JPS6338150B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention automatically corrects the amplitude of an image signal obtained by an image information reading device in a facsimile transmitter or the like in accordance with changes in the light intensity of a light source and the density of the background of a document. This invention relates to an automatic contrast correction circuit.

In general, in image information reading devices such as facsimile transmitters, changes in light intensity due to the rise characteristics of a light source such as a fluorescent lamp that illuminates a document and changes over time,
In order to eliminate the influence of changes in the background density of the original, it is necessary to correct the amplitude of the image signal in accordance with these changes.

FIG. 1 shows a conventional automatic contrast correction circuit that performs such correction. In the same figure, an image signal output from an image sensor (not shown) of an image information reading device has its shading distortion removed and becomes an image signal V _IN . This image signal V _IN is amplified by a variable amplifier circuit 1 whose amplification factor is controlled as described later, thereby producing a corrected image signal V _OUT.
is converted into and output from this correction circuit.

At the same time, the corrected image signal V _OUT is inputted to the peak value detection circuit 3 through the sampling section selection circuit 2 . This peak value detection circuit 3 detects a peak value from the input corrected image signal V _OUT and holds the same peak value. Here, the sampling section selection circuit 2 selects a section to be sampled in one line of the corrected image signal V _OUT according to the section selection signal S ₁ inputted to the circuit 2 (normally, the sampling section selection circuit 2 selects the section to be sampled in one line of the corrected image signal V OUT. The detection of the peak value is performed in the interval selected by the sampling interval selection circuit 2.

The peak value held in the peak value detection circuit 3 is fed back to the variable amplifier circuit 1, and the amplification factor of the variable amplifier circuit 1 is controlled according to the peak value. Note that the peak value held in the peak value detection circuit 3 is attenuated by a predetermined time constant, so the peak value is
It is updated sequentially, including when a lower peak value appears than before (the dotted line in FIG. 3 indicates the change in the peak value).

Therefore, the corrected image signal V _OUT has its amplitude corrected for changes in the light amount of the light source and the density of the background of the document.

However, in such a conventional contrast correction circuit, if the time constant is set large, it becomes impossible to follow sudden changes in the amount of light or the density of the background of the document, while if the time constant is set small, the contrast correction circuit shown in FIG.
Multiple peak values such as V _B and V _C are detected in one line, and the amplification factor of the variable amplifier circuit 1 changes rapidly, resulting in a disadvantage that a stable corrected image signal V _OUT cannot be obtained. . Also, regardless of the size of the time constant, as shown in V _A in Figure 3, 1
Since the peak value held in the peak value detection circuit 3 changes in the middle of the line, the amplification factor is also 1.
There was a drawback that it changed in the middle of the line.

Contrast correction circuits have also been proposed in which the above-mentioned contrast correction operation is performed digitally to improve accuracy, but these require expensive A/D converters and increase the circuit scale. Therefore, there were drawbacks that made it impractical.

The present invention has been made in order to eliminate the above-mentioned drawbacks of the conventional art, and by digitalizing the gain control processing, it is possible to easily follow sudden changes in the density of the background of the document and the amount of light, and the gain can be controlled in one line. To provide an automatic contrast correction circuit which does not change, provides a stable corrected image signal, and is inexpensive.

The present invention will be described below based on embodiments shown in the drawings.

FIG. 2 shows an embodiment of the invention. Reference numeral 4 denotes an input terminal for an image signal, into which an image signal V _IN from which shading distortion has been removed, similar to that of the conventional circuit, is input. Reference numeral 2 designates a sampling interval selection circuit similar to that of the conventional circuit, and reference numeral 3 designates a peak value detection circuit which inputs the image signal V _IN from the image signal input terminal 4 via the sampling interval selection circuit 2, and detects the held peak. By clearing the value line by line by the clear signal _S2 , the peak value of each line is detected. 5
6 is a resistance ladder type D/A converter, and 6 is an analog switch. During the original reading period, the image signal input terminal 4 is connected to the reference voltage input terminal of the D/A converter 5, while during the blanking period, the image signal input terminal 4 is connected to the reference voltage input terminal of the D/A converter 5. teeth,
The output terminal of the reference voltage source 7 is connected to the reference voltage input terminal of the D/A converter 5. 8 is a comparator that compares the output of the peak value detection circuit 3 and the output of the D/A converter 5, and 9 is a successive approximation register.
Together with the A converter 5, it constitutes a successive approximation type A/D conversion circuit.

A memory 10 receives the output of the successive approximation register 9 as an address input, and stores correction coefficients corresponding to various values that the output of the successive approximation register 9 can take. Note that this memory 10 constitutes a correction coefficient generation circuit in this embodiment.
A data selector 11 selectively inputs the output of the successive approximation register 9 and the output of the memory 10 to the D/A converter 5. 12 is an amplifier that amplifies the output of the D/A converter 5.

Next, the operation of this embodiment will be explained. During the document reading section, the image signal V _{IN is input} from the image signal input terminal 4.
is inputted to the reference voltage input terminal of the D/A converter 5 through the analog switch 6, and a correction coefficient, which will be explained in detail later, is inputted from the memory 10 to the D/A converter 5 through the data selector 11. Therefore, the image signal from the D/A converter 5 is
A signal obtained by multiplying V _IN by the correction coefficient is output. This signal is further amplified by the amplifier 12 to obtain a corrected image signal V _OUT .

At the same time, the image signal V _IN is input to the peak value detection circuit 3 through the sampling section selection circuit 2. As a result, the peak value detection circuit 3 detects the peak value in one line of the image signal V _IN (the peak value detection circuit 3 is assumed to hold the peak value with a sufficiently large time constant). do).

