JPS6047567A - Binary coding device for picture signal - Google Patents

Abstract

PURPOSE:To improve resolution without increasing errors in binary coding by providing an image signal binary coding device with an A/D converting circuit, a maximum and minimum level detecting circuit, and a level converting circuit, and converting an image signal after level conversion into a binary signal on the basis of a prescribed level as a threshold value. CONSTITUTION:A picture of an original is formed on a CCD image sensor 3 through the lens 2 of a picture reader and converted by the sensor 3 into an electric picture signal, which is amplified by an amplifier 4 and inputted to a picture signal binary coding device 13. The maximum and minimum level detecting circuit 15 of this device 13 detects and holds the maximum level WB and minimum level BB of the picture signal when a prescribed black and white pattern is read, and sets the conversion characteristics of the level converting circuit 16 to an intermediate level between those levels WB and BB. Then, the picture signal inputted to the device 13 is converted by an A/D converting circuit 14 into a binary signal, which is applied to the circuit 16 to perform level conversion with a signal (a) from the circuit; and an encoder 17 codes the picture signal into a binary signal by using a 50% level as the threshold value to suppress an increase of errors.

Description

[Detailed description of the invention] Industrial applications The present invention is directed to a document reading section of a facsimile IJ device, etc. The present invention relates to an image signal binarization device that can be used for.

Conventional Structure and Problems Therein FIG. 1 is a schematic block diagram showing a document reading section of a conventional facsimile machine. In this figure, lamp 1. The lens 2, CCD image sensor 3, and amplifier 4 constitute an image reading device. The image of the original 5 illuminated by the lamp 1 is transmitted to the CCD image sensor 3 by the lens 2.
An image is formed on the light-receiving surface of the image sensor 3, and converted into a υ image signal by the CCD image sensor 3. The image signal is amplified by the amplifier 4 and output.

The comparator 6 constitutes an image signal binarization device together with a variable resistor 7 and a resistor 8, and an image signal is applied to one input of the comparator 6. A voltage set by a variable resistor 7 is applied to the other input of the comparator 6 via a resistor 8 as a threshold value. The comparator 6 binarizes the input image signal by comparing it with a threshold value, and outputs the binary image signal to the terminal 9.

The image reading device as described above has a frequency response characteristic in which the average level of the image signal increases as the spatial frequency of the read image increases. For example, 8/
When the magnification of lens 2 is set so as to be able to resolve more than 1, the waveform of the image signal for the black and white test chart (lattice pattern) of 1 line/body, 4 lines/group, and 8 lines/Wrnl is the second waveform, respectively. The result will be as shown in the figure.

As is clear from this figure, the threshold value of comparator 6 is set to 5.
When set to the 0-related signal level, the image signals for the black and white test patterns of 1 line/Ki and 4 lines/Tsugu are binarized normally, but the image signals for the black and white test pattern of 9 lines/Yu are all at white level. The image signal will be binarized. Therefore, in order to correctly binarize the image signal for the black and white test pattern of 8 lines/gate, the threshold value was set to, for example, 70% signal level.This time, the spatial frequency of the read image was low and the average level of the image signal decreased. In this case, an image signal that should be binarized to a white level is binarized to a black level. Specifically, a portion of the black background with a density of around 50 e/- turns into a white portion after binarization.

In this way, the above-mentioned image signal binarization device sets a threshold value to correctly binarize an image signal for an image with a high spatial frequency, but a binarization error occurs for an image with a low spatial frequency. There was a problem in which the dark value of the digitization had to be decided based on the balance between the binarization error and the resolution, which tended to lead to a decrease in resolution.
0 and a capacitor 11 to detect the average level of the image signal, and apply the average level to the comparator 6 as a threshold value. However, since the threshold value changes during the binarization period, another problem arises in that the image quality tends to become unstable.

OBJECTS OF THE INVENTION The present invention solves the above-mentioned conventional problems, and provides an image signal binarization device that can improve resolution without making the image quality unstable or increasing binarization errors. With the goal.

Structure of the Invention The present invention provides a means for detecting and holding the maximum level of an image signal when a black and white test pattern having the highest spatial frequency to be resolved is read by an image reading device, and a means for detecting and holding the maximum level of an image signal, A level conversion means is provided for converting a signal level between the intermediate signal level of The purpose is to achieve the above objective by binarizing the data as follows.

Description of examples Embodiments of the present invention will be described below with reference to the drawings.

FIG. 4 is a block diagram showing the configuration of an image signal binarization device according to an embodiment of the present invention. In this figure, since 1 to 4 are the same as in FIG. 1, their explanation will be omitted, and only the image signal binarization device 13 will be explained. This image signal binarization device 1
3 is composed of an analog/digital conversion circuit 14, a maximum/minimum level detection circuit 16, a level conversion circuit 16, and an encoder 176 base 2'. Analog/digital conversion circuit 1
4 and the maximum/minimum level detection circuit 15 are inputted with image signals output from the image reading device, and the respective output signals are inputted into the level conversion circuit 16. The output signal of this level conversion circuit 16 is input to an encoder 17, and the output signal of this encoder 17 is sent to a terminal 18.

