JPH03289876A - Video clamp circuit - Google Patents

Abstract

PURPOSE:To easily prevent the oscillation of a loop which might be caused by input of a noise component or absence of a video signal by providing a circuit discriminating a synchronizing signal of a video signal from the noise component and a switch closing/opening a feedback loop. CONSTITUTION:A synchronizing regeneration circuit 10 separates a synchronization component in a video signal inputted from a terminal 12 whose level is below a reference voltage and amplifies the synchronization component to form a sampling pulse. The sampling pulse closes a switch, a peak level of an outputted synchronizing signal is stored in a capacitor 11 and an envelope of a peak of the synchronizing signal being a distortion component is extracted from a terminal 13. The synchronizing signal and noise extracted by a synchronizing separator circuit 6 are given to a pulse width discrimination circuit 7, and when the pulse width of noise component is detected, the circuit 7 outputs a control signal controlling the switch 1. Upon the receipt of the control signal, a fixed bias V is fed to a video amplifier 2 to interrupt a negative feedback loop.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a video clamp circuit, and more particularly to a video clamp circuit that stabilizes the circuit against external noise when almost no video signal is input.

[Conventional technology]

As shown in the configuration diagram of FIG. 3, the conventional video clamp circuit includes a video signal input terminal 5, a video amplifier 2 having a feedback input terminal, and a waveform distortion detection circuit for detecting waveform distortion of the output signal of the video amplifier 2. 3, a low frequency amplifier 4 for setting the feedback loop gain, and an output terminal 8 which is the output of the video amplifier 2, and the feedback loop is connected to a waveform distortion detector 3, It is connected to the feedback input terminal of the video amplifier 2 via the low frequency amplifier 4.

Next, the operation of the conventional example will be explained. In FIG. 3, a video signal input to an input terminal 1 is amplified by a video amplifier 2 and then branched into two. One of the branched signals is output from the output terminal 8.

The other branched signal is input to the waveform distortion detector 3,
Here, waveform distortion is detected by taking the envelope of the voltage in the synchronization section of the video signal. The detected envelope voltage is amplified by the low frequency amplifier 4 to a suitable loop gain, converted to the opposite polarity, and then fed back to the feedback input terminal of the video amplifier 2. This negative feedback loop compresses the low frequency distortion of the video amplifier 2, so the waveform of the video signal at the output terminal 8 is shaped and its characteristics are improved.

[Problem to be solved by the invention]

The conventional video clamp circuit described above has a video transmission path S/N
If it is used in a section with a poor ratio, the video signal will almost disappear like when unmodulated waves are demodulated, and noise components, especially impulsive noise, will become large, and this type of noise will activate the feedback loop and cause the loop to fail. There is a drawback that loop oscillation is likely to occur due to noise within the band. Therefore, this oscillation waveform outputs a rectangular wave at the output level of the final stage amplifier, which has a disadvantage in that it adversely affects subsequent equipment.

[Means to solve the problem]

The video clamp circuit of the present invention includes a video amplifier, a feedback loop that negatively feeds back an output signal of the video amplifier to an input, and a feedback loop that is inserted into the feedback loop and disconnects or connects the feedback loop depending on the presence or absence of an input control signal. and a switch driving section that extracts a noise component including a synchronization signal and an external impulse noise from the output video signal of the video amplifier, and sends out the control signal when the noise component is extracted.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of the present invention, and FIG. 2 is a circuit diagram of a waveform distortion detector. In the embodiment shown in FIG. 1, a negative feedback loop similar to the conventional example consisting of a video amplifier 2, a waveform distortion detector 3, and a low frequency amplifier 4 is provided with a switch 1 for opening and closing the negative feedback loop. Further, for driving this switch, there is provided a switch driving section comprising a synchronization separator 6 for video signals and a pulse width discrimination circuit 7 for discriminating noise components and synchronization signals.

Next, the operation of this embodiment will be explained. First, waveform distortion detector 3
The operation will be explained with reference to FIG. The synchronous reproducing circuit 10 separates the synchronous component of the video signal input from the terminal 12 and which is below the reference voltage, and then amplifies it to generate a sampling pulse. With this sampling pulse, switch 9 is closed, and the peak value voltage of the synchronization signal output is transferred to capacitor 1.
1, and the envelope of the synchronization signal peak, which is a distortion component, is taken out from the terminal 13.

Next, when only impulsive noise is input, a sampling pulse is created in the synchronous reproducing circuit 10 by this noise component. The instantaneous noise peak value is fed back with the time constant of the feedback loop, so it may become a positive feedback, making the loop unstable and prone to oscillation.
Therefore, switch 5 is inserted into the negative feedback loop to stabilize the circuit by preventing the loop from closing when impulsive noise is input. The synchronization separator 6 sets a synchronization signal and a predetermined level from the video signal, and separates and outputs noise above this level, especially impulse noise. This predetermined level setting is set while testing the degree of suppression of unstable oscillation of the circuit. The synchronization signal and noise extracted by the synchronization separator 6 are further processed by the switch 1 when the pulse width discrimination circuit 7 detects the pulse width of the noise component.
Outputs a control signal to control the Normally, among the noise components, impulse noise may have a higher amplitude level than the synchronization signal, but the noise pulse width is almost always narrower than the pulse width of the synchronization signal, so it is possible to effectively detect the noise component. It is. When the switch 1 receives the aforementioned control signal, the A of the switch 1 is changed as shown in FIG.
It supplies a fixed bias (1) to the video amplifier 2 and cuts off the negative feedback loop on the B side of the switch 1. Therefore, when the pulse width discrimination circuit 7 is not sending out a control signal, it is connected to the B side of the switch 1 and performs a normal negative feedback operation.

〔Effect of the invention〕

As explained above, the present invention provides a conventional video clamp circuit with a circuit that discriminates between a synchronization signal and a noise component of a video signal, and a switch that opens and closes a feedback loop. This has the effect of easily preventing loop oscillation that may occur when there is no signal. Therefore, the S/N of satellite communication reception, etc.
It is extremely effective to use the video clamp circuit of the present invention on lines with poor ratios.

12° 3...Terminal.

Claims (1)

[Claims] 1. A video amplifier, a feedback loop that negatively feeds back an output signal of the video amplifier to an input, and a video amplifier inserted into the feedback loop,
A switch that disconnects or connects this feedback loop depending on the presence or absence of an input control signal, and extracts a noise component including a synchronization signal and external impulse noise from the output video signal of the video amplifier, and when the noise component is extracted. A video clamp circuit comprising: a switch driving section that sends out the control signal. 2. A sync separator in which the switch driving section extracts a sync signal and a noise component from the output video signal of the video amplifier; and a pulse width discrimination circuit that detects the pulse of the noise component with a pulse width and sends out the control signal. The video clamp circuit according to claim 1, characterized in that it has: