JPS586356B2 - Color signal demodulation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION In every other horizontal period, the carrier color signal Sc in the PAL color television system has a carrier color signal component E8 of a red color difference signal (RY) and a blue color difference signal (B- The carrier color signal component EB caused by Y) is a signal S0 having a phase relationship as shown in FIG. 1A, and in every other remaining horizontal period, a signal having a phase relationship as shown in FIG. 1B. S-, and the signal component ER is phase-inverted every horizontal period.

Then, the burst signal Bo is converted into a signal S as shown in FIG.
In the horizontal period when c becomes the signal S0, the signal B0 has a phase advanced by 135 degrees with respect to the (B-Y) axis, and in the horizontal period when the signal Sc becomes the signal S1, it has a phase advanced by 225 degrees. signal B
It is one.

Even when such a carrier color signal is subjected to phase distortion by a signal transmission system, the present invention can correct the phase distortion and perform synchronous detection or frequency conversion of the carrier color signal, and The present invention aims to provide a reference signal forming circuit that solves the problems caused by correction of the phase distortion.

Let's explain one example below.

In FIG. 3, the PAL color video signal is input to input terminal 1.
The carrier color signal Sc is supplied to the band pass filter 2 through the filter 2 and taken out, and this signal Sc is supplied to the addition circuit 3 and the subtraction circuit 4, and is also supplied to the delay circuit 5.
The delayed carrier color signal Sc is delayed by half a period and is supplied to an addition circuit 3 and a subtraction circuit 4.

Therefore, if the phase of the (B-Y) axis is shown as shown in Figure 4A (in Figure 4, in order to clarify the phase relationship of the signals, even if it is a sine wave, the waveform is expressed as a rectangular wave. ), since the signals S+ and S- are added in the adder circuit 3,
From the adder circuit 3, as shown in FIG. 4B (the solid line is when the phase distortion is OO, the broken line is when the phase distortion is 45°).

4C), the carrier color signal component EB is obtained, and since the subtraction circuit 4 subtracts the signals S1 and S1, the carrier color signal component ER is obtained from the subtraction circuit 4. A signal component -ER having an opposite phase to this is obtained alternately every construction horizon period.

These signal components EB, ER, -ER are supplied to the demodulation circuits 6B, 6R, and a reference signal for synchronous detection is supplied from the reference signal forming circuit described below to the demodulation circuits 6B, 6R.
The signal components EB, ER, -ER are synchronously detected, and the demodulated color difference signals (B-Y) are sent to terminals 7B and 7H.
, (RY) are extracted.

A circuit for forming a reference signal for synchronous detection is configured as follows.

That is, the signal component EB from the adder circuit 3 is supplied to the multiplier circuit, in this example, the multiplier circuit 11, and squared to form the signal E2B. The signal components ER and -ER of are supplied to the multiplication circuit 12 and squared to form the signal E21, and this signal E2 is supplied to the subtraction circuit 13, where the subtraction of (E2R-E2B) is performed. 4
A signal Sg having a frequency component twice the carrier frequency is extracted.

Therefore, EB=(B-Y)sin(ωct+dp)±E
Since R=±(RY)cos(ωct+dp),
The signal E2ByE2B from the multiplier circuit 11.12 is E”B={(BY)sin(ωct+dp)}2=1
/2(B-Y)2(1-cos2(ωct+dp))B
2R=(±(RY)cos(mct+dp)}2=1
/2H(RY)2{cos2(ωct+dp)+1)
Therefore, the output signal Sg of the subtraction circuit 13 is Sg=(E2
B-E2B) frequency 2ωc component = Acos2(ωc
t+dp) A=1/2((RY)2+(B-)Y21.

This signal Sg is then supplied to the limiter 15 through the clip circuit 14, set to a constant amplitude as shown in FIG. 4c, and then supplied to the AND circuit 24.

Further, the signal component EB from the adder circuit 3 is supplied to the burst gate circuit 21, and as shown by the broken line in FIG.
A burst signal Bo that is in phase with the -Y) axis is extracted, and this burst signal Bo is supplied to an injection lock type oscillation circuit 22 to be converted into a continuous wave signal that is in phase with the -(B-Y) axis. The signal is supplied to the phase shift circuit 236 and converted into an alternating signal Sp delayed by 900 with respect to the (BY) axis as shown in FIG. 4D, and this signal Sp is supplied to the AND circuit 24.

Therefore, from the AND circuit 24, a signal Sd as shown in FIG. 4 E-G is obtained as an AND output of the signals Sg and Sp.

That is, as shown by the solid line in FIG. 4B, the signal component EB
When the phase distortion of is 00 (dp=00), Fig. 4E
As shown in , the signal Sd has a frequency ωc and (B-Y
) The signal rises for a period of 90 degrees from the fall of the axis, and as shown by the broken line in Figure 4B, the signal component E
When the phase distortion of B is 45° (dp = 45°), as shown in Figure 4F, the signal Sd has a frequency of ωC and is a signal that is 45° more advanced than in the case of Figure 4E. Furthermore, the phase distortion of signal component EB is -45° (dp=-45°)
In this case, as shown in FIG. 4G, the signal Sd is
The signal is delayed by 45° compared to the case in Figure E.

That is, the frequency of the signal Sd is equal to the carrier frequency ωC of the carrier color signal component EB, and the phase distortion is also equal to dp.

