JPS603260B2 - color demodulation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a color demodulation circuit for PAL color television signals.

More specifically, the multipipeter that drives the PAL switch stably generates a switching signal at half the line frequency (referred to as fH/2) that has the correct phase with the transmit signal. This relates to a demodulation circuit. In a PAL color television signal receiver, a PAL switch that operates at fH/2 is used, and to drive this, a multipipulator that divides the horizontal oscillation pulse by two is used, and the fH/2 switching signal is It is well known that this occurs.

It is also well known to control the multipipulator so that the phase of the fH/2 switching signal has an appropriate relationship with the transmission line information transmitted by the color synchronization signal. As this control means, when the fH/2 switching signal is in an incorrect phase relationship, a DC string pressure is generated that temporarily stops the operation of the multipipelator, and the multipipelator is returned to the proper phase. is commonly used.
However, if an error occurs in the phase relationship between the reference signal from the local subcarrier oscillator in the receiver and the burst signal, the operation of the multipipulator will be permanently stopped and the correct operation of the PAL switch will be interfered with. There was a drawback. The present invention aims to provide a color demodulation circuit for color television signals which eliminates such drawbacks. A feature of the present invention is that a demodulator that demodulates a color signal whose magnetism is alternately inverted every horizontal period in a PAL color television signal is provided with a subcarrier signal of correct polarity in synchronization with the polarity of the color signal. A multipipulator is provided for controlling switching of the changeover switch, and a carrier color information amplifier for supplying a color signal to the demodulator. The circuit is configured to generate a line identification signal using this signal and control the multipipelator, and when the line identification circuit generates an error indicating that the over-reading changeover switch is in an incorrect state, the operation of the multipipelator is controlled. The purpose is to stop the carrier color signal amplifier, and then reduce the amplification degree or stop the operation of the carrier color signal amplifier.

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 shows an embodiment of the present invention. Carrier color signal amplifier 1
A portion of the carrier color signal amplified in is sent to a burst extraction circuit 5, where only that burst signal is extracted. FIG. 2 shows an example of waveforms of various parts of the embodiment of the present invention shown in FIG. 1, and FIG. 3 shows a vector diagram showing the phase relationship of signals. The burst signal output from the burst sampling circuit 5 shown in FIG. 2 is synchronously detected by the detector 7 using the reference subcarrier from the local subcarrier oscillator 8. As is well known, the phase of this burst signal is 1
This is an alternating burst whose phase changes between 1350° and -135° every horizontal period IH, and the phases are shown in FIGS. 56 and 57 in FIG. Further, assuming that the phase of the reference subcarrier from the local subcarrier oscillator 8 is as shown in FIG. 358, the output of the detector 7 becomes a burst detection output as shown in FIG. 252. That is, I
Positive and negative outputs are obtained for each H. Unidirectional circuit 6
Now, the polarity of the output of the detector 7 is made unidirectional using the switching signal output of the frequency of fH/2 from the multipipulator 2. For example, the output from multipipulator 2 is shown in Figure 2.
In the case shown by the solid line 3, the output of the one-sided circuit 6 is the second
As shown by the solid line in FIG. 54, the output is of negative polarity. on the other hand,
When the fH/2 switching signal output from the multipipulator 2 has a phase relationship with the burst detection output as shown by the broken line in FIG. 2, the output from the unidirectional circuit 6 is as shown in FIG.
As shown by the broken line 1, the output is of positive polarity. Now, the fH/2 switching signal output of multipipulator 2 is shown in Fig. 253.
If the phase relationship shown by the solid line is correct, then when the output shown by the broken line in FIG. In order to obtain the identification signal, the line identification signal generation circuit 4, which takes the output of the unidirectional circuit 6 as its input, generates a DC reset signal only when the input is a positive polarity signal, and stops the operation of the multipipelator 2. . The line identification signal generating circuit 4 can be realized, for example, by a smoothing circuit and a threshold circuit, and outputs a reset pulse when the DC component of the input is positive. When the operation of the multipipulator 2 stops, the output of the unidirectional circuit 6 becomes the output shown by the solid line in FIG. The occurrence was stopped and Multiplayer 2 was restarted. However, at this time, if the phase of the reference subcarrier from the local subcarrier oscillator 8 is shifted from 900 as shown by the broken line 59 in FIG. The output is as shown by the broken line 62 in Figure 2, and its DC component becomes more positive than the base level without the burst signal, and the line identification signal generating circuit 4 continues to generate the reset signal, and the multipipelator remains stopped forever. Put it away. Therefore, in the present invention, the output of the line identification signal generation circuit 4 is added to the control circuit 3 of the carrier chrominance signal amplifier 1. 1
Decrease or stop the operation. This eliminates the burst output from the burst extraction circuit 5. As a result, the DC component of the output of the unidirectional circuit 6 disappears, and the line identification signal generating circuit 4 stops generating the reset signal. As described above, it is an object of the present invention to provide a color demodulation circuit for a color television signal that has excellent features that do not have the drawback that the multipiperator stops permanently even if an error occurs in the phase of a reference subcarrier. It is something that can be done.

[Brief explanation of drawings]

FIG. 1 is a diagram of a color demodulation circuit in one embodiment of the present invention. Fig. 2 is a waveform example to explain the device, Fig. 3 is a waveform example to explain the device.
The figure is a vector diagram showing the phase relationship of signals according to the present invention.
1...Carrier color signal amplifier, 2...Multi/
Control circuit, 4...
- Line identification signal generation circuit, 5...burst sampling circuit, 6...unidirectional circuit, 7...detection circuit, 8...local subcarrier oscillator. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1. A demodulator that demodulates a color signal whose polarity is alternately inverted every horizontal period in a PAL color television signal is provided with a changeover switch that synchronizes with the polarity of the color signal and provides a subcarrier signal of the correct polarity. , a multivibrator that controls switching of the changeover switch, and a carrier color signal amplifier that supplies a color signal to the demodulator, and a phase difference between the PAL alternating burst signal being output from the carrier color signal amplifier and the output of the multivibrator. a line identification signal generating circuit is provided which generates a line identification signal indicating that the multivibrator output is in an incorrect phase relationship with the PAL alternating burst signal and the changeover switch is in an incorrect state by comparing the A control circuit is provided for stopping the operation of the multivibrator when the identification signal is larger than a certain predetermined threshold value and thereafter reducing the amplification degree of the carrier color signal amplifier, and after stopping the operation of the multivibrator, A color demodulation circuit characterized in that the suspension of operation of the multivibrator is released by lowering the output amplitude of the PAL alternating burst signal of the carrier color signal amplifier and lowering the output amplitude of the line identification signal below the threshold value.