JPH0535668Y2 - - Google Patents

Description

[Detailed description of the invention] (a) Industrial application field The present invention relates to a muting circuit for video tape recorders (VTRs) and the like.

(b) Conventional technology: Called 8mm video or high-fidelity video
A VTR records an FM modulated audio signal on a video track. For this reason, when a blank tape is played back, FM noise is generated.

On the other hand, Japanese Patent Laid-Open No. 57-202186 discloses that a horizontal synchronizing signal in a video signal is detected to determine whether or not the received radio wave is a regular video signal, and if the received radio wave is not a regular video signal, an audio signal is output. A mutating circuit for muting is disclosed. Therefore, if you use the muting circuit in the above publication, when playing back a blank tape, since there is no horizontal synchronization signal, the audio signal will be muted without being distinguished from a regular video signal, and the FM when playing back a blank tape will be muted. Prevent noise.

However, in the case of the above-mentioned conventional example, a gate circuit, a resonance circuit, a detection circuit, etc. are required to determine whether or not a regular video signal is input from the horizontal synchronization signal, resulting in high cost.

(c) Problems to be solved by the invention The present invention has been devised in view of the above-mentioned drawbacks of the conventional example, and provides a muting circuit that prevents noise that occurs when playing back unrecorded tapes and achieves this at a low cost. The purpose is to

(d) Means for solving the problem: an AFC circuit that compares a horizontal synchronization signal in a video signal accompanied by an audio signal with a variable frequency oscillation output, and controls the variable frequency oscillator based on the comparison output;
Compare the output of the AFC circuit and the horizontal synchronization signal,
The comparison result is output to the AFC circuit, and the AFC
AFC-ID circuit that speeds up the circuit pull-in and the corresponding
A comparison circuit for comparing the output voltage of the AFC-ID circuit with a reference voltage is provided, and the audio signal is configured to be muted by the output of the comparison circuit.

(E) Effect As mentioned above, since the audio signal is muted by the output of the comparator circuit, a separate gate circuit, resonance circuit,
A signal discrimination circuit consisting of a detection circuit or the like is not required.

(F) Embodiment Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. The figure is a block diagram of an audio/video signal processing system for an 8 mm VTR using the muting circuit of this embodiment.

In the figure, 1a and 1b are recording and reproducing heads, respectively.
2 and 7 are YC synthesis circuits that combine luminance signals and color signals; 4 and 5 are YC separation circuits that separate luminance signals and color signals from video signals; and 3 are luminance signal processing circuits that record and reproduce luminance signals. , 6 is a color signal processing circuit for recording and reproducing color signals, 8 is an AGC circuit, and 9 is a tuner. Further, SW1 is a switch for switching the signal output from the video signal output terminal 10 between recording and reproduction.

SW2 is a switch that is switched between recording and reproduction, and 11 is a synchronization separation circuit that extracts a horizontal synchronization signal from the luminance signal input via the switch SW2. The horizontal synchronization signal output from the synchronization separation circuit 11 is composed of a closed loop of a phase comparator 12, a voltage controlled oscillator 13, and a 1/n frequency divider 14.
The output signal of the oscillator 13, which is input to the AFC circuit 15 and whose phase is locked, is sent to the color signal processing circuit 6.
and used as a signal for frequency conversion. Further, the horizontal synchronization signal output from the synchronization separation circuit 11 is passed through a 1/m frequency divider 16 to the AFC-
Input to ID circuit 17.

The AFC-ID circuit 17 controls the oscillator 1 when the oscillation output of the oscillator 13 deviates by more than nH.
This is to perform pull-in in a short time so that the oscillation frequency of No. 3 returns to a range that can be controlled by the AFC loop. The AFC-ID circuit 17 detects how many output signals of the oscillator 13 are input during one period of a signal obtained by dividing the horizontal synchronization signal by 1/m, and determines that the number is within the operating range of the AFC loop. Outputs an H (high) signal when there is too much, an L (low) signal when it is small, and an M (middle) signal when it is within the operating range.
It is composed of a value output circuit, and its output is applied to the oscillator 13 to control the oscillator 13.

18 is a hysteresis comparator which takes the output of the AFC-ID circuit 17 as one input and the reference voltage Vr as the other input;
It is set between the H signal and the M signal output from the AFC-ID circuit 17. Reference numeral 19 denotes an OR gate which inputs the output of the hysteresis comparator (comparison circuit) 18 and a forced mute signal when mute is forcibly applied from the outside.

Further, 20 is an audio signal recording circuit, 21 is an audio signal reproducing circuit, and 22 is a filter. 23 is a switch for selecting the audio signal from the tuner 9 and the audio signal from the audio signal reproducing circuit 21. Said switch SW3 and audio signal output terminal 2
5 is provided with a mute circuit 24 which cuts off the audio signal when the H signal is input from the OR gate 19. In addition, the switches SW1 and SW
2. SW3 is set to the P side during playback and to the R side during recording.
Switch to the side.

Next, the operation of this embodiment will be explained. During playback, the playback head 1b extracts both video and audio signals recorded on the video track, and the YC separation circuit 5 separates the signals into a luminance signal and a color signal. The luminance signal is processed by a luminance signal processing circuit 3 and the chrominance signal is processed by a chrominance signal processing circuit 6, and then combined into a video signal by a YC synthesis circuit 7. Then, a video signal is output from the video signal output terminal 10 via the switch SW1.

