention will be omitted, and the same reference numerals will be used for the same parts.

Here, the direct current level shift circuit 6 is provided with an independent variable direct current level shift means 18 capable of independently setting the level shift amount. In this case, the independent variable DC level shift means 18 includes a D / A conversion circuit 19 and an input terminal 20 for the shift amount setting data.

Therefore, by providing the independent variable DC level shift means 18 as described above, the A / D conversion circuit 7 is always operated in response to all the DC level fluctuation factors that occur before the DC level shift circuit 6. It becomes possible to obtain the signal of the optimum input DC level. Although the method of creating the shift amount setting data is omitted here,
Similar to the method of creating the gain setting data, it is possible to pursue the optimum data by looking at the difference between the maximum and minimum values after A / D conversion in advance.

Next, a modified example of the circuit configuration according to claim 1 will be described with reference to FIGS. 6 and 7. First, the 1st modification is described based on FIG. Here, since the CCD sensor 1 is an odeven configuration sensor, the capacitances 2a and 2b are separately provided for the two output values, respectively.
The correlation dual S / H circuits 3a and 3b are connected. An analog multiplexer 21 having a switch that is variable according to the odd-even switching signal OE is connected to the subsequent stage of the correlated double S / H circuits 3a and 3b. Other configurations are the same as in the case of FIG.

In such a structure, the CCD sensor 1
By performing correlated double sample and hold on each of the two output values from the above, the signals are converted into a single column of signals corresponding to the space read by the analog multiplexer 21, and thereafter processed in the same manner as the circuit of FIG. It is possible to obtain the same effect as the case of the invention described in Item 1. The inventions according to claims 2 and 3 can also be applied to this configuration.

Next, a second modification will be described with reference to FIG. Here, since the CCD sensor 1 is an odeven sensor, the capacitors 2a and 2b and the correlated dual S / H circuit 3 are separately provided for the two output values.
a, 3b, variable gain DC amplifiers 4a, 4b, D /
A conversion circuits 5a, 5b, gain setting data input terminals 14a, 14b, and DC level shift circuits 6a, 6b
, The semi-fixed resistors 15a and 15b, and the A / D conversion circuit 7
a, 7b, shading correction circuits 8a, 8b, and correction memories 9a, 9b are connected. An analog multiplexer 22 having a switch variable according to the odd-even switching signal OE and an output terminal 16 are connected to the subsequent stages of the shading correction circuits 8a and 8b.

In such a configuration, processing from correlated double sample hold to shading correction is performed on each of the two output values from the CCD sensor 1, and one row corresponding to the space read by the analog multiplexer 22 is performed. By converting the signal to a signal and thereafter processing the same as in the circuit of FIG. 1, the same effect as in the case of the invention of claim 1 can be obtained. Even in this configuration,
The invention described in claims 2 and 3 can be applied.

[0027]

According to the first aspect of the invention, the light reflected or transmitted from the document surface is condensed by the imaging lens and is incident on the photoelectric conversion element, and the light is reflected or transmitted from the document surface according to the reflectance or the transmittance. In an image reading apparatus that obtains image information by detecting an electric signal as an image signal, a sample hold circuit in which the output of the photoelectric conversion element is AC-coupled or DC-coupled is provided, and each sample-hold circuit is DC-coupled in the subsequent stage. A DC amplifier with a variable gain that samples and holds the feedthrough period and the light output period for each pixel and outputs the voltage difference is provided, and a DC level shift circuit that is DC-coupled is provided at the subsequent stage of the DC amplifier. Since the A / D conversion circuit is connected after the DC level shift circuit, the level change of the line sensor output can be adjusted accordingly.
Quantizer error can be minimized by effectively utilizing the conversion dynamic range in the converter, and the zero level can be accurately detected, stored and digitized, and pixel-by-pixel correction can also be performed. Is.

According to a second aspect of the present invention, in the first aspect of the present invention, the DC level shift circuit includes variable DC level shift means capable of setting a level shift amount according to a set gain of a variable gain DC amplifier. Therefore, by correcting the DC level that changes according to the set gain of the DC amplifier with the DC level shift circuit according to the value, it is possible to always obtain the optimum input DC level signal of the A / D conversion circuit. Is.

According to a third aspect of the invention, in the first aspect of the invention, the DC level shift circuit is provided with an independent variable DC level shift means capable of independently setting a level shift amount. It is possible to always obtain the signal of the optimum input DC level of the A / D conversion circuit in correspondence with all the DC level shift factors generated before the circuit.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an image reading apparatus according to an embodiment of the present invention.

FIG. 2 is a block diagram showing an internal configuration of a correlated double sample hold circuit.

FIG. 3 is a waveform diagram.

FIG. 4 is a block diagram showing an embodiment of the invention described in claim 2.

FIG. 5 is a block diagram showing an embodiment of the invention according to claim 3;

FIG. 6 is a block diagram showing a first modification of the invention described in claim 1.

FIG. 7 is a block diagram showing a second modification of the invention described in claim 1.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photoelectric conversion element 3 Sample hold circuit 4 DC amplifier 6 DC level shift circuit 7 A / D conversion circuit 17 Variable DC level shift means 18 Independent variable DC level shift means

Claims (3)

[Claims] 1. The light reflected or transmitted from the document surface is condensed by an imaging lens and is incident on a photoelectric conversion element, and an electric signal corresponding to the reflectance or the transmittance from the document surface is detected as an image signal. In the image reading device for obtaining image information by the above, the output of the photoelectric conversion element is AC-coupled or DC-coupled, and the sample is obtained by sample-holding the feed-through period and the optical output period of each pixel and outputting the voltage difference thereof. A hold circuit is provided, a direct-current-coupled variable-gain direct-current amplifier is provided at the subsequent stage of the sample-hold circuit, a direct-current-coupled direct-current level shift circuit is provided at the subsequent stage of the direct-current amplifier, and A is provided at the subsequent stage of the direct-current level shift circuit. An image reading apparatus having a / D conversion circuit connected thereto. 2. The image reading apparatus according to claim 1, wherein the DC level shift circuit includes a variable DC level shift means capable of setting a level shift amount according to a set gain of a variable gain DC amplifier. . 3. The image reading apparatus according to claim 1, wherein the DC level shift circuit includes an independently variable DC level shift means capable of independently setting a level shift amount.