JPH01865A - Image signal correction circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Summary] This is an image signal correction circuit that stores in advance data that corrects the input and output characteristics of each image sensor element, and adjusts the input and output of each image sensor element based on these data. Align characteristics.

FIELD OF THE INVENTION The present invention relates to an image signal correction circuit for an image scanner, and more particularly to an image signal correction circuit for an image scanner using an image sensor consisting of a plurality of image sensors or a plurality of chips.

Image scanners are required to read documents of larger size and higher resolution, but conventional image sensors do not have enough pixels, so multiple image sensors are used to read documents. Furthermore, when using a contact sensor, there is a limit to the size of one chip and the configuration uses a plurality of chips, which is the same as in the case of a plurality of image sensors.

In such cases, differences in sensitivity linearity between each sensor or each chip may cause differences in output images between each chip when reading a document with the same degree of a degree, and this needs to be corrected. .

[Prior Art] FIG. 3 is a block diagram of a conventional image signal correction circuit.

The analog video signal output from the image sensor element enters the A/D converter 1 and is converted into digital data, but prior to actual conversion, full scale adjustment is performed. That is, first, a level corresponding to the output of the image sensor element when reading a pure white image is input from the white level holding and supplying circuit 2 to the A/D converter 1 and converted into digital data.

Next, a signal of a level corresponding to the output of the image sensor element when reading a pure black image (that is, when light is blocked) is input from the black level holding and supplying circuit 3 to the A/D converter 1, and is converted into digital data.

Here, the digital conversion data of the output of the white level holding and supplying circuit 2 is converted to 01. Assuming that the digitally modified data output from the black level holding and supplying circuit 3 is D2, the dynamic range (full scale) of this A/D converter 1 is expressed as (DI-D2>. Therefore, the HA/D converter 1 is , this dynamic range is determined by the required number of gradations (for example, 16 m)
The input analog video signal is A/D converted using the number of bits corresponding to the number of gradations (4 bits for 16 gradations).

[Problems to be Solved by the Invention] According to the conventional correction circuit, there is no difference between the white level and black level of each image sensor element because they are normalized. For example, as shown in FIG.
, C, even if there is a difference in the dynamic range or output level between the white level and black level, the A/D converter adjusts the white level output of each element to 1. Since the black level output is normalized to 0, the white level output and black level output of each element all end up being the same value.

However, in the halftone region of white and black, the outputs of each image sensor element do not have the same value.

FIG. 5 is a diagram showing the output characteristics of each image sensor element. The horizontal axis is the concentration, and the vertical axis is the output, which is normalized to full scale 1. rA, fB, and fC are image sensor elements △ and B in FIG. 4, respectively.

The output characteristic of C is a straight line drawn from the origin 0 to the full scale point.

In these three characteristics rΔ to fC, the outputs at the origin 0 and at the full scale point K are the same. However, each image sensor element exhibits a unique bend (nonlinear characteristic) along the way. Therefore, the outputs of the density I are respectively 01 to 03 as shown in the figure, and are not the same. Taking the case shown in the figure as an example, for the same density ■, when read by image sensor element A, the output is faint, and B
When read in C, the output is dark, and when read in C, the output is even darker.

The present invention has been made in view of these points, and
It is an object of the present invention to provide an image signal correction circuit that can correct the gradation characteristics of each image sensor and obtain a uniform image signal (video signal).

[Means for Solving the Problems] FIG. 1 is a block diagram of the principle of the present invention. In the figure, 11 is an A/D converter that converts the output (analog video signal) of each image sensor element into digital data;
2 is a reference level holding and supplying circuit that holds and supplies reference levels of white and black levels to these A/D converters 11; 13 is a reference level holding and supplying circuit that receives each output of the A/D converter 11 and responds to a switching signal; The multiplexer 14, which outputs the data of the selected channels, is a gradation conversion circuit that receives the output of the multiplexer 13 and its switching signal as an address, and stores data whose gradation characteristics are corrected for each image sensor element.

[Function] The A/D converter 11 outputs a level corresponding to the image sensor element output when reading a pure white image outputted from the reference level holding and supplying circuit 12 (hereinafter simply referred to as white level) and a level corresponding to the output when reading a pure black image. Upon receiving the outputs of the respective levels (hereinafter simply referred to as black levels) corresponding to the outputs of the image sensor elements, digital data of these levels is obtained. A/
The D converter 11 calculates the dynamic range of the image sensor element from the difference between these two digital data, and sets this dynamic range to the required number of gradations. Thereafter, the A/D converter 11 converts the analog video signal output from the image sensor element into digital data.

