JPS6129591B2 - - Google Patents

Description

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

In the PAL system, the modulation axis for one of the two color difference signals, that is, the (RY) color difference signal, is 180 pixels per horizontal scanning period (hereinafter referred to as 1H).
It is well known that the signal is sent with a phase shift of 1°. It is sent like this (R-
Y) In order to demodulate the color difference signal with the correct polarity,
A switch is used that operates at a frequency that is half the horizontal frequency (hereinafter referred to as H/2) and is switched every 1H. To drive this switch, a flip-flop that divides the horizontal pulse by two is used, Two line switching signals are generated. It is also well known to control flip-flops so that the phase of the H/2 line switching signal has an appropriate relationship to the transmission line information transmitted by the color synchronization signal.
In such well-known circuits, the
As stated in the publication, there was a drawback in that the PAL switch was permanently stopped in order to control the phase of the H/2 line switching signal.

Therefore, the present invention aims to provide a color demodulation device for a PAL color television receiver that can accurately demodulate (RY) color difference signals without controlling the phase of the H/2 line changeover switch. This is the purpose.

EMBODIMENT OF THE INVENTION Hereinafter, the present invention will be described in detail with reference to the drawings showing one embodiment thereof.

In the figure, 1 is an input terminal for a chroma signal extracted from a PAL color television signal. A carrier color signal (indicated by a broken line) and a burst signal (indicated by a solid line) whose polarity is reversed with respect to the (RY) axis every 1H are added here. This input carrier color signal is delayed by a 1H period in a 1H delay circuit 2, and is added to an adder 3 and a subtracter 4 together with the original input carrier color signal, where they are added to and subtracted from each other. As a result, the (BY) axis carrier signal is obtained from the adder 3 and is added to the (BY) demodulator 5. on the other hand,
From the subtracter 4, the polarity is reversed every 1H.
Since a carrier color signal of the Y) axis is obtained, it is applied to the (RY) demodulator 6 via a switching circuit to be described later. Such delay and addition/subtraction means are well known. 7 and 8 are output terminals for the (BY) signal and the (RY) signal. However, in this device,
Before adding the input carrier color signal and the delayed carrier color signal to the adder 3 and the subtracter 4, burst removal gates 9 and 10 are provided to remove the burst signal in advance,
Burst components are prevented from appearing in the outputs of the adder 3 and subtracter 4.

Then, the polarity of the ±(RY) axis carrier color signal output from the subtracter 4 is inverted by a phase inverter 11,
The direct carrier color signal and the inverted carrier color signal are respectively applied to the two input terminals A and B of the first switch 12, and the output carrier color signal is (RY).
It is added to the demodulator 6. This first switch is a static switch that is switched only once when necessary by a switching signal from flip-flop 13.

On the other hand, only the burst signal is extracted from the input carrier color signal applied to the input terminal 1 by the burst gate 14. This burst signal also has a phase inverter 15.
The direct burst signal and the burst signal with the reversed polarity are applied to the two input terminals A and B of the second switch 16, respectively. This second switch 16 is also switched in conjunction with the first switch 12 by a switching signal from the flip-flop 13.

At this time, the first and second switches 1
2 and 16 are set so that a burst signal of a polarity corresponding to the polarity of the carrier color signal output from the first switch 12 appears from the second switch 16. For example, when the first switch 12 outputs a carrier color signal of the +(RY) axis, a burst signal having a +(RY) axis component also appears from the second switch 16. be. By doing so, the polarity of the burst signal output from the second switch 16 can represent the polarity of the carrier color signal output from the first switch 12. These first and second switches 1
The flip-flop 13 that switches between the switches 12 and 16 is inverted only once when it is detected that the demodulation polarity is incorrect by a phase discriminator 17, which will be described later.
16 cannot be switched.

Next, a section that generates color subcarriers for demodulation will be explained. First, only the burst signal is extracted from the input chroma signal applied to the input terminal 1 by the burst gate 18 and applied to an oscillation phase control loop consisting of a phase discriminator 19, a local color subcarrier oscillator 20, and a 90° phase shifter 21. This circuit is well known, and the oscillator 20 is controlled to continuously oscillate at the average phase of the burst signal, and generates a -(BY) axis color subcarrier. Therefore, by inverting the polarity of this signal using the phase inverter 22 and adding it to the (BY) demodulator 5 as a color subcarrier of the (BY) axis, the (BY) signal can be demodulated.

