JPH0636555B2 - Image A / D conversion circuit - Google Patents

Description

The present invention relates to an image A / D conversion circuit, and is most suitable for use in a still image gradation correction device.

[Conventional technology]

An electronic still camera incorporating a CCD and a magnetic sheet (disk) drive is used as one of image media. For example, in a photo transmission system of a newspaper company, an image (still image) taken by an electronic still camera and recorded on a disc is applied to a reproducing device, and the reproduction output is transmitted to the head office by using a wireless or wired line.

Usually, an electronic still camera is provided with an automatic squeezing function, and the automatic squeezing adjustment allows recording at an appropriate brightness level. However, the automatic squeezing function is not sufficient, and for an object to be imaged with a large dynamic range of white-black (bright-dark), the effect of squeezing adjustment is biased to either light or dark, and the resolution of both white and black parts deteriorates. easy. Further, the dark current of the CCD used in the image pickup device is not stable, and the DC component of the video signal largely drifts.

[Problems to be solved by the invention]

In order to correct such an image in which white-intermediate color-black is not uniformly distributed and obtain an image with good contrast, a method of detecting a white peak of an image and applying AGC is conventionally used. . In this method, since the brightness level of the image is expanded according to the white peak value, there is a problem that the black level rises and the reproducibility and resolution on the black side are adversely reduced, and there is no direct current drift of the CCD output. I couldn't correct it.

In view of this problem, the present invention can obtain an image having a uniform gradation distribution from white to black, and can eliminate the unstable DC drift of the CCD output, which can substantially improve the image resolution. An object is to provide an image correction means.

In Japanese Patent Laid-Open No. 56-77575, a signal obtained by A / D-converting and then D / A-converting an image signal in a facsimile is peak-held and bottom-held, and these hold values are used to perform A / D conversion. A facsimile apparatus is disclosed in which the conversion range is changed. This conventional technique requires two A / D converters, a peak hold circuit and a bottom hold circuit to control the upper limit and the lower limit of A / D conversion, and has a problem that the circuit configuration becomes complicated.

[Means for solving problems]

The image A / D conversion circuit of the present invention converts an input video signal into an A / D signal.
The A / D converter 3 for D conversion, the digital output converted by the A / D converter 3 and the reference value of the black level are compared for each sample, and when they match each other, a black output of a coincidence pulse signal is made. A level comparator 8,
The digital output converted by the A / D converter 3 and the reference value of the white level are compared for each sample, and a white level comparator 4 that outputs a coincidence pulse signal when they match each other and the black level comparator 8 The integration circuit 9 which widens the pulse width of the coincident pulse signal of the output of the above and then integrates it into a direct current, and the addition of the output of the integration circuit 9 and the input video signal and the addition to the input terminal of the A / D converter. And a integrating circuit 6 for widening the pulse width of the coincidence pulse signal output from the white level comparator 4 and integrating it to convert it into a direct current, which is applied to the reference input terminal of the A / D converter 3. To do.

With this configuration, even if the gradation of the input video signal is biased to either the black side or the white side, the video signal can be corrected to have a uniform gradation distribution with a simple circuit.

〔Example〕

FIG. 1 is a circuit diagram showing an embodiment in which the image A / D conversion circuit according to the present invention is used in a transmitter of an electronic still camera system, and FIG. 2 is a waveform diagram showing the operation of FIG.

In FIG. 1, a video signal source 1 derives a reproduced video signal of a still image captured by an electronic still camera. The input video signal is basically the A / D converter 3
Is converted into an 8-bit digital signal and derived as a transmission output. This A / D conversion output is compared with the reference white level and black level, respectively, and the error information is fed back before the A / D conversion and as shown in FIG. 2, 0 _IRE (0% white) to 1
An image with a gradation uniformly distributed in the range of 00 _IRE (100% white) is obtained.

First, the output of the A / D converter 3 is given to the digital comparator 8 and compared with the output of the reference value generator 7 corresponding to 0 _IRE (all zeros of 8-bit digital value) for each sample value. The comparator 8 generates a pulse when the A / D conversion output is equal to 0 _IRE, and does not generate an output when it is large. The binary comparison output is converted into DC by an integrating circuit 9 including a pulse stretch circuit (pulse width expander), an integrator and a filter, and then the A / D converter 3
It is supplied to the adder 2 inserted on the input side of and is added as DC component to the input video signal.

If the input video signal is in the state shown in FIG. 2A, there is no output pulse from the comparator 8 and therefore the output D of the integrating circuit 9 is D.
The C level gradually decreases with a predetermined time constant. Therefore, A /
The DC level of the video signal input to the D converter 3 gradually decreases. When the black peak of the video signal falls below 0 _IRE , a pulse is output from the comparator 8 at that portion, and this is integrated by the integrating circuit 9, so the integrated output rises. Therefore, the DC of the input video of the A / D converter 3
The level will increase. This feedback loop stabilizes at the point where the decrease amount of the integrated output at a constant slope and the pulse integration amount of the comparator output are balanced, and in this stable state, the black peak coincides with 0 _IRE as shown in FIG. 2B. .

