JPS586357B2 - 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 is composed of a carrier color signal component ER of a red color difference signal (R-Y) and a carrier color signal component ER of a blue color difference signal (B- The carrier color signal component EB caused by Y) is a signal S+ having a phase relationship as shown in FIG. 1A, and in the remaining every other horizontal period, a signal S+ has a phase relationship as shown in FIG. 1, the signal component ER is phase inverted every horizontal period, and the burst signal B.

As shown in Fig. 2, during the horizontal period when the signal Sc becomes the signal S0, the signal B+ has a phase that is 135 degrees ahead of the (B-Y) axis, and the signal Sc becomes the signal S1. During the horizontal period, the signal B is advanced in phase by 225°.

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.

An example of this will be explained below.

In FIG. 3, a PAL color video signal is supplied to a bandpass filter 2 through an input terminal 1, and a carrier color signal S
c is taken out, and this signal So is supplied to the addition circuit 3 and the subtraction circuit 4c, and is also supplied to the delay circuit 5.
This delayed carrier color signal Sc is delayed by a horizontal period.
The signal 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, the signals S+ and S- are added from the adder circuit 3 as shown in FIG. When the angle is 45°.

4C), the carrier color signal component EB is obtained, and the subtraction circuit 4 subtracts the signals S+ and S-, so the subtraction circuit 4 outputs the carrier color signal component ER and this A signal component ER having a phase opposite to that of ER is obtained alternately every horizontal period.

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

The reference signal forming circuit 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 is squared to form the signal EB2.This signal EB2 is supplied to the subtracter circuit 13, and is also The signal components ER and -ER are supplied to a multiplier circuit 12 and squared to form a signal ER2, and this signal ER2 is supplied to a subtraction circuit 13, where a subtraction of (ER2-EB2) is performed. After that, 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 signals EB2 and ER2 from the multiplier circuit 11.12 are as follows: EB2=((BY)sin(ωct+dp) }2=
1/2(B-Y)2 {1-cos2(ωct+dp)}
ER2={±(RY)cos(ωct+dp)}2=
1/2(RY)2{cos2(ωCt+dp)+1F
Therefore, the output signal Sg of the subtraction circuit 13 is Sg-(ER
2-EB2) frequency 2ωC component = Acos2(ω(
t+dp) 82 1/2 {(RY)2+(B-Y)2).

This signal Sg is then supplied to the limiter 14 and the fourth
As shown in Figure C, after the amplitude is set to a certain level, the OR circuit 1
5 to the AND circuit 2j.

In addition, the signal component EB from the adder circuit 3 is supplied to the burst gate circuit 21 as shown by the broken line in FIG.
A burst signal Bo that is in phase with the -Y axis is extracted.

is supplied to an injection lock type oscillation circuit 22 and converted into a continuous wave signal in phase with the -(B-Y) axis, and this signal is supplied to a phase shift circuit 23 as shown in FIG. 4D. The alternating signal Sp is delayed by 90° with respect to the (BY) axis, 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
The case number when the phase distortion of is 00 (ctp=o°) is the fourth
As shown in Figure E, the signal Sd has a frequency ωC and (B
-Y) axis rises for a period of 90° from the falling edge, and as shown by the broken line in Fig. 4B, if the phase distortion of the signal component EB is 45° (dp = 45°), As shown in FIG. 4F, the signal Sd has a frequency ωC and is a signal with a 45° base sum compared to the case of FIG.
°), as shown in FIG. 4G, the signal Sd is
The signal is delayed in phase by 45° compared to the case shown in FIG. 4E.

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 having a frequency and a 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 multivibrator 2 so that the duty ratio is 5.
This signal is made into a 0% alternating signal, and 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 used as the carrier color signal components ER, -BR from the subtraction circuit 4. Synchronized, the phase is advanced or delayed by 900 every 1,000 periods, and this phase-shifted signal is sent to the demodulation circuit 6R.
is supplied as a reference signal for synchronous detection.