On the other hand, the audio signal extracted by the reproduction head 1b is separated from the video signal by a filter 22, and is output to a mute circuit 24 via an audio signal reproduction circuit 21 and a switch SW2.

Now, if you play back a tape on which an audio signal has been recorded, in the case of an 8mm VTR, the audio signal is recorded superimposed on the video signal, so the video signal is necessarily extracted from the playback head 1b, and the brightness is The signal processing circuit 3 outputs a luminance signal. At this time, the synchronization separation circuit 11 separates the horizontal synchronization signal from the luminance signal, and inputs the horizontal synchronization signal to the AFC-ID circuit 17 via the 1/m frequency divider 16. Said AFC
- As mentioned above, the ID circuit 17 has three circuits: H, M, and L.
A value signal is output, but if the AFC circuit 15 is operating normally, an M signal is output. The hysteresis comparator 18 receives the M signal and outputs an L signal to the OR gate 19. When the OR gate 19 is not inputting the forced mute signal, it outputs an L signal to the mute circuit 24, the mute circuit 24 does not operate, and an audio signal is output from the audio signal output terminal 25.

Furthermore, when a tape on which no audio signal is recorded is played back, no video signal is extracted from the playback head 1b, and no brightness signal is input to the synchronization separation circuit 11. For this reason, the AFC-ID circuit 17
Since no horizontal synchronizing signal is input to , it determines that the oscillation frequency of the voltage controlled oscillator 13 is infinite, and outputs an H signal to the hysteresis comparator 18 . The hysteresis comparator 18 outputs an H signal based on this H signal, and the OR gate 19 outputs an H signal. Therefore, the mute circuit 24 operates, and the FM noise generated during playback of the unrecorded tape is not output from the audio signal output terminal 25.

Next, the time of recording will be explained. The video signal received by the tuner 9 is brought to a constant level by the AGC circuit 8, and is output from the video signal output terminal 10 via the switch SW1. At the same time, the above
The video signal, which is the output signal of the AGC circuit 8, is separated into a luminance signal and a color signal by a YC separation circuit 4. The luminance signal is processed by a luminance signal processing circuit 3, and the color signal is processed by a color signal processing circuit 6.
The combining circuit 2 combines the signals into a video signal. On the other hand, the audio signal received by the tuner 9 is sent to the switch SW
3 to the mute circuit 24 and simultaneously output to the audio signal recording circuit 20. and,
The audio signal output from the audio signal recording circuit 20 is superimposed on the video signal output from the YC synthesis circuit 2 and recorded on the video track by the recording head 1a.

Now, if the tuner 9 is in a normal receiving state, the synchronization separation circuit 11 separates and outputs the horizontal synchronization signal from the luminance signal input via the switch SW2, as in the case of recording described above. The AFC-ID circuit 17 inputting this horizontal synchronization signal outputs the M signal, so the mute circuit 24 inputs the L signal,
No mute operation is performed, and an audio signal is output from the audio signal output terminal 25.

Further, when the tuner 9 is out of synchronization and in a non-receiving state, the synchronization separation circuit 11 does not input a luminance signal and therefore does not output a horizontal synchronization signal. The AFC-ID circuit 17 outputs an H signal in the same way as when playing back a non-recorded tape, and the mute circuit 2
The H signal is input to 4. Therefore, the mute circuit 24 operates, and the noise that occurs when no reception is received is not output from the audio signal output terminal 25.

In the above-described muting circuit, since the horizontal synchronizing signal is not input when a blank tape is being played or when there is no reception, the AFC circuit 15 is pulled in faster by the 1/m frequency-divided signal of the horizontal synchronizing signal.
The output of the AFC-ID circuit 17 is used to determine the presence or absence of a horizontal synchronizing signal, and to mute the noise that occurs when a non-recorded tape is played back and when no reception is received.

(g) Effects of the invention According to the invention, since muting is performed by determining the presence or absence of a horizontal synchronization signal from the output of the AFC-ID circuit provided to accelerate the pull-in of the AFC circuit, a separate horizontal synchronization signal is required. It is possible to provide a muting circuit that prevents noise that occurs when playing back a non-recorded tape, when there is no reception, etc., at low cost, without providing a circuit for determining the presence or absence of a signal.

[Brief explanation of the drawing]

The drawing shows 8 using the muting circuit of the present invention.
FIG. 2 is a block diagram of the audio/video signal processing system of the millimeter VTR. 15...AFC circuit, 17...AFC-ID circuit,
18...Hysteresis comparator (comparison circuit),
24...Myuto circuit.

Claims (1)

[Scope of utility model registration request] an AFC circuit that compares a horizontal synchronization signal in a video signal accompanied by an audio signal and a variable frequency oscillator output, and controls the variable frequency oscillator based on the comparison output;
Compare the output of the AFC circuit and the horizontal synchronization signal,
The comparison result is output to the AFC circuit, and the AFC
AFC-ID circuit that speeds up the circuit pull-in and the corresponding
A muting circuit that includes a comparison circuit that compares the output voltage of the AFC-ID circuit with a reference voltage, and mutes the audio signal using the output of the comparison circuit.