On the other hand, the output of the A / D converter 3 is also given to the digital comparator 4 and compared with the output of the reference value generator 5 (all 1s of 8-bit digital value) corresponding to 100 _IRE for each sample value. The comparator 4 generates a pulse if the A / D conversion output is equal to 100 _IRE ,
If it is 0 _IRE or less, no output is generated. The comparison output is integrated (converted to DC) by the integration circuit 6 and given to the reference input terminal ref of the A / D converter 3. As a result, the A / D conversion ratio, that is, the quantization level width is changed.

The integration circuit 6 includes a pulse stretch circuit, an integrator, and a filter, and adjusts the pulse width of the input pulse before performing DC regeneration by integration. When the input video signal is in the state shown in FIG. 2A, there is no comparator output, and the integrated output drops with a constant time constant. Therefore, the A / D converter 3
The reference input voltage of is decreased, and the analog input level corresponding to the full-bit converted digital value gradually decreases. That is, when this A / D conversion system is viewed from the outside, the dynamic range of conversion is changed and amplitude expansion is performed on the input as shown in FIG.

Due to the amplitude extension, the white peak becomes 100 as shown in FIG. 2B.
_{When IRE} is reached, the comparison pulse output is output and the integral output rises. The feedback loop stabilizes at the point where this increase and the decrease of the integrated output at a predetermined time constant are balanced, that is, when the white peak matches 100 _IRE .

In this way, no matter what the white peak level and the black peak level of the input video signal are, each peak is controlled from 0 _IRE to 10 by the feedback control of the A / D output.
A / D conversion is performed so that it falls within the range of 0 _IRE .
As a result, even with any input signal, a uniform gradation image quality is always obtained. Especially when making a hard copy of an image,
Generally, since the dynamic range of the image is reduced, the hard copy contrast can be improved by performing the correction in which the gradation is uniformly distributed in the range of 0 to 100 _IRE as in the embodiment. Further, since the dynamic range of the corrected image is widened, the apparent resolution is increased. Furthermore, since all bits (0 to 255 levels) of digital data are valid data, the resolution of information is good and the S / N of signal processing at the time of transmission and reception is good.

Moreover, since the black peak value matches 0 _IRE after correction,
The DC drift of the image pickup device (CCD) is also corrected at the same time.

In FIG. 1, the comparators 4 and 8 are disabled within the blanking interval (horizontal and vertical blanking) so that the comparators 4 and 8 do not operate in the synchronizing signal portion. The blanking signal is formed by a blanking (BLK) generation circuit 10 which receives an input video signal.

It is also possible to perform the correction process only on a certain area of the still image to expand the gradation difference in this portion. In this case, a blanking signal representative of the outside of this area is generated, and the comparators 4 and 8 are disabled in this blanking signal portion.

〔The invention's effect〕

As described above, the present invention uses the comparator that forms the coincidence pulse when the digital conversion output of the input video signal is compared with the reference values of the black side and the white side and coincides,
After widening the pulse width of the comparison pulse of the comparator comparison output, it is integrated and converted into a direct current, and the black level direct current signal is added to the input video signal and the white level direct current signal is
By applying it to the reference input of the A / D converter,
The conversion range of the converter is variably controlled. Therefore, according to the present invention, an input video signal whose gray scale is biased to the white side or the black side can be converted into a digital video signal having a uniform gray scale distribution by a simple circuit, whereby an apparent resolution is obtained. The correction of the black level can eliminate the drift of the input video signal due to the dark current of the image sensor.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment in which an image A / D conversion circuit according to the present invention is used in a transmitter of an electronic still camera system,
FIG. 2 is a waveform diagram showing the operation of FIG. In the reference numerals used in the drawings, 1 ... video input source 2 ... adder 3 ... A / D comparator 4 ... comparator 5 ... reference value generator 6 ... integration circuit 7 ... reference value generator 8 …… Comparator 9 …… Integrator circuit 10 …… Blanking generator circuit.

Claims (1)

[Claims] 1. An A / D converter for A / D converting an input video signal, a digital output converted by the A / D converter and a black level reference value are compared for each sample, and both are in agreement. When the black level comparator that outputs a coincidence pulse signal and the digital output converted by the A / D converter and the white level reference value are compared for each sample, a coincidence pulse signal is output when the two coincide. A white level comparator, an integration circuit that widens the pulse width of the coincidence pulse signal output from the black level comparator and then integrates it into a direct current, and adds the output of the integration circuit and the input video signal to the above A / An adder applied to the input terminal of the D converter and the pulse width of the coincidence pulse signal of the output of the white level comparator are widened and then integrated to convert to DC Image A and a integrating circuit applied to the reference input terminal of the A / D converter
/ D conversion circuit.