JPH0810924B2 - Dropout compensator - Google Patents

Description

The present invention relates to a dropout compensator for a VTR.

[Outline of Invention]

The present invention detects the sync tip level or pedestal level of a reproduced video signal in a VTR dropout compensation device and clamps the output signal of a delay circuit to this level to determine the average level of the input video signal. The output signal that has passed through the delay circuit can be controlled in accordance with the change, and the clogging of the sink can be prevented.

[Conventional technology]

Conventionally, a dropout compensation circuit of a VTR detects a dropout in a playback video signal by a dropout detection circuit, and when a dropout is detected, the dropout detection circuit replaces the playback video signal by 1H (1 horizontal section) before. It is designed to output a reproduced video signal. In other words, the playback video signal and the playback video signal that has passed through the 1H delay circuit are supplied to the switch circuit, and when dropout is detected in the playback video signal by the dropout detection circuit, 1H
A reproduced video signal that has passed through the delay circuit is output.

By the way, the average level of the reproduced video signal varies every 1H. Therefore, as shown in FIG. 3, the DC level varies every 1H. In this way, the DC level of the playback video signal fluctuates for each 1H, so if the playback video signal and the playback video signal via the 1H delay circuit are supplied as they are to the switch circuit, the brightness level will change. As a result, dropout compensation cannot be performed correctly. Further, this DC level difference is added up in the case of a dropout reaching several H, which also causes a disturbance of synchronization.

Therefore, the conventional dropout compensation circuit is provided with a clamp circuit that clamps the reproduced video signal and the reproduced video signal of 1H before to a predetermined level. Then, for example, the pedestal level of the reproduced video signal and the reproduced video signal of 1H before is clamped to a predetermined level to make the DC levels equal.

[Problems to be solved by the invention]

In this way, the playback video signal and the playback video signal before 1H are clamped and supplied to the switch circuit, and the switch circuit is switched by the output of the dropout detection circuit to perform dropout compensation. , Output is always clamped regardless of the presence or absence of dropout. When the reproduced video signal is passed through the clamp circuit, sync clogging occurs and the sync level becomes smaller than the video signal level.

When the sink is clogged in this way, there arises a problem that the characteristics during dubbing are deteriorated. In other words, recording side VTR
In the sync AGC circuit of, when the sync level is low, the input signal level is low, and control is performed in the direction of increasing the gain. For this reason, the gain of the video signal rises too much, and waveform distortion and overmodulation due to white clipping easily occur. It is also disadvantageous for moire.

Therefore, an object of the present invention is to provide a dropout compensator capable of performing accurate dropout compensation by tracking the fluctuation of the average level of the reproduced video signal and controlling the reproduced video signal level via the 1H delay circuit. To do.

Another object of the present invention is to provide a dropout compensator in which sink clogging does not occur.

[Means for solving problems]

In the present invention, the reproduced video signal is supplied to one input terminal 5A of the switch circuit 5, and the output signal of the switch circuit 5 is passed through the delay circuit 8 to the other input terminal 5B of the switch circuit 5.
Is supplied to the switch circuit 5 and the dropout detection circuit.
In the dropout compensating device controlled by the output of 12, the level detection circuit 20 for detecting the sync tip level or pedestal level of the reproduced video signal and the output signal of the delay circuit 8 are clamped to the detection level of the level detection circuit 20. The dropout compensating device is provided with a clamp circuit 9.

[Action]

The reproduction video signal and the reproduction video signal that has passed through the 1H delay circuit 8 are supplied to the switch circuit 5. Switch circuit 5
Are controlled by the output of the dropout detection circuit 12. When the dropout does not occur, the input terminal 5A and the output terminal 5C of the switch circuit 5 are connected, and the reproduced video signal is output from the output terminal 7. When dropout occurs, the input terminal 5B and the output terminal 5C of the switch circuit 5 are connected, and the reproduced video signal 1H before is output from the output terminal 7.

The output of the 1H delay circuit 8 is clamped by the clamp circuit 9. The clamp level is a level obtained by sampling and holding the sync tip level or the pedestal level of the input reproduction video signal. Therefore, the DC level of the output signal of the 1H delay circuit 8 is always equal to the DC level of the input reproduced video signal.

〔Example〕

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

In FIG. 1, reference numeral 1 denotes an input terminal, to which the FM modulated luminance signal reproduced by the rotary head is supplied. This FM modulated luminance signal is supplied to the FM demodulation circuit 2.
The output of the FM demodulation circuit 2 is supplied to the input terminal 5A of the switch circuit 5 via the low pass filter 3 and the amplifier 4.

The output of the output terminal 5C of the switch circuit 5 is output from the output terminal 7 via the amplifier 6 and is also supplied to the 1H delay circuit 8. The output of the 1H delay circuit 8 is supplied to the input terminal 5B of the switch circuit 5 via the clamp circuit 9.

The FM modulated luminance signal reproduced by the rotary head is supplied to the input terminal 10, and the reproduced luminance signal is supplied to the dropout detection circuit 12 via the limiter 11. The dropout detection circuit 12 detects a dropout by peak detection. The output of the dropout detection circuit 12 is supplied to the comparator 13, the output of the comparator 13 is supplied to the switch circuit 5 as a switch control signal, and is also supplied to the AND gate 15 via the inverter 14.

When dropout occurs, the output of the comparator 13 becomes high level. When the output of the comparator 13 is low level, the input terminal 5A and the output terminal 5C of the switch circuit 5 are connected. When the output of the comparator 13 becomes high level, the input terminal 5B and the output terminal 5C of the switch circuit 5 are connected.

