JPS63988B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a contrast correction circuit for a facsimile device, and particularly to a facsimile device that converts a video signal into a black and white binary signal and transmits the signal.
The present invention relates to a circuit that converts a video signal into a binary signal by floating a threshold value in black-and-white binary conversion of the video signal according to the contrast state of the video signal.

Generally, as shown in FIG. 1, a system for converting into a binary signal is a so-called fixed-threshold binarization circuit consisting of a comparison potential generation circuit 1 that generates a constant potential V _c and a binarization circuit 2. It is often used, especially for binary conversion of signals with uniform amplitude due to its simplicity.

However, the originals used in facsimile are of various contrasts,
That is, the density of the written characters relative to the ground density of the original differs from one original to another, and the amplitude of the photoelectrically converted video signal also varies.

In addition, in a photoelectric conversion system generally used in facsimile devices, reflected light from a transmitted document illuminated by a one-dimensional illumination light source such as a fluorescent lamp is converted into a one-dimensional solid-state image such as a CCD image sensor by a light collection lens. A configuration is adopted in which a video signal is projected onto a sensor and a video signal is output by self-scanning of the image sensor. In this type of photoelectric conversion system, there are various problems such as density non-uniformity on the document surface, non-uniformity between the center and periphery of the fluorescent lamp, non-uniformity between the center and periphery of the lens, and one-dimensional non-uniformity of the image sensor. Due to sensitivity non-uniformity across the surface, video signals often exhibit level deviations commonly referred to as shading.

In this way, if the video amplitude is not constant and the video signal affected by shading is converted into white and black binary signals by a fixed threshold binary conversion circuit and then transmitted, the received recorded image will be compared to the contrast of the original. This can result in significantly different contrast conditions and, in severe cases, become unreadable.

For example, one of the image sensors as shown in Figure 2-1.
When converting the video signal 3 output during the time period T by self-scanning twice into a binary signal using the fixed potential of V _c as a reference signal, as the binary signal, the second
- As shown in Figure 2, the interval t _b → t _c becomes 0 (V) even though it is a manuscript area with no characters written on it, and the area that should originally be converted to white is converted to black. A problem occurs. Regarding the video signal 4 of the document with poor contrast shown in FIGS. 2-3,
As a binary signal, the interval t _d → _te in FIGS. 2-4 is always converted to white, and information is completely lost, resulting in an indecipherable situation in the recorded image. This is the second -
As shown in the wavy line waveform showing the envelope of the video signal in Figure 1, the fixed potential V is applied even when the white level of a uniform document lacks uniformity between the outputs V _a to _{V b} or when the contrast condition deteriorates. This is due to binary conversion using _c as a reference signal.

In this way, the binary conversion method that uses a fixed potential as the reference potential has the disadvantage that what should originally be converted to white is converted to black, or when the contrast state changes significantly, information is missing that becomes undecipherable. It has the following disadvantages.

Conventionally, various methods have been considered to solve this drawback.For example, even in a binary conversion system using a fixed threshold, there is a method in which the optimal threshold point is selected from two or three predetermined points by operator operation. Such drawbacks can be alleviated by a method of reducing the influence of shading by floating the threshold value using a shading waveform extraction means from a video signal (for example, the specification of shading correction circuit patent application No. 85451/1983). Measuring methods have been put into practical use.

However, in order to provide operator control, the above methods are prone to operator errors, are trial-and-error based on checking recorded images, or are facsimile machines with a single transmitting function. The latter method has a major disadvantage in that it involves a loss of time and expense, such as having to communicate and receive contact from the recipient, and although the latter method also reduces the effect of shading, it can cause significant contrast differences between documents. Therefore, the current situation is that it must be used in combination with the above-mentioned operator operation, which has the same major drawback.

Furthermore, in recent years, as the amount of information has increased, it has become desirable to transmit documents continuously through a single communication operation. It is completely impossible to perform an artificial contrast switching operation according to the contrast. Thus, the above methods involving artificial manipulation means,
Another major drawback is that it cannot deal with changes in the contrast state of the original documents from one document to another during continuous feeding.

The present invention has been made in order to eliminate the above-mentioned drawbacks inherent in the conventional technology, and therefore, an object of the present invention is to improve the content of the technology described in the specification of Japanese Patent Application No. 54-85451. In addition, it is an object of the present invention to provide a novel contrast correction circuit that can perform automatic correction completely electrically without relying on human operating means.