On the other hand, the output of the oscillator 20 is phase-shifted by a 90° phase shifter 21 and applied to the phase discriminator 19 for oscillation phase control as a color subcarrier with a +(RY) axis phase, and the polarity is changed by a phase inverter 23. Invert -(R-
Y)-axis phase color subcarriers, and their ±(R-
The color subcarrier of the phase of the Y) axis is switched to the third switch 24.
are applied to the two input terminals a and b, respectively. This third switch 24 has a horizontal pulse input terminal 25.
H/2 from flip-flop 26, which is inverted every 1H by a horizontal pulse (horizontal sync pulse or horizontal flyback pulse) applied from
The frequency is switched every 1H by the frequency switching signal, and the color subcarrier of the phase of the + (RY) axis and the -
The color subcarrier of the phase of the (RY) axis is taken out alternately every 1H and applied to the (RY) demodulator 6.

However, unlike conventional devices, this device has a third switch 24 and a flip-flop 26.
Since the switching polarity of the third switch 24 is not controlled in any way, the polarity of the color subcarrier output from the third switch 24 differs depending on which polarity the switching signal output from the flip-flop 26 starts from, as shown in the figure. The polarity can be either a or b2. It is unspecified which one will be. However, in order to correctly demodulate the (RY) signal with the (RY) demodulator 6, the polarity of the carrier color signal applied from the first switch 12 and the color subcarrier applied from the third switch 24 must be changed. It is necessary that the polarities match correctly, but as mentioned above, if it is not known whether the polarity of the color subcarrier from the third switch 24 will be a or b2, the two polarities will not match. condition may occur. Therefore, in this device, the polarity of the color subcarrier applied from this third switch 24 to the (RY) demodulator 6 and the polarity of the burst signal obtained from the second switch 16 (the polarity of this burst signal is switch 12) to (R-
Y) which represents the polarity of the carrier color signal applied to the demodulator 6) is compared by the phase discriminator 17, and if the two polarities do not match, the phase discriminator 17 Switching pulse from 1
The polarity of the two signals is made to match by inverting the flip-flop 13 and switching the polarity of the carrier color signal to the (RY) demodulator 6 by switching the first and second switches 12 and 16 to the opposite side. In order to perform correct (RY) demodulation.

This operation will be explained in more detail. now,
As shown in the figure, +(R
-The chroma signal modulated on the Y) axis, and on the 2nd H, the chroma signal modulated on the -(R-Y) axis...
...assumed to have been input. Then, if the first switch 12 is switched to the input terminal A side, the polarity of the carrier color signal applied from the first switch 12 to the (RY) demodulator 6 will be -, +, -, +( No.
1st H, 2nd H, 3rd H, 4th H, and so on)
So, if it is switched to the input terminal B side, +,
-, +, -. In conjunction with this, the polarity of the burst signal applied from the second switch 16 to the phase discriminator 17 is also changed when the second switch 16
If it is switched to the input terminal B side, it becomes -, +, -, +, and if it is switched to the input terminal B side, it becomes +, -, +, -. On the other hand, from the third switch 24 (RY)
The polarity of the color subcarrier applied to the demodulator 6 and the phase discriminator 17 will be -, +, -, + if the third switch 24 is switched from the input terminal a side first, and from the input terminal b side. If it has been switched first, it will be +, -, +, -.

Therefore, as a first state, consider a case where the first and second switches 12 and 16 are switched to the A side, and the third switch 24 is switched from the A side first. At this time, the polarities of the carrier color signal and burst signal output from the first and second switches 12 and 16 and the polarity of the color subcarrier output from the third switch 24 are both -, +, -, +. Since both match, no switching pulse is generated from the phase discriminator 17, and a correct (RY) demodulation operation is performed with the switching state unchanged. Conversely, when the first and second switches 12 and 16 are switched to the B side, and the third switch 24 is in the second state where the B side is switched first, the polarities of both are similarly changed. Matches +, -, +, - and is correct (R-
Y) Demodulation is performed.