When the input terminal 5A and the output terminal 5C of the switch circuit 5 are connected, the reproduction luminance signal demodulated by the FM demodulation circuit 2 is supplied to the amplifier 6 via the low-pass filter 3, the amplifier 4, and the switch circuit 5, and the output It is taken out from the terminal 7.
When the input terminal 5B and the output terminal 5C of the switch circuit 5 are connected, the reproduction luminance signal of the line before 1H output from the 1H delay circuit 8 is supplied to the amplifier 6 via the clamp circuit 9 and the switch circuit 5, It is taken out from the output terminal 7.

The demodulated reproduction luminance signal is supplied to the input terminal 16, and the reproduction luminance signal is supplied to the sync separation circuit 17. The sync separation circuit 17 separates the sync signal. The separated sync signal is supplied to a mono-multi (monostable multivibrator) 19 after the equalization pulse in the vertical blanking interval is removed via the half H killer circuit 18. Mono Multi
At 19, a timing pulse having a predetermined width is formed from the synchronization signal at a predetermined timing. This timing pulse corresponds to the pedestal level section. The output of the monomulti 19 is supplied to the AND gate 15.

It is supplied to the output sample and hold circuit 20 of the AND gate 15 and the switch circuit 21. AND gate 1
When the output of 5 becomes high level, the output of the amplifier 4 is sampled and held by the sample and hold circuit 20, and
The switch circuit 21 is turned on. When the switch circuit 21 is turned on, the output of the 1H delay circuit 8 changes to the sample hold circuit 20.
It is clamped according to the level sampled and held by.

When the luminance signal shown in FIG. 2A is reproduced, the timing pulse shown in FIG. 2B is formed by the monomulti 19 from the synchronizing signal of the reproduced luminance signal. When the output of the comparator 13 (FIG. 2C) is low level, the output of the AND gate 15 becomes high level corresponding to the pedestal level section of the reproduction luminance signal, as shown in FIG. 2D. AND
When the output of the gate 15 (FIG. 2D) becomes high level, the pedestal level of the reproduction luminance signal output from the amplifier 4 is sampled and held as shown in FIG. 2F. The pedestal level of the reproduction luminance signal is supplied to the clamp circuit 9 via the switch circuit 21. Therefore, the pedestal level of the output of the 1H delay circuit 8 is clamped by the clamp circuit 9 to the pedestal level of the input reproduction luminance signal.

When noise due to dropout occurs as indicated by N in FIG. 2, the output of the comparator 13 becomes high level, as shown in FIG. 2C. When the output of the comparator 13 becomes high level, the input terminal 5B of the switch circuit 5
Is connected to the output terminal 5C, and the output of the 1H delay circuit 8 is supplied to the amplifier 6 via the clamp circuit 9 and the switch circuit 5,
It is taken out from the output terminal 7. The output of the 1H delay circuit 8 is clamped by the clamp circuit 9 so that the pedestal level becomes equal to the pedestal level of the input reproduction luminance signal. Therefore, even if the average level of the reproduced video signal varies, the luminance level does not vary.

The timing pulse output from the mono-multi 19 is supplied to the sample hold circuit 20 and the switch circuit 21 via the AND gate 15 when the dropout occurs, the hold mode is automatically set, and noise is generated. This is because the fluctuation of the clamp potential is suppressed.

In the above-described embodiment, the pedestal level is sampled and held by the sample and hold circuit 20, but the sync tip level may be sampled and held and the sync tip level of the output of the one delay circuit 8 may be clamped. .

Further, although the processing of the luminance signal has been described in the above-described embodiment, the processing of the composite video signal can be performed in the same manner. Furthermore,
It is also possible to replace the transient part with an integrated signal as in an RF switching transient removal circuit.

〔The invention's effect〕

According to the present invention, the sync tip level or pedestal level of the reproduced video signal is sampled and held, and the sync tip level or pedestal level of the video signal that has passed through the 1H delay circuit 8 is clamped to this sampled and held level. . Therefore, the switch circuit 5 is always
The DC levels of the inputs become equal, and accurate dropout compensation can be performed even if the average level of the video signal fluctuates. In this way, the DC level of the video signal that has passed through the 1H delay circuit 8 is always made to follow the DC level of the input reproduction video signal, so that it is stable against temperature changes.

Further, according to the present invention, when the dropout does not occur, the reproduced video signal is output without passing through the clamp circuit. Therefore, sink clogging does not occur, and the characteristics during dubbing are improved.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, FIG. 2 is a waveform chart of each part used for explaining an embodiment of the present invention, and FIG. 3 is a schematic diagram used for explaining a conventional dropout compensation circuit. is there. Description of main symbols in the drawing 1: Input terminal, 5: Switch circuit, 7: Output terminal, 8: 1H delay circuit, 9: Clamp circuit, 12: Dropout detection circuit, 20:
Sample and hold circuit.

Claims (1)

[Claims] 1. A dropout detecting means for detecting dropout of a reproduced video signal, and a switch controlled by an output of the dropout detecting means for selecting the reproduced video signal and the reproduced video signal delayed by one horizontal section. Means, sample and hold means for sampling and holding the sync tip level or pedestal level of the reproduced video signal, and clamp means for clamping the reproduced video signal delayed by one horizontal period based on the output of the sample and hold means. If the dropout is not detected, the reproduction video signal is output as it is through the switch means, and if the dropout is detected, the reproduction video signal delayed by one horizontal section is clamped by the clamp means and output. Dropout characterized by Compensation device.