JPH0767133A - Clamp circuit for secam signal - Google Patents

Abstract

PURPOSE:To provide the clamp circuit which stabilizes a DC level without being affected by noise. CONSTITUTION:This circuit is provided with a first clamp circuit 8 for clamping the video signal of a SECAM system from an input terminal 1, second clamp circuit 9 for clamping the video signal of the SECAM system from the input terminal 1, switch circuit 10 for switching and outputting the output signals of the first and second clamp circuits 8 and 9 in each horizontal synchronizing signal cycle, and third clamp circuit 11 for clamping the output signal of the switch circuit 10, the response sensitivity of the first and second clamp circuits 8 and 9 is set low, and the response sensitivity of the third clamp circuit 11 is set high.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a secum signal clamp circuit for clamping a secum video signal.

[0002]

2. Description of the Related Art As a video signal clamp circuit, a circuit shown in FIG. 2 has been conventionally known. A video signal is applied to the input terminal (1) of FIG. 2 and is led to the output terminal (3) via the differential amplifier (2). On the other hand, the video signal obtained at the output terminal (3) is
The level is compared with the reference voltage V _ref of the reference power source (5), and the signal corresponding to the level difference is smoothed by the smoothing circuit (6),
It is applied to the negative input terminal (-) of the differential amplifier (2).

On the other hand, the comparator (4) operates only during the generation of a BGP (burst gate pulse) from the terminal (7), and the video signal level at the output terminal (3) is the reference voltage V _ref.
Give feedback so that it is equal to. Therefore, the video signal is clamped to the reference voltage V _ref during the BGP generation period, that is, during the pedestal period. BGP occurrence period,
The video signal in the periods other than is superposed on the clamped DC voltage and is led to the output terminal (3).

Therefore, according to the circuit of FIG. 2, the video signal can be clamped.

[0005]

By the way, the secum (S
The video signal of the ECAM system is as shown in FIG.
Color difference signals of RY and BY are 1H because of line-sequential method
(H is one horizontal synchronizing signal period), and the DC levels are different from each other. Therefore, it is necessary to set the response sensitivity of the clamp circuit to be high so that it can follow a rapid signal change. However, if the response sensitivity becomes high, it becomes easy to be affected by the noise in the signal that arrives during the clamp period, so that the response sensitivity cannot be increased so much. Therefore, a clamp circuit that has high response sensitivity and is resistant to noise has been desired as a clamp circuit for the secum system.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and includes a first clamp circuit for clamping a secam type video signal from an input terminal, and a secum type image from the input terminal. A second clamp circuit that clamps a signal; and a switch circuit that switches and outputs the output signals of the first and second clamp circuits for each horizontal synchronization signal period.
A third clamp circuit for clamping the output signal of the switch circuit, wherein the response sensitivities of the first and second clamp circuits are set low and the response sensitivity of the third clamp circuit is set high.

[0007]

According to the present invention, the first and second low response sensitivities are provided.
A video signal is first applied to the clamp circuit, and for example, RY
Only the signal is clamped by the first clamp circuit, and only the BY signal is clamped by the second clamp circuit. Therefore, the first
Also, both the second clamp circuit and the second clamp circuit can suppress the influence of noise without a large DC fluctuation. And the first and second
After the output signals of the clamp circuit are combined by the switch circuit and returned to one video signal, the third clamp circuit having high response sensitivity absorbs the DC fluctuation for every 1H to perform total clamp.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a circuit diagram showing a secum signal clamp circuit according to the present invention, in which (8) is a first clamp circuit for clamping a secam video signal from an input terminal (1),
(9) is a second clamp circuit for clamping the secam type video signal from the input terminal (1), (10) is one horizontal output signal of the first and second clamp circuits (8) and (9) A switch circuit that switches and outputs every synchronization signal cycle, (11) a third clamp circuit that clamps the output signal of the switch circuit (10), and (12) generate every 2H according to BGP and have phases relative to each other. It is a waveform shaping circuit for generating signals A and B which are shifted by 180 degrees and a signal C for inverting the switch circuit (10) every 1H.

The video signal of the secum system of FIG. 3A from the input terminal (1) is simultaneously applied to the first and second clamp circuits (8) and (9). On the other hand, the waveform shaping circuit (1
From 2), the intermittent BGP shown in FIGS. 3 (c) and 3 (d)
Signals A and B are generated and applied to the first and second clamp circuits (8) and (9). Further, the waveform shaping circuit (12) generates the switching control signal of the switch circuit (10) shown in FIG. 3 (e) and keeps the switch circuit (10) in the "H" level period of the signal of FIG. 3 (e). The switch circuit (10) is switched to the b side during the "L" level period.

Therefore, during the RY signal period of FIG. 3A, the output signal of the first clamp circuit (8) passes through the switch circuit (10) and the LPF (13) for removing noise to make a comparator. It is applied to (14) and (15). Here, since only the comparator (14) is operated by the signal A of FIG. 3 (c), only the first clamp circuit (8) is operated, and the reference voltage V _ref level of the reference power supply (5) is set. The RY signal with the pedestal level clamped is applied to the third clamp circuit (11).

Here, the RY signals in FIG.
Since they are continuous signals, they do not have large DC level changes with each other and are usually almost equal. Therefore, the first clamp circuit (8) with low response sensitivity in consideration of noise
Even if the smoothing circuit (16) has a low response sensitivity, it does not hinder the use. Next, in the BY signal period of FIG. 3A, the switch circuit (10) switches to the b side, and the comparator (15) of the second clamp circuit (9).
Works. Then, as in the case of the RY signal, the pedestal level is clamped to the reference voltage V _ref level B.
The -Y signal is applied to the third clamp circuit (11).

R applied to the third clamp circuit (11)
Although the DC levels of the −Y signal and the BY signal are equal to each other, a difference in the DC level may occur between the RY signal and the BY signal. This is because the response sensitivity of the first and second clamp circuits (8) and (9) is low. Therefore, in the present invention, the third sensor with high response sensitivity is used.
A clamp circuit (11) is arranged to eliminate the DC level difference between the RY signal and the BY signal.

Therefore, the output terminal (3) is provided with a secam system video signal which is not affected by noise and is clamped to a constant level (reference voltage V _ref level). The LPF (17) of the third clamp circuit (11) is for removing noise, and the value of the reference power supply (18) is set equal to that of the reference power supply (5).

[0014]

As described above, according to the present invention, it is possible to provide a clamp circuit for a secam signal which is not easily affected by noise and which can stably maintain a constant clamp level.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a secum signal clamp circuit according to the present invention.

FIG. 2 is a circuit diagram showing a conventional clamp circuit for a secum signal.

FIG. 3 is a waveform diagram for use in the description of FIGS. 1 and 2.

[Explanation of symbols]

 (8) First clamp circuit (9) Second clamp circuit (10) Switch circuit (11) Third clamp circuit

Claims (2)

[Claims] 1. A first clamp circuit for clamping a secam type video signal from an input terminal, a second clamp circuit for clamping a secum type video signal from the input terminal, and the first and second clamp circuits. A switch circuit for switching and outputting the output signal every 1 horizontal synchronization signal cycle, and a third clamp circuit for clamping the output signal of the switch circuit. The response sensitivity of the first and second clamp circuits is A clamp circuit for a secum signal, which is set to be lower than the response sensitivity of the third clamp circuit. 2. The first and second clamp circuits repeat each other, when one of them is operating every horizontal synchronizing signal period, the other stops operating. 1. The clamp circuit for the secum signal according to 1.