However, in the third state, if the third switch 24 is first switched from the B side even though the first and second switches 12 and 16 are switched to the A side, the switch remains unchanged. The polarities of the carrier color signals and burst signals output from the first and second switches 12 and 16 are -, +, -, +, but the polarities of the color subcarrier output from the third switch 24 are +, -. ，
+, -, and correct (RY) demodulation cannot be performed. Therefore, in this case, the phase discriminator 17 detects that the polarity of the burst signal from the second switch 16 is - in the first H, and the polarity of the burst signal from the third switch 24 is
detects that the polarity of the color subcarrier from is + and that the two polarities do not match, and generates one switching pulse,
The flip-flop 13 is reversed and the first and second switches 12 and 16 are switched to the B side. Then,
From the 2nd H onwards, the first and second switches 12 and 16
The polarities of the carrier color signal and the burst signal output from the switch 24 become -, +, -, which match the polarity -, +, - of the color subcarrier from the third switch 24, and henceforth are correct (R-Y ) Demodulation can be performed. After all, the polarity of the output of the first switch 12 at this time becomes -, -, +, - as shown in C. From the 2nd H onwards, since the polarities of both match, the switching pulse is no longer generated from the phase discriminator 17, and the correct (RY) demodulation operation continues as in the case of the above-mentioned second state.

Conversely, as a fourth state, even if the first and second switches 12 and 16 are switched to the B side, but the third switch 24 is switched from the A side first, both switches are switched. However, in this case as well, the phase discriminator 17
At 1H, a switching pulse is generated, the flip-flop 13 is inverted, and the first and second switches 12,
16 is switched to the A side, correct (RY) demodulation is performed from the 2nd H onward in the same manner as in the first state described above.

In this way, in this device, the polarity of the carrier color signal and the color subcarrier to the (RY) demodulator 6 can be changed without any control over the switching polarity of the third switch 24, which is switched every 1H. The first and second switches 1 only once when the polarities do not match.
2 and 16, the polarities of both can be matched, and correct (RY) demodulation operation can be easily realized.

In the above embodiment, the color subcarrier output from the third switch 24 is directly applied to the phase discriminator 17 for detecting polarity coincidence. It is also possible to add a switch to take out the color subcarrier whose polarity is inverted from that of the output of the third switch, and compare it with the polarity of the burst signal output from the second switch 16 in addition to the phase discriminator. good. however,
In that case, the phase discriminator must be arranged to generate a switching pulse when both inputs have the same polarity. If such a fourth switch is further added, it is possible to arrange the switches symmetrically during the time course of each signal, and there is an advantage that the phase relationship between each signal can be precisely maintained.

As described above, according to the present invention, a completely new PAL color television receiver can obtain a (RY) color difference signal of the correct polarity simply by generating an H/2 line switching signal whose phase is not controlled. A demodulation circuit can be realized.

[Brief explanation of the drawing]

The figure is a block diagram of a color demodulator according to an embodiment of the present invention. 1...Chroma signal input terminal, 2...1H delay circuit, 3...Adder, 4...Subtractor, 5...(B
-Y) demodulator, 6... (RY) demodulator, 7...
(B-Y) signal output terminal, 8... (R-Y) signal output terminal, 9, 10... burst removal gate, 11... phase inverter, 12... first switch, 13... Flip-flop, 14...Burst gate, 15...Phase inverter, 16...Second switch, 17...Phase discriminator, 18...Burst gate, 19...Phase discriminator, 20...Local color subcarrier Oscillator, 21...90° phase shifter, 22,2
3... Phase inverter, 24... Third switch, 2
5...Horizontal pulse input terminal, 26...Flip-flop, 27...Burst gate pulse input terminal.

Claims (1)

[Claims] 1. First and second switches each having two input terminals and switched in conjunction are provided, and the carrier color signal of the PAL color television signal is applied directly to the input terminal of the first switch, and the carrier color signal of the PAL color television signal is applied directly to the input terminal of the first switch. The carrier color signal and the carrier color signal whose polarity is inverted with respect to the (RY) axis are added, the output carrier color signal of this first switch is applied to the (RY) demodulator, and the carrier color signal of the second switch is applied to the (RY) demodulator. A burst signal corresponding to the carrier color signal applied to one and the other input terminals of the first switch is applied to one and the other input terminals, respectively, and a first switch having two input terminals is switched every horizontal scanning line. 3 switches are provided, and the color subcarrier of the (RY) axis is added to one input terminal of the third switch, and the color subcarrier of the -(RY) axis is added to the other input terminal of the third switch. The output color subcarrier of the second switch is added to the (RY) demodulator to demodulate the carrier color signal, and the output burst signal of the second switch is added to the (RY) demodulator. When a color subcarrier or a color subcarrier with its polarity inverted is added to a phase discriminator, and the polarity of the output burst signal does not match the polarity of the color subcarrier applied to the (RY) demodulator. 1. A color demodulating device, characterized in that a switching signal is generated at a time, and the first and second switches are switched by the switching signal.