JP2508470B2 - Video detection circuit - Google Patents

Description

The present invention relates to a video detection circuit in video equipment.

[Outline of Invention]

When synchronously detecting the video intermediate frequency signal, the PLL circuit that obtains the reproduced image carrier is automatically switched so that the loop time constant becomes longer during audio multiplex broadcasting, and the loop time constant was optimized according to the received content. It is a video detection circuit.

[Conventional technology]

A detection circuit for performing video detection and audio detection by synchronous detection using a PLL circuit is known (for example, Japanese Patent Publication No. 60-41).
493 publication). This detection circuit is configured to obtain a video detection output by locking the PLL to the video carrier (intermediate frequency) and synchronously detecting the video intermediate frequency signal using the PLL output as a local oscillation signal. The audio detection output is obtained by extracting the beat signal (4.5 MHz) of the difference between the video carrier and the audio carrier as an intermediate frequency from the video detection output, and further performing FM detection.

[Problems to be solved by the invention]

The PLL circuit has a loop filter that determines the speed of following the input phase, but the detection performance changes as follows depending on the time constant.

First, in order to improve the voice quality, it is necessary to make the loop time constant sufficiently long to suppress the interference due to the second harmonic component 2f _H of the horizontal synchronizing signal.

On the other hand, when receiving a video carrier that contains a lot of residual phase modulation noise (ICPM) from some satellite stations or video game devices, do not keep the loop filter time constant short enough so that the PLL circuit can sufficiently follow the ICPM. Then, the 4.5MHz voice intercarrier is shaken by ICPM and noise leak occurs. In other words, if the PLL circuit follows the video carrier that has been phase-modulated by noise at high speed, the intercarrier frequency is kept at 4.5 MHz and the audio quality is good.

As described above, the time constant of the PLL of the synchronous detection circuit may be longer or may be shorter. Therefore, conventionally, the time constant is fixed to an intermediate value, or
A time constant changeover switch is provided.

However, the time constant switching type is troublesome because it is necessary to perform a switching operation one by one depending on the received content, and it is premised that the user has a certain degree of expertise regarding noise. Further, if the time constant is fixed, the reception quality cannot be optimized.

In view of this problem, an object of the present invention is to eliminate the need for switch operation so that the PLL time constant is always optimally set.

[Means for solving problems]

It is a video detection circuit for synchronously detecting a video intermediate frequency signal based on a reproduced video carrier obtained from a PLL circuit.

Switching means for switching the loop time constant of the PLL circuit between long and short is provided.

The switching means is controlled by the audio multiplex discrimination signal so that the loop time constant becomes long when the audio multiplex broadcast is received.

[Action]

It suppresses the second harmonic component of the horizontal synchronizing signal from leaking to the PLL output when receiving audio multiplex broadcasting, and improves the quality of subchannel audio. During reception other than audio multiplex broadcasting, PL is set to sufficiently follow the residual phase modulation noise of the video.
The L loop time constant is shortened and the voice intercarrier is set to 4.5 MHz.
Keep it at z to reduce the noise modulation component.

〔Example〕

FIG. 1 shows a video detection circuit according to an embodiment of the present invention. The video IF signal IF-converted by the tuner circuit 1 is supplied to the balanced demodulator 3 via the IF amplifier 2 and is subjected to synchronous detection based on the reproduced video carrier supplied from the PLL circuit 4. The detection output is led to the amplification stage as a video signal and is also supplied to a 4.5 MHz bandpass filter 5 for extracting an audio intercarrier. The extracted voice intercarrier is demodulated by the FM demodulation circuit 6 into a voice signal.

The PLL circuit 4 includes the IF amplifier 2 to the bandpass filter 7
It is equipped with a VCO8 that is phase-synchronized with the video carrier (f _P = 58.75 MHz) supplied via. The oscillation output of VCO8 is the phase discriminator 9
At, the phase comparison is performed with the input video carrier, and the error output is supplied to the oscillation control input of the VCO 8 via the loop filter 10.

