JPH04622Y2 - - Google Patents

Description

[Detailed explanation of the idea] The present invention relates to a synchronization signal level control circuit.

Conventionally, in a VTR (video tape recorder), an AGC circuit is provided in the video signal processing circuit to ensure that the level of the recorded video signal is always constant. And the AGC circuit generally has
SYNC, AGC is adopted, and this SYNC,
AGC is a system that focuses only on the horizontal synchronization signal level, which is unrelated to the brightness of the video signal, and attempts to keep the video signal level constant by controlling this synchronization signal level so that it is always constant. It is. For example, if you take a 1VPP video signal and a synchronization signal of 0.3VPP as a reference, if the synchronization signal portion is 30% of the entire video signal, the output will always be 1VPP regardless of the input signal level. becomes.

Generally, the ratio of the synchronization signal level to the video signal level (synchronization signal ratio) is set at 30% as mentioned above, but in normal television broadcasting, the synchronization signal level varies somewhat depending on the broadcasting station, and , video detection circuit and high frequency amplification of the receiver,
In an intermediate frequency amplification circuit, the synchronization signal of the video signal is susceptible to nonlinear distortion of the circuit, and generally the detection output is often output with the synchronization signal portion shrunk compared to the input signal. Therefore, as mentioned above,
If we take a signal with a 1VPP video signal and a synchronization signal of 0.3VPP as a reference, even if the video signal is 1VPP, if the synchronization signal is shrunk to only 0.15VPP, then AGC
The circuit amplifies the video signal so that the 0.15VPP synchronization signal becomes 0.3VPP, and as a result, the video signal becomes
Since it becomes 2VPP, overmodulation may occur during FM modulation.

Therefore, if the video signal output exceeds 1VPP, the peak at which the white peak is detected is
AGC is activated to keep the video signal within 1VPP, but when this peak AGC is activated, the above-mentioned synchronization signal ratio changes.

This invention was made to eliminate the above-mentioned drawbacks, and detects the output level of the AGC circuit, and if this level is higher than a predetermined value, the
The present invention provides a synchronization signal level control circuit that brings the synchronization signal ratio of a video signal input to an AGC circuit close to a constant value.

An embodiment of the present invention will be described below with reference to the drawings.

The drawing shows a schematic block diagram of the circuit of the present invention, in which 1 is an input terminal into which a composite video signal is input, 2 is an input terminal to which a composite video signal is input;
3 is a synchronization level variable expansion circuit that expands and controls the sync signal ratio to approach the normal value, that is, 30%, when the level of the horizontal synchronization signal of the composite video signal from input terminal 1 decreases; 3 is an AGC control The SYNG and AGC circuits are formed by a closed loop of an amplifier 31, a sync separation circuit 32, and a sync level detection circuit 33, and operate to keep the sync signal level constant. 4 is an amplification shaping circuit that amplifies and shapes the output of the sync separation circuit 32 and inputs it to the sync level variable expansion circuit 2; 5 is a peak when the output of the SYNC and AGC circuit 3 is above a predetermined value, for example 1 VPP or above; A peak level detection circuit 6 is a rectifier circuit that rectifies the peak level detection output and inputs it to the synchronous level variable expansion circuit 2, which forms a closed loop together with the synchronous level variable expansion circuit 2 and the peak level detection circuit 5. Form. 7 is an output terminal.

Next, the operation of the circuit having the above configuration will be explained.

Now, assuming that the regular video signal level is 1VPP and the synchronization signal ratio is 30%, if the synchronization signal ratio of the video signal input from input terminal 1 is 30%, then
The output of the SYNC and AGC circuit 3 becomes a 1VPP regular video signal and is led out to the output terminal 7.

If only the synchronization signal of the next input video signal is compressed, that is, if the synchronization signal ratio is small, the SYNC/AGC circuit 3 amplifies the entire video signal so that the synchronization signal becomes the normal level. SYNC, AGC circuit 3 output will be 1VPP or more. Therefore, when the SYNC and AGC circuit 3 output exceeds 1VPP, the peak level detection circuit 5 operates to detect the peak level, and this detection output is rectified by the rectifier circuit 6, and the synchronization level can be adjusted together with the amplification shaping circuit 4 output. input to the expansion circuit 2, the synchronization signal period,
Control is performed so that only the synchronization signal level is expanded to approach the normal synchronization signal ratio.

Therefore, the video signal whose synchronization signal level has been expanded by this synchronization level variable expansion circuit 2 is SYNC,
The AGC circuit 3 converts it to 1VPP and outputs it to the output terminal 7.

As described above, according to the present invention, a video signal in which only the synchronization signal has been compressed in the video detection circuit of a television receiver is SUNC, and only the synchronization signal is expanded in the previous stage of the AGC circuit to bring the synchronization signal ratio to a normal value. By bringing them close together, the SYNC and AGC circuit outputs will not exceed the normal values, and overmodulation during FM modulation can be prevented.

[Brief explanation of the drawing]

The drawing is a schematic block diagram of a synchronizing signal level control circuit according to an embodiment of the present invention. Explanation of main drawing numbers, 2... Synchronous level variable expansion circuit, 3 ... SYNC, AGC circuit, 5... Peak level detection circuit.

Claims (1)

[Scope of utility model registration request] A variable synchronization level expansion circuit that variably expands the horizontal synchronization signal level of the composite synchronization signal is installed before the AGC circuit that detects the horizontal synchronization signal level and controls the level of the video signal, and an AGC circuit is installed after the previous AGC circuit. It is equipped with a peak level detection circuit that detects a peak level when the circuit output exceeds a predetermined level, and the first stage synchronous level variable expansion circuit inputs the output of the peak level detection circuit and a synchronous signal, and detects the synchronous signal. A synchronization signal level control circuit that extends only the synchronization signal level during a period to bring a synchronization signal ratio of a video signal close to a normal value.