In this way, a signal Sd with a frequency and phase equal to the carrier frequency ωC and phase distortion dp of the carrier color signal component EB is taken out from the AND circuit 24, and this signal Sd is supplied to the monostable multi-by-break 25 so that the duty ratio is 50. % alternating signal, and this signal is supplied to the demodulation circuit 6B as a reference signal for its synchronous detection, and is also supplied to the variable phase circuit 26 to be synchronized with the carrier color signal components ER, -ER from the subtraction circuit 4. The phase is advanced or delayed by 900 in one horizontal period, and this phase-shifted signal is sent to the demodulation circuit 6.
It is supplied to R as a reference signal for its synchronous detection.

Therefore, there is a phase distortion a in the carrier color signal components EB, ER, -ER.
Even if p occurs, the phase of the reference signal for synchronous detection changes in the same way, so the reference phase of signal components EB, ER, and ER and the phase of the reference signal for synchronous detection are different from each other due to phase distortion. Even if there is ap, it is relatively always constant, so even if there is a phase distortion dp, a color image with the correct hue can be reproduced.

Similar correction methods include methods using a crystal oscillator and methods using a flip-flop circuit, but when using a crystal oscillator, the rapidly changing phase distortion ap cannot be corrected due to its flywheel effect. If a flip-flop circuit is used, a circuit for determining whether the phase distortion dp is 1 or 1 is required, and the structure thereof is complicated.

However, according to the present invention, since no element having a response delay is included in the signal path of the signal Sc→Sg→Sd, the response speed is fast and the phase distortion dp, which changes rapidly, can be corrected.

Further, there is no need for a circuit for determining whether the phase distortion dp is 10 or 1, making the configuration simple and inexpensive.

Furthermore, since the clip circuit 14 is provided, a reproduced image with a good S/N ratio can be obtained.

That is, if there is no clip circuit 14, phase distortion dp is corrected even when the level of the carrier color signal Sc is low (color saturation is low); If phase distortion is corrected when it is small, the reproduced image will have a poor S/N ratio.

However, in the present invention, when the level of the carrier color signal Sc is low, the level of the signal Sg from the subtraction circuit 13 is low, and therefore, by setting the clip level of the clip circuit 14, in this case, the clip circuit 1
4, the signal Sg is no longer obtained, so the signal Sd is also not obtained, and the signals (B-Y) and (R-Y) are no longer demodulated.

Therefore, a color image with a poor S/N ratio will not be reproduced.

In this case, since Sd is not obtained, the reproduced image becomes partially black and white, but this black and white portion is originally a portion with low color saturation, so there is no problem.

Furthermore, as is clear from Fig. 4, the range in which the phase distortion dp can be corrected is -45° to +45° or more.
It is sufficiently wide and does not change the color saturation by correcting the phase distortion dp.

FIG. 5 shows another example of a circuit for forming a signal Sg from a carrier color signal Sc and a circuit for cutting off a signal Sd when the level of the signal Sc is small. The signal Sc from the delay circuit 5 and the signal Sc from the delay circuit 5 are connected to a multiplier circuit 16 which outputs a component of frequency 2ωC.
is supplied to form a signal Sg.

That is, in this case, the signal Sc from the filter 2 and the delay circuit 5 is Sc = Ecsin (ωct + θ + dp) Sc = ECSin (ωCi - θ + dp), so in the multiplier circuit 16, E2oSin (ωct + θ + dp) sin (ωct - θ
−dp) = 1/2E2C{cos2θ−cos2(ωct+dp
) }, the first term in {} is a DC component whose level changes depending on the hue θ, and the second term is the signal Sg.

Then, this signal Sg is supplied to the AND circuit 24 through the limiter 15, and is also supplied to the rectifier circuit 17 to be converted into a DC signal with a level corresponding to the level of the signal Sg, and this DC signal is supplied to the level discrimination circuit 18. , the discrimination output is supplied to the AND circuit 24 through the inverter 19, and when the level of the signal Sg is low, the inverter 1
9 becomes "0", the output signal Sd of the AND circuit 24 is cut off.

Note that when a continuous wave signal synchronized with the average phase of the burst signal BO is formed by a PLL or APC circuit instead of the oscillation circuit 22, the phase of the continuous wave signal is 90 degrees higher than the average phase of the burst signal BO. Since the phase is delayed by .degree., the phase shift circuit 23 is not necessary in this case.

Furthermore, in FIG. 3, the positions of circuits 14 and 15 can be interchanged.

[Brief explanation of the drawing]

1 and 2 are vector diagrams for explaining PAL color video signals, FIG. 3 is a system diagram of an example of the present invention, and FIG.
The figure is a waveform diagram for explaining the same, and FIG. 5 is a system diagram showing a part of another example of the present invention. 5 is a delay circuit, 14 is a clip circuit, 15 is a limiter,
22 is an oscillation circuit.

Claims (1)

[Claims] From the carrier color signal of the I PAL system, a signal with a frequency and phase twice the carrier frequency and phase distortion is obtained, and from the burst signal, a signal with a frequency equal to this burst signal and a phase with respect to the (B7Y) axis is obtained. Obtain continuous wave signals that differ by 90 degrees, and supply this continuous wave signal and a signal with twice the frequency and phase above to an AND circuit to obtain a signal with a frequency and phase equal to the carrier frequency and phase distortion of the carrier color signal. Obtain an alternating signal, perform color demodulation using this alternating signal as a reference signal for synchronous detection of the carrier color signal, and cut off the alternating signal when it is detected that the level of the carrier color signal is below a predetermined level. A color signal demodulation circuit designed to do this.