JPS5935554B2 - color demodulation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a color demodulation circuit for 20 PAL color television receivers.

In the PAL system, one of the two color difference signals, for example, the modulation axis for the RY color difference signal, is set for one horizontal scanning period (
It is well known that the signal is sent with 1800 phase changes every time (hereinafter referred to as IH). In order to demodulate, for example, the R-Y color difference signal transmitted in this way with the correct polarity, a switch that operates at half the horizontal frequency (hereinafter referred to as fH) is used, and in order to drive this A multivibrator that divides the horizontal pulse frequency by 2 is used, fH/2
It is well known to generate 30 line switching signals. It is also well known to control the multivibrator so that the phase of the line switching signal of f/2 is in proper relationship with the transmission line information transmitted by the color synchronization signal. In such a well-known circuit, 35
As described in the No. 566 publication, there was a drawback that the PAL switch was permanently stopped in order to control the phase of the fH/2 line switching signal. The present invention aims to provide a color demodulation circuit for a color television receiver that demodulates an RY color difference signal of correct polarity without controlling the phase of the FH/2 line switching signal.

Hereinafter, the present invention will be explained in detail with reference to the drawings. FIG. 1 shows a block diagram showing one implementation example of the present invention. A carrier color signal (simple PAL
It is well known that this method is not processed in the first method).
is applied to one input terminal of a switch circuit 2, and a carrier color signal with inverted polarity is applied to the other input terminal through an 1800 phase shifter 1.

The first switch circuit 2 is controlled by a control signal from a control circuit 3, and supplies one of two inputs to its output terminal. Its output signal is applied to the R-Y demodulator 5. On the other hand, one input terminal of the second switch circuit 7 having two input terminals receives the R-Y reference subcarrier, that is,
If the PAL alternating burst average phase is 180, then 9
A subcarrier with a zero-cross phase is added. A subcarrier whose polarity is inverted, ie, has a phase of -90 phase, is applied to the other input terminal through a 180 phase shifter 8. Second
The switch circuit 7 generates a reference subcarrier for R-Y with a phase difference of 1806 every 1H at its output, which is controlled by a multivibrator 6 which divides the frequency of the horizontal pulse by 2 and generates a line switching signal of FH/2. do. However, the phase that changes for each line is not synchronized with the transmission line information transmitted by the alternating burst signal. In order for the R-Y demodulator 5 that demodulates the output of the first switch circuit 2 to supply the R-Y color difference signal of the correct polarity by using the R-Y reference subcarrier of the second switch circuit 7, ,
The control circuit 3 receives the burst signal output from the first switch circuit 2 from the burst sampling circuit 4 and the second switch circuit 7.
The first switch circuit 2 is controlled by comparing the phase with the output of the switch. The control circuit 3 will be explained in more detail. FIG. 2 shows an implementation example of the control circuit 3. The phase discrimination circuit 11 inverts the polarity of the alternating burst signal from the burst extraction circuit 4 every 1H from the second switch circuit 7.
- Perform synchronous detection using the Y reference subcarrier. When the phase of the alternating burst signal is +1359 line, the phase of the R-Y reference subcarrier is -900 in the 1H period of the line +900-135 (this case is considered to be the correct phase relationship with the transmission line information). The burst detection output of the phase discrimination circuit 11 has positive polarity, and conversely, the RY reference subcarrier phase has a phase of -900 during the 1H period on the line with a burst phase of +135 bars, and the phase of +90 waves during the 1H period on the line 135. In this case (assuming that the phase relationship is incorrect), the burst detection output has negative polarity. The DC component detection circuit 12 is a circuit that detects the DC component of the burst detection output realized by a low-pass filter or a peak holding circuit. If the phase relationship is as follows, a DC signal that is more negative than the reference level is obtained. In order to control the state of the first switch circuit 2 using this information, a trigger circuit 13 shown in FIG.
and a flip-flop circuit 14. The trigger circuit 13 generates a trigger signal that changes the state of the flip-flop circuit 14 only when the output of the DC component detection circuit 12 is negative, and when it is supplied to the flip-flop circuit 14 is positive, the state of the flip-flop O'7 circuit 14 does not change. . By controlling the first switch circuit 2 with the output of the flip-flop circuit 14, a correct phase relationship can always be obtained. This trigger circuit 13 is a so-called OR circuit that applies a horizontal pulse, vertical pulse, or FH/2 line switching signal to the flip-flop circuit 14 as a trigger signal only when a signal indicating a negative output from the DC component detection circuit 12 is generated. It can be realized. Further, since the output of the DC component detection circuit 12 corresponds to the level of the burst signal, the well-known automatic color gain control signal (AC
C signal). Furthermore, by activating a so-called Kira-ichi operation that stops the operation of the color demodulation circuit when the output of the DC component detection circuit 12 is below a certain level positive than the reference level, when the burst signal level becomes small, the first When the state of the switch circuit 2 is not appropriate and the polarity of the R-Y color difference signal output is incorrect, the Kiraichi circuit will also operate, causing inappropriate color reproduction on the screen of a receiver equipped with this circuit. This has the advantage of preventing this. As described above, according to the present invention, a completely new PAL color television receiver is provided, in which an R-Y color difference signal of correct polarity can be obtained simply by generating an FH/2 line switching signal whose phase is not controlled. A color demodulation circuit can be realized.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a color demodulation circuit according to an embodiment of the present invention, and FIG. 2 is a block diagram showing an implementation example of a control circuit that can be used in the color demodulation circuit of the present invention. 2...First switch circuit, 3...Control circuit, 5...RY demodulator, 7...
2nd switch circuit, 4... Burst extraction circuit, 11... Phase discrimination circuit, 12... DC component detection circuit, 13... Trigger circuit, 14...
...Flip-flop circuit.

Claims (1)

[Claims] 1. A carrier color signal of a PAL color television signal is directly applied to one end of a first switch circuit having two input ends, and a carrier color signal having a phase 180° different from the carrier color signal is applied to the other end. A chrominance signal is applied, and one of the carrier chrominance signals selected by the control signal from the control circuit is applied to the R-Y demodulator, and the polarity is reversed every horizontal scanning period from the second switch circuit. The burst signal of the first switch circuit output detected by the reference subcarrier detected by the burst sampling circuit and the phase of the reference subcarrier from the second switch circuit are compared to determine whether the burst signal is correct. Polar R
- A color demodulation circuit, wherein the control circuit controls the first switch circuit to obtain a Y color difference signal. 2. A phase discrimination circuit in which the control circuit synchronously detects the burst signal using the reference subcarrier from the second switch circuit; a DC component detection circuit that detects the DC component of the burst detection output of the phase discrimination circuit; The color demodulation circuit according to claim 1, characterized in that it is constituted by a trigger circuit controlled by the output of the detection circuit and a flip-flop circuit triggered by the trigger circuit. . 3. The color demodulation circuit according to claim 2, wherein the output of the DC component detection circuit is used as an automatic color gain control signal and a color killer signal. 4. The color demodulation circuit according to claim 2, wherein the trigger circuit is constituted by an OR circuit of a DC component detection circuit output and a horizontal pulse. 5. The color demodulation circuit according to claim 2, wherein the trigger circuit is constituted by an OR circuit of a DC component detection circuit output and a vertical pulse. 6 The trigger circuit is the DC component detection circuit output and the horizontal frequency.
3. The color demodulation circuit according to claim 2, wherein the color demodulation circuit is constituted by an OR circuit with a line switching signal having a frequency of 1/1.