Digital data output from each A/D converter 11 enters the multiplexer 13. The multiplexer 13 sequentially switches a plurality of inputs according to a separately input switching signal and outputs the same. The data selected by the multiplexer 13 enters the gradation conversion circuit 14. The gradation conversion circuit 14 receives the multiplexer 13 and the switching signal as an address, and outputs gradation data stored at an address corresponding to the input address. This output data is linear with corrected gradation characteristics between 0 and full scale. When the multiplexer 13 switches to the next channel, the input address of the gradation conversion circuit 14 also outputs gradation conversion data for the corresponding channel. In this manner, according to the present invention, a corrected image signal is output for each image sensor element, and the outputs of all image sensor elements are made uniform.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 2 is a block diagram showing an embodiment of the present invention. The embodiment shown in the figure is a circuit that performs gradation correction of the outputs of three image sensor elements, and obtains 4-bit data from 8-pit data.

Components that are the same as those in FIG. 11 are designated by the same reference numerals. In the figure, 12a is a white level holding/supplying circuit, and 12b is a black level holding/supplying circuit. The white level holding and supplying circuit 12a and the black level holding and supplying circuit 12b constitute a reference level holding and supplying circuit 12.

Reference numeral 20 denotes a threshold value setting means for setting a threshold value for gradation conversion, and its output is given to the gradation conversion circuit 14 as an address.

The numbers attached to the signal lines in the figure indicate the number of pits. That is, the circuit shown in the figure converts the image sensor element output (analog video signal) into 8-bit digital data using the A/D converter 11, and converts the 8-bit digital data into 4-bit digital data through the gradation conversion circuit 14. Convert to data. For example, a sample hold circuit is used as the white level holding and supplying circuit 12a and the black level holding and supplying circuit 12b. As the threshold value setting means 20, for example, a 4-bit dip switch is used, and as the gradation conversion circuit 14, for example, a ROM is used.

The operation of the circuit configured as described above will be explained as follows.

Prior to the actual gradation conversion operation, the white level holding and supplying circuit 12a samples and holds the output (analog video signal) when the image sensor element reads a pure white image, and the black level holding and supplying circuit 12b In this case, the output of the image sensor element in a state where light to the image sensor element is blocked is sampled and held.

The A/D converter 11 first converts the holding voltages of these white level and black level holding and supplying circuits 12a and 12b into digital data, sets the difference as a dynamic range, and uses it in the subsequent actual gradation conversion operation. is between 2
Digitize into 56 gradations (8 bits).

The image signal thus converted into digital data is input to the multiplexer 13. The multiplexer 13 then switches and outputs the channel according to the input switching signal, and the output is input to the gradation conversion circuit 14. On the other hand, the gradation conversion circuit 14 is also given a switching signal and a setting value of the threshold value setting means 20 as an address. The gradation conversion circuit (ROM>14 stores data for gradation conversion from 8 to 4 bits calculated in advance for each pit for each image sensor element.The gradation conversion circuit 14 Multiplexer 13 output, switching signal and threshold setting means 2
It receives an output of 0 as an address and outputs the data stored at the corresponding address.

When the multiplexer 13 is switched, it outputs data obtained by gradation-converting the image signal of the next channel. Hereafter, the same operation is repeated.

According to this embodiment, by changing the setting value of the threshold value setting means 20, the gradation threshold value can also be changed. According to this embodiment, the number of bits of the A/D converter 11 (8 bits here) is set to the number of bits of the required image signal (4 bits here).
It is thicker than the bit. Therefore, even if there is a difference in the dynamic range of each image sensor element, it is possible to avoid being affected by the difference.

Although the above-mentioned embodiment has been described using an example of three image sensor elements, it goes without saying that the present invention is not limited to this.

[Effects of the Invention] As described in detail above, according to the present invention, a gradation conversion circuit is provided in which data corrected for the gradation characteristics of each image sensor element is provided, and the output of each image sensor element is It is possible to provide an image signal correction circuit that can correct the gradation characteristics of each image sensor and obtain a uniform image signal using the data as an address and the fill conversion circuit. Further, according to the present invention, since the gradation threshold value can also be changed, an image of any gradation can be obtained without variation among the sensors.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of the principle of the present invention, Fig. 2 is a block diagram showing an embodiment of the present invention, Fig. 3 is a block diagram of the structure of a conventional circuit, and Fig. 4 shows the output characteristics of each image sensor element. FIG. 5 is a diagram showing the output characteristics of each image sensor element. In FIG. 1, 11 is an A/D converter, 12 is a reference level holding and supplying circuit, 13 is a multiplexer, and 14 is a gradation conversion circuit.

Claims (3)

[Claims] (1) An A/D converter (11) that converts the output of each image sensor element into digital data, and standards for white level and black level during A/D conversion for these A/D converters (11). a reference level holding and supplying circuit (12) that holds and supplies a level; a multiplexer (13) that receives each output of the A/D converter (11) and outputs channel data according to a switching signal; (13) A gradation conversion circuit (14) which receives the output and its switching signal as an address and stores data corrected for the gradation characteristics of each image sensor element.
14) An image signal correction circuit whose output is the output. (2) The image signal correction circuit according to claim 1, wherein threshold data for further changing the gradation threshold is given to the gradation conversion circuit (14) as an address. (3) The image signal correction circuit according to claim 1, wherein the number of bits of the A/D converter (11) is made larger than the number of bits for obtaining the required gradation. .