JP3076997B2 - Chroma signal clipping circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit for clipping a chroma signal in a composite video signal used in a video camera or the like.

[0002]

2. Description of the Related Art FIG. 2 illustrates a high-level clip (hereinafter, referred to as a high clip) and a low-level clip (hereinafter, referred to as a low clip) of a composite signal obtained by multiplexing and combining chroma signals of a conventional color television signal. FIG.

[0003] The balanced modulator 11 the color difference signals R- Y 及 beauty <br/> B-Y in, and modulated by the subcarrier signals SC1, SC2 each having a 90 ° phase difference, and a burst signal to mission-box The chroma signal (B) is obtained and supplied to the adder 12.
The adder 12 adds the luminance signal (A) including the chroma signal (B) and the periodic signal to generate a composite video signal (C).
Get. The composite video signal (C) is clipped by the high clip circuit 13 to a signal having a high clip level α or higher, and is converted into a signal (d). Further, the low clip circuit 14 outputs a signal having a low clip level β or lower except for a blanking period. Is clipped to become a signal (e).

At this time, the processing order of the high clip circuit 13 and the low clip circuit 14 may be reversed.

As described above, conventionally, a composite video signal can be subjected to high clip and low clip processing, thereby suppressing a VTR blurring phenomenon, overmodulation, and out-of-synchronization.

[0006]

However, in the above-described conventional chroma signal clipping circuit, as shown in FIG.
By clipping above or below the subcarrier frequency indicated by the sine wave at the clip level α or β,
The waveform is distorted or the DC component changes.

There is another problem that the chroma phase changes due to the distortion of the waveform due to the clipping operation.

An object of the present invention is to solve such a conventional problem, and an object of the present invention is to provide a highly accurate chroma signal clipping circuit for preventing a chroma signal distortion or phase change caused by clipping. .

[0009]

SUMMARY OF THE INVENTION The present invention provides a luminance signal Y, R, clipped at a high clip level α and a low clip level β by a high luminance knee characteristic and a quantization dynamic range obtained by digital signal processing. -Y and BY color difference signals are converted to a luminance signal by frequency conversion.
Frequency converter and two kinds of color difference signals of RY and BY
A second frequency converter for converting the frequency of
Of the two types of color difference signals frequency-converted by the converter,
The first color difference signal having a large width and the first frequency converter
An adder for adding the number-converted luminance signal and a first color
The difference signal and the luminance signal frequency-converted by the first frequency converter
And the maximum of the output signal from the adder
The value and the minimum value of the output signal from the subtractor are within a predetermined range.
A comparison / judgment circuit for judging whether or not there is an output
An arithmetic circuit that calculates a first color difference signal based on
Two types of color difference signals that have been frequency-converted by the frequency converter
The second chrominance signal having a small amplitude is converted into a first color signal by an arithmetic circuit.
A multiplier that multiplies the difference signal by the same ratio as the calculated change amount;
The configuration with the luminance signal Y and color difference signals R-Y, B
−Y so that the addition result with the larger one of the high clip level α or the subtraction result from the luminance signal Y with the larger one of the color difference signals RY and BY is the low clip level. After controlling the color difference signal amount so that the amplitude ratio between the color difference signals RY and BY becomes the same before and after the control so that the color difference signal does not become lower than the level β, the color difference between RY and BY is adjusted. It is characterized in that a chroma signal is obtained by digitally balanced modulation of the signal.

[0010]

According to the present invention, a CCD (Charge Coupled Device) is provided.
ce) If the digital luminance signal Y at the drive frequency and the digital color difference signal at half the frequency are converted into digital signals at the quadruple subcarrier frequency and then operated, the high level and low level of the composite video signal are equivalently obtained. Can be determined. The reason is that the chroma signal is obtained by switching the color difference signals RY and BY having the quadruple subcarrier frequency and their two complements-(RY) and-(BY). It is.

By controlling the gain of the color difference signal, high clipping and low clipping of the composite video signal can be equivalently performed. Further, by controlling the color difference signal gain so that the amplitude ratio between the color difference signals RY and BY becomes the same, it is possible to prevent a change in chroma phase caused by clipping.

Since the above gain control is performed using the color difference signal before the balanced modulation, the circuit scale is reduced.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to embodiments with reference to the drawings.

FIG. 1 shows a configuration of a chroma signal clipping circuit according to an embodiment of the present invention. The digital color difference signals, RY and BY, which are half the driving frequency of a CCD (charge coupled device) are shown.
Is converted by a frequency converter 31A into a digital signal of a quadruple subcarrier frequency, and the converted color difference signal is
The selector 32 determines the larger one (hereinafter referred to as _Cmax ) of the RY and BY of the two color difference signals by an adder 33 and a subtractor 34.
, And the smaller one (hereinafter C _min ) is supplied to the multiplier 36.