Next, the operation of this image signal binarization device 13 will be explained.
It is assumed that the position and magnification of the lens 2 are adjusted so that the image reading device can resolve a black and white test pattern of 8 lines/throat. In other words, when a test chart with 8 black and white test patterns drawn in the main scanning direction is set as document 6, each grid of the black and white test pattern corresponds one-to-one to a pixel of the COD image sensor. It shall be adjusted as follows.

First, eight black and white test patterns are read by an image reading device. The maximum/minimum level detection circuit 16 detects and holds the maximum level W8 and the minimum level B8 of the image signal input at this time, and holds the maximum level W8 and the minimum level B8 of the image signal input at this time.

A signal a indicating this is output. This signal a sets the level conversion characteristics of the level conversion circuit 16 as described later.

Next, a document to be actually read is set as document 5 and read by an image reading device. At this time, the maximum/minimum level detection circuit 16 does not perform any detection operation. The image signal output from the image reading device is multivalued quantized for each pixel by the analog/digital conversion circuit 14, inputted as a digital image signal to the level conversion circuit 16, subjected to level conversion processing, and then sent to the encoder 17. is input. The encoder 17 binarizes the input image signal using the tso% signal level as a threshold, and outputs the binarized image signal to the terminal 18.

FIG. 5 is a level conversion characteristic diagram of the level conversion circuit 160. As is clear from this figure, the conversion straight line has a slope such that the signal level W+B □, which is intermediate between the maximum level W detected by the maximum/minimum level detection circuit 16 and the minimum level B8, becomes a 60% signal level corresponding to the threshold value. 19, level conversion of the image signal is performed. The effect of such level conversion will be explained next.

Figure 6 shows when the black and white test patterns of 1 line/song, 4 lines/TMl, and 8 lines/deaf are read by an image reading device.
3 is a waveform diagram of an image signal obtained on the output side of the level conversion circuit 16. FIG. However, the image signal is expressed as an analog signal. As is clear from comparing this figure with Figure 2, fluctuations in the average level of the image signal are suppressed, and the threshold value for the image signal for any black and white test pattern is approximately halfway between its maximum and minimum levels. Therefore, any image signal can be correctly binarized, and the resolution can be improved compared to the conventional method. Furthermore, since the threshold value does not change, the image quality is also stable.

Note that in this embodiment, the threshold value is set to 50% signal level, but it is not necessarily limited to this. However, it is generally preferable to set the threshold value to a signal level of around 50%. Further, in this embodiment, the encoder 17 is used as a means for binarizing the image signal after level conversion, but a comparator may be used instead. Further, in this embodiment, the level conversion and binarization are performed after converting the image signal into a digital signal, but the analog image signal may be directly processed.

Effects of the Invention As described above, according to the present invention, the maximum level and minimum level of the image signal obtained by reading the test pattern are detected and held by the maximum/minimum level detection means, and the level conversion characteristic of the level conversion means is adjusted to the maximum level. It is set to convert a signal level between the level and the minimum level to a predetermined signal level, and the image signal obtained by reading the original is subjected to level conversion processing by the level conversion means, and then converted to a binarization means. Since the image is binarized using the predetermined signal level as a threshold, the resolution can be improved without increasing the binarization error, and the image quality can be stabilized.

[Brief explanation of the drawing]

Figure 1 shows a 1-obe used in the document reading section of a facsimile machine.
Figure 2 is a waveform diagram for explaining the frequency response characteristics of an image reading device, and Figure 3 is a schematic block diagram showing a conventional image signal binarization device. Block diagram illustrating an example of an image signal binarization device, No. 4
The figure is a block diagram showing a document reading section of an image signal binarization device according to an embodiment of the present invention, FIG. 5 is a level conversion characteristic diagram of a level conversion circuit, and FIG. 6 is for explaining the effect of level conversion processing. FIG. 1... Lamp, 2... Lens, 3...
... CCD image sensor, 4 ... amplifier, 13 ... image signal binarization device, 14 ...
- Analog-digital conversion circuit, 16... Maximum/minimum level detection circuit, 16... Level conversion circuit, 17... Encoder. Name of agent: Patent attorney Toshio Nakao and one other person
Shokutoga 11Δn□

Claims (1)

[Claims] An image signal binarization device that converts an image signal output from an image reading device into a binary image signal, the image signal binarization device comprising: a maximum level and a minimum level of an image signal obtained by reading a black and white test pattern with the image reading device; Detect and hold maximum,
minimum level detection means; level conversion means for performing level conversion processing on an image signal obtained by reading a document with the image reading device; and binarization of the image signal after the level conversion processing using a predetermined signal level as a threshold value. binarization means, wherein the level conversion characteristic of the level conversion means is automatically set to convert a signal level intermediate between the maximum level and the minimum level to the predetermined signal level. Binarization device.