JPS5828793B2 - Eizoushingo no Isouhenchiyouhouhou - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION When transmitting a video signal by recording it on a magnetic tape, the video signal may be phase modulated.

Phase modulation has the advantage of reducing moire and the like by reducing the modulation index and suppressing deviation, compared to frequency modulation.

In this case, in order to demodulate the video signal on the reproduction or reception side, there is a method of configuring an APC loop having a sampling system.

Figure 1 shows the case where such a demodulation method is used, and the input terminal 1
A phase-modulated video signal from the voltage-controlled variable frequency oscillator 3 is supplied to the phase detection circuit 2, an oscillation signal from the voltage-controlled variable frequency oscillator 3 is supplied to the circuit 2, a demodulated video signal is obtained from the circuit 2, and this is It is taken out through a pass filter 4 to an output end 5.

The output of the filter 4 is the sampling hold circuit 6.
The horizontal synchronization signal taken out from the synchronization separation circuit 7 is also supplied to the circuit 6, which detects the level of the horizontal synchronization signal in each horizontal section and supplies the detection output to the oscillator 3. By controlling the oscillation frequency, the level of the horizontal synchronizing signal of the demodulated video signal is kept constant.

However, in this case, in order to remove the effects of jitter and drift in the transmission system, the phase-modulated video signal from the input terminal 1 is supplied to the frequency discriminator 8 as shown in the figure, and the discriminator output is supplied to the oscillator 3. It is also necessary to provide an AFC loop for controlling the oscillation frequency.

Therefore, this method has the disadvantage that the entire circuit configuration becomes complicated.

On the other hand, an APC loop without a sampling system,
For example, there is a method of configuring a PLL.

In this case, FIG. 2 shows that the phase-modulated video signal from the input terminal 11 is supplied to the phase detection circuit 12, the oscillation signal from the voltage-controlled variable frequency oscillator 13 is supplied to the circuit 12, and the phase-modulated video signal from the input terminal 11 is supplied to the circuit 12. A demodulated video signal is obtained and taken out through a low-pass filter 14, and the output of the filter 14 is supplied to an oscillator 13 through a time constant circuit 15 to control its oscillation frequency.

In this case, due to the phase detection characteristics of the PLL, the response of low frequency components in the demodulated video signal decreases, so the output of the PLL is taken out to the output end 17 through the clamp circuit 16, as shown in the figure. It is necessary to perform DC regeneration.

By the way, in PLL, the time constant τ1 in the time constant circuit 15
By reducing the value of τ1 to speed up the response, the lock range can be expanded, and the time constant τ1 can be reduced to approximately 1.5X10.
If it is about sec, for example, when the carrier frequency during phase modulation is 4 h, a lock range of approximately ±1% can be guaranteed.

When the time constant τ1 is selected to be approximately 1.5 x 10-'sec, the phase detection characteristics of the PLL, that is, the phase detection circuit 1
The ratio of the output of the filter 14 to the input of 2 is as shown by the curve 18 in FIG. 3, and its cutoff frequency 1 f (=) is approximately 10 kHz.

However, the time constant τD in the clamp circuit 16 is determined by the horizontal frequency fH and cannot be made lower than about lXl0 sec. The ratio is curve 19 in the same figure.
The cutoff frequency fD (=27c, rD)
does not exceed approximately 1.5 kHz.

Therefore, as is clear from the figure, in the reproduced output characteristic, a valley portion occurs in a predetermined frequency region centered around the frequency fM around 3.5 kHz, and the reproduced output is attenuated in this portion.

If the time constant τ is made larger than the above value and the phase detection characteristic is shifted to the lower frequency side, that is, the cutoff frequency fp is made lower than the above value, the reproduced output characteristic can be flattened. If this is done, the lock range in the PLL will become narrower, which is not preferable.

In view of this problem, the present invention is designed to flatten the reproduced output characteristics by particularly making improvements on the modulation side.An example of the method of the present invention will be explained below with reference to FIG. 4.

In the present invention, on the modulation side, pre-emphasis is applied with characteristics corresponding to the phase detection characteristics and DC reproduction characteristics on the demodulation side.

FIG. 4 shows a case in which the present invention is applied when recording a video signal on a tape and reproducing it. The video signal from the input terminal 21 is supplied to a pre-emphasis circuit 22, and its output is sent to a phase modulator 23. The modulated video signal is supplied to the recording amplifier 24 to be phase-modulated, and the modulated video signal is sent to the recording/playback switch 2 through the recording amplifier 24.
It is supplied to the head 26 through the recording side contact point R of No. 5 and recorded on the tape 27.

During reproduction, the reproduction signal from the head 26 is supplied through the reproduction side contact P of the switch 25 to the PLL 29 having the above-mentioned configuration through the reproduction amplifier 28 for demodulation, and the demodulated video signal is supplied to the clamp circuit 16 to generate a DC signal. The output is outputted to the output terminal 17 through the amplifier 30.

The pre-emphasis circuit 22 on the modulation side applies pre-emphasis to emphasize the above-mentioned predetermined frequency region determined by the phase detection characteristics and DC reproduction characteristics on the demodulation side. As shown by the broken line 20 in FIG.

In this case, at the frequency fM that should be emphasized the most, approximately 5
It may be emphasized by about dB.

As described above, according to the present invention, since pre-emphasis is applied on the modulation side to compensate for the attenuation of the output on the demodulation side, the reproduced output characteristics become flat.

Moreover, the configuration on the demodulation side is simplified, and since it does not have a sampling system with a quick response, it is resistant to noise mixed in the modulated video signal.

[Brief explanation of the drawing]

1 and 2 are system diagrams of an example of a phase demodulation circuit for explaining the present invention, FIG. 3 is a characteristic curve diagram for explaining the present invention, and FIG. 4 is a system diagram of an example of the method of the present invention. It is a diagram. 21 is a video signal input terminal, 22 is a pre-emphasis circuit, 23 is a phase modulator, and 29 is a PLL. 16 is a clamp circuit, and 17 is an output end for a reproduced signal.

Claims (1)

[Claims] 1. A video signal phase modulation method in which pre-emphasis is applied on the modulation side with characteristics corresponding to both of the above characteristics in order to compensate for output attenuation in a predetermined frequency range caused by the phase detection characteristics and DC reproduction characteristics on the demodulation side. .