On the other hand, the digital luminance signal Y of the CCD driving frequency is converted into a quadruple subcarrier frequency by the frequency converter 31B and supplied to the adder 33 and the subtractor 34.
The digital luminance signal Y has been subjected to high luminance knee processing in the preceding luminance signal processing system, and does not fall below the high clip level α and below the low clip level β from the dynamic range of quantization. Further, the frequency-converted digital luminance signal Y is for controlling the color difference signal amount, and the luminance component of the composite video signal is a signal before frequency conversion as shown in FIG.

In the adder 33, the luminance signal Y and the color difference signal R-
The color difference signal _Cmax of the larger of Y and BY is added and compared with the high clip level α by the comparison / determination circuit 35A.
Is greater than 0 and the difference is smaller (A = Y + C
_max- α) is output. Then, the high clip is performed by subtracting from _Cmax by the subtractor 37. That is, when the luminance signal + color difference signal becomes higher than the high clip level α, the color difference signal amount is reduced by that amount so that the sum of the luminance signal and the color difference signal does not exceed the high clip level α. .

The subtractor 34 subtracts the larger color difference signal _Cmax from the luminance signal Y, and compares it with the low clip level β in the comparison / determination circuit 35B. If β is smaller, 0 is set.
Is larger, the difference (B = β− (Y−C _max )) is output. Then, low clipping is performed by subtracting the larger color difference signal C _max by the subtracter 37. That is, when the luminance signal-color difference signal becomes smaller than the low clip level β, the color difference signal amount is reduced by that amount so that the luminance signal-color difference signal does not fall below β.

In the above description, the clip levels α and β can be variably set, for example, by controlling from a microcomputer.

In the multiplier 36, (C) is used to reduce the smaller color difference signal C _min at the same rate as C _max is reduced.
multiplied by the _max -A-B) / C _max to C _min. However, since the difference A and B from the clip level cannot occur when both are positive, (C _max -A) / C
multiplied by _max, and multiplied by (C _max -B) / C _max at the time of low clipping. Since A = B = 0 in other cases, it is x1. By doing so, the two color difference signals RY, B-
Only one of Y is clipped, so that no phase rotation occurs.

In the digital balanced modulator 38, for example, NT
In the case of the SC system, RY, BY,-(RY),
Switching is performed in the order of-(BY) to obtain a chroma signal. In the case of the PAL method, RY and-(R-
Y) is reversed. As described above, balanced modulation is performed after the above-described chroma clip. Note that the luminance signal Y
The low clip judgment is made by subtracting the color difference signal RY or BY from the following equation:-(RY)
And-(BY).

The chroma signal obtained as described above is
The signal is converted into an analog signal by a D / A converter 39, passes through a sub-carrier frequency band-pass filter (BPF) 41,
The luminance signal Y converted into an analog signal by the D / A converter 40 and the synchronizing signal are mixed to form a composite video signal.

At the time of low clipping, the output is set to 0 in the blanking period (BLK) by the comparison / determination circuit 35B so as not to clip the burst signal. FIG. 1 does not show the delay for time adjustment in each of the systems C _max , C _min , and Y, but it goes without saying that the delay is adjusted.

[0023]

As described above, according to the present invention, the high clip and the low clip of the composite video signal are equivalently controlled by controlling the gain of the color difference signal according to the corresponding luminance signal level by digital signal processing. Then, the composite video signal is subjected to high clip and low clip without distortion of the chroma component and fluctuation of the DC component.

Further, since the gains of the RY and BY color difference signals are controlled in conjunction with each other, there is no change in the phase of the chroma signal due to clipping, and the picture quality is not deteriorated.
It is possible to prevent blur, overmodulation, and synchronization disorder in the TR image.

Further, abnormal coloring caused by the dynamic range of an image pickup device such as a CCD or a circuit at the time of high-luminance incidence can be eliminated.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a high clip and low clip circuit according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a conventional high clip and low clip.

FIG. 3 is an enlarged view showing a chroma component of a composite video signal.

[Explanation of symbols]

31A, 31B: frequency converter, 32: selector, 33: adder, 34, 37 ... subtractor, 35A, 35B: comparison and judgment circuit,
36 ... Multiplier, 38 ... Digital balancer, 39,40 ... D /
A converter, 41 ... Band pass filter (BPF).

Claims (1)

(57) [Claims] 1. A first frequency for frequency-converting a luminance signal
A converter and two kinds of color difference signals of RY and BY are frequency-converted.
A second frequency converter for converting, and the second frequency converter
Of the two types of color difference signals frequency-converted by the
The large first color difference signal and the first frequency converter
An adder for adding the number-converted luminance signal;
Color difference signal and frequency-converted by the first frequency converter.
A subtractor for subtracting the luminance signal from the adder, and an output from the adder.
The maximum value of the force signal and the minimum value of the output signal from the subtractor.
A comparison / determination circuit for determining whether or not is within a predetermined range;
The first color difference signal is performed based on the output of the comparison / determination circuit.
An arithmetic circuit for calculating the frequency and the frequency conversion by the second frequency converter.
Of the two kinds of converted color difference signals, the second one having a small amplitude is used.
The first color difference signal is calculated by the calculation circuit using the color difference signal.
And a multiplier for multiplying the chroma signal by the same ratio as the change amount .