The loop filter 10 may be a low pass filter including a resistor R and a capacitor C1. Capacitor C2 controlled by transistor 11 to switch the filter time constant
Is provided in parallel with the capacitor C2.

The transistor 11 is turned on by the voice multiplex discrimination signal MPX obtained from the FM demodulation circuit 6. That is, at the time of audio multiplex broadcasting, the capacitor C2 is additionally connected to the loop filter 10 to lengthen the loop time constant so as to sufficiently suppress the double harmonic of the horizontal synchronizing frequency. As a result, the noise equivalent to the sub-channel carrier (2f _H ) of the voice multiplex is suppressed from leaking to the inter carrier, and high quality received voice can be obtained. That is, the output of VCO8 is 2 based on the video carrier f _P.
If f _H fluctuation component is included, 2f _H fluctuation by the video that are not included in the original sound carrier f _S transmitted from the broadcasting station to 4.5MHz sound intercarrier output of 4.5MHz band-pass filter 5 The components are mixed, and the sub-audio L-R component of the output of the FM demodulation circuit 6 is deteriorated by the fluctuation component due to this image. Therefore, if the PLL loop time constant is increased, the VCO
Since the output of the 8 2f _H fluctuation component is removed, no longer contaminated with 2f _H fluctuation component by the video during sound intercarrier, deterioration of the sub-audio of L-R signal demodulated is alleviated. In the audio multiplex broadcasting, the broadcasting station manages the residual phase modulation noise ICPM of the video to be sufficiently low, so that there is no problem even if the PLL loop time constant is lengthened to slow the tracking speed.

When receiving a monaural broadcast other than the audio multiplex broadcast or a video game image, the audio multiplex determination signal MPX disappears, the transistor 11 is turned off, and the time constant of the loop filter 10 becomes short. Therefore, the PLL circuit 4 operates sufficiently following the residual phase modulation noise of the input image,
Noise leakage to the voice intercarrier is suppressed to a low level, and high quality voice can be obtained. Note that the second harmonic of the horizontal frequency may leak, but since the sub-channel output is cut in the FM demodulation circuit 6 during monaural broadcasting, deteriorated sound does not come out of the speaker.

〔The invention's effect〕

As described above, according to the present invention, since the PLL loop time constant of the PLL type video synchronous detection circuit is lengthened at the time of audio multiplex broadcasting, and automatically switched by the audio multiplex determination signal so as to be shortened in other cases, the content of the received image is changed. The loop time constant is selected accordingly. Especially, in the case of audio multiplex broadcasting, the PLL loop time constant becomes long, so that it is possible to sufficiently suppress the second harmonic noise of the horizontal synchronizing frequency due to the video contained in the video carrier from being mixed into the VCO output of the PLL circuit. The channel sound can be demodulated with high quality. In addition, except for audio multiplex broadcasting, the PLL loop time constant becomes sufficiently short, the synchronous detection carrier follows the residual phase modulation noise, and noise modulation of the audio intercarrier can be reduced.

[Brief description of drawings]

FIG. 1 is a block circuit diagram of a PLL type video detection circuit showing an embodiment of the present invention. In the symbols used in the drawings, 1 ... Tuner circuit 2 ... IF amplifier 3 ... Balanced demodulator 4 ... PLL circuit 5 ... Bandpass filter 6 ... FM detection circuit 7 ... Bandpass filter 8 ... VCO 9 ... Phase discriminator 10 ... Loop filter 11 ... Transistor.

Claims (1)

(57) [Claims] 1. A video detection circuit for synchronously detecting a video intermediate frequency signal based on a reproduced video carrier obtained from a PLL circuit, comprising switching means for switching the loop time constant of the PLL circuit to short or long, and audio multiplex broadcast reception. A video detection circuit characterized in that the switching means is controlled by an audio multiplex discrimination signal so that the loop time constant becomes longer at times.