JPS6322765Y2 - - Google Patents

Description

[Detailed description of the invention] This invention is an automatic variable gain device that prevents the white level peak portion from being clipped and the information in the white level portion from being lost when converting an optical image into an electrical video signal. It is related to.

Generally, when the white level of a video signal from a television camera becomes excessive, the peak portion of the white level of the video signal is clipped by a white clip circuit. Conventional white clip circuits are as follows:
There is a knee (KNEE) circuit which has a fixed characteristic, and as shown in Figure 1, the signal at the peak part of the white level is bent along the knee characteristic A according to the magnitude of the input level of the video signal. This clips the peak white level of the video signal. Characteristic curve A shown in Figure 1 shows the video input level versus video output level characteristic after gamma correction when using a conventional fixed knee slope circuit, but the video output level is 2.3 times the maximum specified input level. The above video input level simply reached the white clip level and was clipped, and all higher level inputs are treated as saturated at the white level, and therefore video reproduction is impossible. Ta.

The purpose of this invention is to adjust the knee slope in response to the peak level of the input signal so that even if the peak part of the white level of the video signal changes significantly, it will not be clipped and the information of the video signal in the white level part will not be lost. An object of the present invention is to provide an automatic variable gain device that allows appropriate changes.

That is, the present invention includes a high level selection means for selecting a maximum value from a plurality of types of input color information signals, and a high level selection means for selecting a maximum value from a plurality of types of input color information signals, and a method for selecting a maximum value from each of the plurality of types of input color information signals based on the maximum value selected by the high level selection means. a plurality of variable gain amplifiers that independently amplify each of the plurality of types of input color information signals and an output signal of the variable gain amplifier corresponding to the input color information signal, whichever has a lower level; The present invention is characterized by comprising a plurality of low-level selection means for selecting.

Hereinafter, the present invention will be described in detail with reference to the drawings.

In the present invention, the characteristic curves B ₁ , B ₂ ,
As in B ₃ and B ₄ , the slope of the knee characteristic is changed according to the peak level of the video signal so that the peak portion is near the white clip level. Note that the curve A' in FIG. 2 shows an example of the conventional input/output characteristic before gamma correction, which corresponds to the curve A in FIG. 1.
Curve B in FIG. 1 shows the characteristics obtained by the present invention after gamma correction.

FIG. 3 shows an example of the configuration of the automatic variable gain device of the present invention, in which red signal input R, green signal input G and blue signal input B are supplied to variable gain amplifiers 1, 2 and 3, respectively. 3 and each signal input R, G, and B to black non-addition mixing circuits (black NAM circuits) 4, 5, and 6;
Outputs the lower level of both input signals.
The outputs from the black NAM circuits 4, 5, and 6 are supplied to gamma corrections 7, 8, and 9, respectively, to output gamma-corrected red, green, and blue color signals R',
Take out G′ and B′. Furthermore, each signal input R, G
and B as white peak non-addition mixing circuit (white peak
The maximum peak level of each signal input R, G, and B is detected here, and the output is passed through a peak level hold circuit 11 having a predetermined time constant to a variable gain amplifier 1,
2 and 3 and control their gains.

The operation of the circuit shown in FIG. 3 will be described by taking as an example the case where the red signal input R is a sawtooth wave larger than the other color signal inputs G and B. The red signal input R in this case is represented by the waveform shown in FIG. 4A. By changing the gain of the variable gain amplifier 1 using the gain control input from the peak level hold circuit 11,
For a red signal input R supplied to the variable gain amplifier 1, a constant amplitude signal is generated between a predetermined output level L1 and a white clip level L2, as shown in FIG. Make sure you get the output. The signal input R and the fourth
By supplying the amplifier output signal shown in FIG. 4B to the black NAM circuit 4, the lower level of both signals is selected in the black NAM circuit 4, and a signal having the waveform shown in FIG. 4C is obtained. Thereby, even if the video input signal fluctuates significantly, the slope angle of the output signal of the variable gain amplifier changes, thereby making it possible to change the compression ratio in the high level range of the video signal output.

According to the circuit shown in FIG. 3, a characteristic having a knee slope similar to the red signal conversion characteristic can be obtained for the green signal input G and the blue signal input B as well. More black NAM
By inputting the output signals of circuits 4, 5 and 6 to gamma circuits 7, 8 and 9, respectively,
Red, green and blue signal outputs R', G' and B' with variable slope knee characteristics with smooth slopes can be obtained.

As mentioned above, by processing the video input using the automatic variable gain device of the present invention, it is possible to reproduce details in the white peak region at a level of 10 to 20 times, which is much higher than before, in the same way as in film imaging. be able to. Furthermore, in a camera that uses an auto iris (automatic diaphragm), information on white areas can be reproduced by eliminating skipping of white areas during iris operation, thereby reducing the sense of abnormality. Furthermore, with the present invention, it is possible to freely select the imaging conditions for empty sky photography and backlight photography, which have traditionally been considered difficult.

[Brief explanation of the drawing]

Figures 1 and 2 are characteristic curve diagrams comparing the video input level vs. video output level characteristics after gamma correction and before gamma correction between the conventional and the present invention, and Figure 3 is the characteristic curve diagram for the automatic variable gain of the present invention. A block diagram showing an example of the configuration of the device, and FIGS. 4A, B and C are signal waveform diagrams for explaining the operation of the device shown in FIG. A... Video input level versus video output level characteristic curve after gamma correction when using a conventional fixed knee slope circuit, B... Video input level versus video output level after gamma correction when changing the knee slope according to the present invention Video output level characteristic curve, B ₁ , B ₂ , B ₃ , B ₄ ... Video input level versus video output level characteristic curve before gamma correction according to the present invention when the peak level of the video signal changes, L1 ... Predetermined output level, L2
...White clip level, R, G, B... Color signal input, R', G', B'... Color signal output, 1, 2,
3... Variable gain amplifier, 4, 5, 6... Black non-addition mixing circuit, 7, 8, 9... Gamma correction circuit, 10
...White peak non-addition mixing circuit, 11...Peak hold circuit.

Claims (1)

[Claims for Utility Model Registration] High-level selection means for selecting a maximum value from a plurality of types of input color information signals; and a maximum value selected by the high-level selection means for each of the plurality of types of input color information signals. a plurality of variable gain amplifiers that independently amplify with a gain based on a value; and a lower level of each of the plurality of types of input color information signals and an output signal of the variable gain amplifier corresponding to the input color information signal. and a plurality of low level selection means for selecting one signal from the other.