Therefore, there is a phase distortion d in the carrier color signal components EB, ER, and 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 a phase distortion dp, it is always relatively constant, so even if there is a phase distortion dp, a color image with the correct hue can be reproduced.

However, with this configuration, when the level of the carrier color signal SC is small, the level of the signal Sg from the subtraction circuit 13 becomes small, and the signal Sd is formed from the limiter noise from the limiter 14 and the signal Sp from the transfer circuit 23. As a result, color demodulation is performed, and at this time, the S/N of the reproduced image deteriorates.

Therefore, in order to prevent this S/N reduction, in the present invention, when the level of the carrier color signal Sc is small, the signal S
A signal Sp is used instead of d.

That is, in this example, the signal Sg from the subtraction circuit 13 is supplied to the rectifier circuit 16 and 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 17 to convert the signal Sg into a DC signal. When the level of is lower than a predetermined level, a signal of "1" is sent to the OR circuit 15.
and when it is above a predetermined level,
A signal of 0'' is supplied to the OR circuit 15.

Therefore, when the level difference between the signals Sc is large, the signal Sg
Since the level of is large and the output signal of the discrimination circuit 17 is "0", as described above, the signal Sd has a phase equal to the phase distortion ctp of the carrier color signal Sc, and a color image with the correct hue is reproduced.

When the level of the signal Sc is low, the level of the signal Sg is low, and the output signal of the discrimination circuit 1γ becomes "■". Therefore, regardless of the signal Sg from the limiter 14, the signal Sg from the OR circuit 15 is "1". '', therefore, the signal Sp is taken out from the AND circuit 24 as the signal Sd, and color demodulation is performed thereby.

Therefore, since color demodulation is not performed by the signal Sd formed from the noise from the limiter 14 and the signal Sp, a color image with a good S/N ratio is reproduced.

In this case, since the signal Sd is the signal Sp, the phase distortion dp is not corrected, and the reproduced image becomes an image with a partially shifted hue, but this part is originally a part with low color saturation. Therefore, there is no problem even if the hue is off.

Thus, according to the present invention, even if there is a phase distortion dp in the carrier color signal Sc, it is possible to correct this, and in this case, even when the level of the carrier color signal Sc is small, the reproduced color image S /N never gets worse.

In addition, as a similar correction method for phase distortion dp, limiter 1
There is a method of supplying the output of limiter 14 to a crystal oscillator to obtain a predetermined reference signal, and a method of supplying the output of limiter 14 to a flip-flop circuit to obtain a predetermined reference signal. Due to the flyhole effect, phase distortion dp, which changes quickly, cannot be corrected, and if a flip-flop circuit is used, a circuit is required to determine whether phase distortion dp is 10 or 1, and The disadvantage is that the configuration 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 quickly, can be corrected.

Further, there is no need for a circuit for determining whether the phase distortion dp is 10 or 1, and the configuration is simple and inexpensive. Furthermore, as is clear from Figure 4, the range in which phase distortion dp can be corrected is sufficiently wide, from -45° to +45° or more, and color saturation can be changed by correcting phase distortion ap. I have nothing to do.

FIG. 5 shows another example of the circuit for forming the signal Sg from the carrier color signal Sc; in this example, the filter 2
The signal Sc from the delay circuit 5 and the signal Sc from the delay circuit 5 are supplied to a multiplication circuit 18 which outputs a component of the frequency 2ωC to form a signal Sg.

That is, in this case, the signal Sc from the filter 2 and the delay circuit 5 is as follows.
+dp)=1/2Ec2(cos2θ-cos2(ωc
t0dp)), the first term in {} is a DC component whose level changes depending on the hue θ, and the second term is the signal Sg.

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

[Brief explanation of drawings]

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 limiter, and 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 (BY) 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. An alternating signal is obtained, color demodulation is performed using this alternating signal as a reference signal for synchronous detection of the carrier color signal, and when the level of the carrier color signal is below a predetermined level, the continuous wave signal is used instead of the alternating signal. A chrominance signal demodulation circuit which extracts the chrominance signal as a reference signal for the above-mentioned synchronous detection.