The above object of the present invention is to provide a white level envelope waveform extraction means for extracting a white level envelope waveform from a video signal, and a white level envelope waveform extracting means for extracting a white level envelope waveform from a video signal, in a contrast correction circuit of a facsimile device that converts a video signal into a black and white binary signal and transmits the same. a white level envelope waveform extraction circuit having white level extraction range limiting means for limiting a white level extraction range of the extraction means; and black level peak value extraction means for extracting a black level peak value from the video signal; and the black level peak value. a black level peak value extraction circuit having a black level extraction range limiting means for limiting a black level extraction range of the extraction means, the white level potential extracted by the white level envelope waveform extraction circuit and the black level peak value extraction circuit; A comparison potential generation circuit that generates a fixed divided potential with respect to the black level potential extracted by the circuit, and a binarization circuit that converts the video signal into white and black binary signals using the comparison potential generated by the generation circuit as a reference. This is achieved by a contrast correction circuit characterized by comprising:

According to the present invention, the influence of non-uniformity of the white level generated in the photoelectric conversion system is reduced, and a stable contrast compensation effect is exhibited against differences in the contrast state of each document.

Hereinafter, a preferred embodiment of the present invention will be explained in detail with reference to the drawings.

FIG. 3 is a block diagram showing an embodiment of the contrast correction circuit according to the present invention, and FIG. 4 shows waveforms of various parts for video signals having different contrast states of originals. In the figure,
Reference number 5 is a one-scan video signal input terminal output from the image sensor, and the black reference level is 0.
(V) potential, and the maximum amplitude of the white background level of the document is +V (V) potential. 6 and 8 are white level envelope waveform extraction circuits, which are integral circuits that charge with a fast time constant that follows the highest image frequency and discharge with a slow time constant that corresponds to the slope of the white level envelope waveform; 16 from the input video signal shown in figure 15.
A white level envelope waveform 20 shown in is extracted. 7 and 9 are black level peak value extraction circuits, which are integral circuits that discharge with a fast time constant that follows the highest image frequency and have an extremely slow charging time constant that spans multiple scanning times; A black level waveform 22 shown in is extracted.

Specifically, such a circuit can be realized, for example, by a circuit shown in FIG. That is, in the figure, the white level envelope waveform extraction circuit can be realized by the configuration of 6 and 8, and the black level peak value extraction circuit can be realized by the configuration of 7 and 9, and the fast time constant when charging the capacitor C1 is as follows. It is determined by the parallel resistance value of resistors R1 and R2 and capacitor C1. The slow time constant is determined by resistor R2 and capacitor C1 since the diode D1 blocks the discharge current through resistor R1 when capacitor C1 is discharged.

Also, the fast discharge time constant of capacitor C2 is determined by the parallel resistance value of resistors R3 and R4 and capacitor C2, and when charging capacitor C2, the charging current passing through resistor R3 is blocked by the diode of D2. To achieve this, a very slow time constant is determined by resistor R4 and capacitor C2. Here, the switch SW has the fourth black level extraction.
This is controlled so that it is performed only during the document scanning section T in the figure.

In this way, the potential generated in the capacitor C1 is received by the high input impedance amplifier OP1,
By outputting, a white level envelope waveform is extracted. On the other hand, extraction of the black level peak value is realized by receiving the potential generated in the capacitor C2 by the amplifier OP2 having a high input impedance.

Here, circuits 8 and 9 are constituted by clamp circuits, 8 is a limiting circuit that stops the white level from falling, and 9 is a limiting circuit that stops the black level from rising. This circuit 8 is connected to a fixed potential corresponding to the minimum potential of the output level of the white background of the document +
The output of the circuit 6 is clamped at V _L (V), and the fixed potential +V _L (V) is +V (V).
Zener diode from higher Vcc potential
Obtained by subtracting the step-down potential of ZD1,
Even in such a case, the white level envelope waveform is +V _L
(V) to stop the drop in white level.

In addition, the circuit 9 outputs character information (black signal) during one scan.
is not included at all, and when this state continues for many scanning times, the potential generated in the capacitor C2 gradually starts to rise, but when it reaches the step-down potential of the Zener diode ZD2 or higher, the charge in the capacitor C2 is zeroed out through the diode ZD2. (V) discharge to a potential;
This limits the rising of the black level to a fixed potential V _B ′ (V) (V _B ′<V _L ).