Next, when the one-line document reading section ends and a blanking section begins, the analog switch 6 is switched by the section switching signal _S1 , and a constant reference voltage V _Ref is applied from the reference voltage source 7 through the analog switch 6. At the same time, the selector 11 is also switched so that the output of the successive approximation register 9 is input to the D/A converter 5, and the D/A Converter 5, comparator 8 and successive approximation register 9
The A/D conversion circuit consisting of starts A/D conversion operation.

That is, the A/D conversion circuit successively compares the peak value of the previous line held in the peak value detection circuit 3 with the output of the D/A converter 5, thereby determining the contents of the successive approximation register 9. The value is determined starting from the most significant digits, and finally the contents of the register 9 are set to an N-bit digital value corresponding to the peak value. In other words, the content of this register 9 is a digital value representing the ratio of the peak value to the reference voltage V _Ref .

Next, the contents of the register 9 are inputted to the memory 10 as an address, so that a correction coefficient corresponding to the contents of the register 9 is output from the memory 10. Here, this correction coefficient is determined by the output of the D/A converter 5 when the same value as the peak value is input to the reference voltage input terminal of the D/A converter 5.
It is set to a value that indicates a predetermined damping ratio with respect to V _Ref .

Then, when the reading section of the next line begins, the analog switch 6 and selector 11 are switched, and the image signal V _IN is again input to the reference voltage input terminal of the D/A converter 5 through the analog switch 6. The correction coefficient is input from the memory 10 to the D/A converter 5 through the selector 11. As a result, the image signal V _IN of this line has its amplitude corrected by the correction coefficient determined from the peak value of the previous line, and is inputted from the amplifier 12 as the image signal V _OUT .

To explain this more specifically, suppose that the fourth
As shown in the figure, if the peak value of the image signal V _IN in a certain line is V _D , at the end of the reading section of this line, the peak value detection circuit 3 holds V _E. Then, in the next blanking interval, this V _E is A/D converted by the A/D conversion circuit, and the result is input into the memory 10.

As a result, when V _E is input to the reference voltage input terminal of the D/A converter 5, the D/A converter 5
The memory 10 outputs a correction coefficient whose output indicates a predetermined damping ratio with respect to V _Ref . That is, for example, if the damping ratio is set to 1/2, then (1/2
A correction coefficient of ×V _Ref /V _E ) is output from the memory 10. Therefore, if the peak value of the next line is V _F as shown in FIG. 4, the peak value of the output of the D/A converter 5 on that line is (V _F ×
1/2×V _Ref /V _E ).

Further, the amplifier 12 amplifies the output of the D/A converter 5 by a reciprocal multiple of the predetermined attenuation ratio. That is, when the attenuation ratio is set to 1/2 as described above, the amplifier 12 performs twice the amplification. Therefore, in the end, the peak value V _F in the image signal V _IN
In the corrected image signal V _OUT , (V _F × 1/2 × V _Ref / V _E ) × 2 = V _F × V _Ref / V _E , and if V _F = V _E , this corrected image signal V _OUT
The peak value of will be equal to V _Ref .

In this way, in this circuit, each line of the image signal V _IN is corrected one after another according to the peak value of the previous line.

In this circuit, the amplitude of the image signal is corrected in this way, so not only when the background density and light intensity of the original are stable, but also when these changes suddenly, the image signal is always within two lines. It can be followed, and the gain is always constant during one line. Further, since the correction calculation is performed by a D/A converter in a successive approximation type A/D conversion circuit that A/D converts the peak value of each line, costs can be reduced.

As is clear from the above description, the automatic contrast correction circuit according to the present invention obtains peak values for each line, A/D converts these peak values using a successive approximation type A/D conversion circuit during the blanking interval, By obtaining a correction coefficient for the peak value as a digital value from this A/D conversion result and calculating this correction coefficient to the image signal of the next line,
The amplitude of the image signal is corrected, and the calculation of the correction coefficient and the image signal is carried out by the successive approximation type A/D.
By using the D/A converter in the conversion circuit, it has excellent followability to changes in the density of the original document and the amount of light, and the gain does not change during one line, making it stable. This provides an excellent effect in that a corrected image signal can be obtained and the manufacturing cost can be reduced.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a conventional automatic contrast correction circuit, FIG. 2 is a block diagram of an automatic contrast correction circuit according to an embodiment of the present invention, FIG. 3 is a signal waveform diagram in the conventional circuit, and FIG. 4 is a block diagram of the automatic contrast correction circuit according to an embodiment of the present invention. It is a signal waveform diagram in an Example. 3... Peak value detection circuit, 5... D/A converter, 6... Analog switch, 7... Reference voltage source, 8... Comparator, 9... Successive approximation register, 10... Memory, 11... data selector.

Claims (1)

[Claims] 1. A peak value detection means for detecting the peak value of each line of the read image signal to be corrected, a successive approximation type A/D conversion means, and a digital value correction coefficient according to the output of this A/D conversion means. the D/A in the successive approximation type A/D conversion means during the blanking period of the read image signal;
With the constant reference voltage input from the reference voltage source to the reference voltage input terminal of the conversion means, the A/D conversion means A/D converts the peak value of the n-th line detected by the peak value detection means. Converted,
During the image signal section of the read image signal on the n+1th line, a correction coefficient corresponding to the A/D conversion result is input from the correction coefficient generation means to the D/A conversion means, and the D/A conversion means An automatic contrast correction circuit characterized in that the read image signal of the n+1st line is inputted to the reference voltage input terminal of the conversion means, and the D/A conversion means performs a correction calculation on the read image signal of the n+1th line. .