As is clear from the above explanation, the waveform at the point 10 in Fig. 3 is the maximum potential of the video signal +V (V).
to a fixed potential V _L (V), forming an envelope waveform of a white level. On the other hand, the waveform at point 11 in FIG. 3 extracts the peak value of the black level while fluctuating from 0 (V) to +V _B '(V), and the video signal is always at point 10 in FIG. and 11
It is included within the potential level of the point indicated by . This is illustrated by 16 and 18 in FIG. In particular, 1
8 shows that the same effect is performed even when the original contrast condition is poor compared to 16.

Here, the white level envelope waveform potential output 10 and the black level peak potential output 11 extracted from the video signal are applied to both ends of the variable resistor RV, and are divided into a constant voltage between the white level potential and the black level potential. ,
The divided voltage output 12 is input as a reference potential to the binarization circuit 13. This reference potential 12 and the video signal 5
are compared and output to 14 as white and black binary signals. That is, 20, 20' of FIG. 4 16 or 18
The reference potential 21 is a constant divided potential of the white level potential indicated by 22 or 22' and the black level potential indicated by 22 or 22'.
or 21', and converts the video signal of the dotted line waveform in FIG.
Alternatively, ideal binary white and black signals can be obtained without being affected by changes in the contrast state of the original.

The features of the present invention include a white level envelope waveform output 10 having a white level envelope waveform extraction means as shown in FIG. The purpose is to convert the video signal 5 into a binary conversion output 14 using a fixed divided voltage potential 12 between the black level peak value output 11 and the black level peak value output 11 as a reference potential. In addition, even for a video signal with a long all-white signal section, the reference potential 12 is inverted with the lowest potential level of the non-uniform part of the white level,
This prevents the contrast state from being significantly different from the actual document contrast state.

As is clear from the above explanation, in facsimile machines that convert video signals into white and black binary signals and transmit them using the contrast correction circuit of the present invention, the biggest problem in recording image quality has been the It has the effect of completely eliminating the negative effects caused by differences in contrast, and the unevenness of the white level of the video signal due to non-uniformity of the density of the original, and the gradual change in the contrast between the density of the original and the density of the characters. As a result, even when a video signal undergoes amplitude fluctuations, both the white level and the black level follow the changes, operating as a so-called floating slice, and achieving optimal binarization.

Although the present invention has been described above with reference to one preferred embodiment thereof, this is merely an illustrative example, and it goes without saying that the present invention is not limited only to the embodiment described herein.

[Brief explanation of the drawing]

Figure 1 is a block diagram of a commonly used fixed threshold binary conversion system, and Figures 2-1, 2-2, 2-3, and 2-4 are typical optical systems. One-scanning output signal diagram of a one-dimensional image sensor, No. 2-1
A general binary conversion system output diagram of the output signal shown in the figure,
One scanning output diagram of an image sensor with a poor original contrast condition and a general binary conversion system output diagram of the output signal shown in Figures 2-3. Figure 3 shows an embodiment of the contrast correction circuit according to the present invention. 4 is a waveform diagram of each part of the circuit shown in FIG. 3 for video input signals with different contrast states;
FIG. 5 is a diagram showing a specific example of the circuit configuration of the main components of the present invention. 1... Comparison potential generation circuit, 2... Binarization circuit,
5... Video signal input terminal from the image sensor, 6... White level envelope waveform extraction circuit, 7... Black level peak value extraction circuit, 8... White level extraction range limiting circuit, 9... Black level extraction range limiting circuit. circuit,
12... Comparison reference potential, 13... Binarization circuit, 1
4...Binary conversion output terminal.

Claims (1)

[Claims] 1. White level envelope waveform extraction means for extracting a white level envelope waveform from the video signal, and white level extraction by the white level envelope waveform extraction means, in a contrast correction circuit of a facsimile device that converts a video signal into a black and white binary signal and transmits it. a white level envelope waveform extraction circuit having a white level extraction range limiting means for limiting a range; a black level peak value extracting means for extracting a black level peak value from the video signal; and a black level extraction circuit for the black level peak value extracting means. a black level peak value extraction circuit having a black level extraction range limiting means for limiting the range of the white level potential extracted by the white level envelope waveform extraction circuit and the black level extracted by the black level peak value extraction circuit; It is characterized by including a comparison potential generation circuit that generates a fixed divided potential with respect to a level potential, and a binarization circuit that converts the video signal into a white and black binary signal using the comparison potential generated by the generation circuit as a reference. Contrast correction circuit.