JPS6141289A - Color demodulation circuit of paln system - Google Patents

Abstract

PURPOSE:To attain the stable demodulation even with a PAL color video signal having an inferior S/N by performing a demodulating action synchronously with the subcarrier signal produced based only a color burst signal and according to the detection output of a detection means which detects a low level of a chroma signal at an effective part of scan line of the color video signal. CONSTITUTION:Noise limiters 30 and 31 set before 2-multiplication circuits 10 and 11 for detection of chroma signal levels. These limiters 30 and 31 cut the signals lower than a prescribed limiting level and transmit the chroma signals to the next stage in case a color video signal having an inferior S/N is supplied in a weak electric field mode, etc. In this case, the chromaticity signals are also cut at an area where the chromaticity signal levels are kept at low levels. Here the output signals of limiter circuits 12 and 13 are erased and the subcarrier signal given from a color synchronizing circuit 9 is led as it is to the output of a phase modulator 15. Thus the procedure proceeds automatically to the PALD demodulation. Then the demodulation of chromaticity signals is possible. Furthermore the color density change, etc. due to the PALD demodulation are not detected owing to a thin color screen.

Description

DETAILED DESCRIPTION OF THE INVENTION [111] The present invention relates to a PALN color demodulation circuit.

BACKGROUND ART Conventionally, there has been a color demodulation circuit of this type as shown in FIG. In the figure, the PAL color video signal has luminance/
The chromaticity separation circuit 1 separates the chromaticity (Y) signal and the chromaticity (C) signal, the luminance signal is supplied to the matrix circuit 2, and the chromaticity signal is supplied to the one-day delay circuit 3 and burst gate circuit 4, respectively. Ru. The chromaticity signal supplied to the one-day delay circuit 3 is delayed by the circuit 3, added to and subtracted from the original chromaticity signal by an adder 5 and a subtracter 6, and separated into two carrier color signals. Since the phase of the subcarrier modulated by R-Y is reversed every 1H, only the subcarrier modulated by B-Y is taken out as the output of the adder 5.

Still, only the subcarrier modulated by ±(B−Y) is extracted as the output of the subtracter 6. The two carrier color signals thus obtained are demodulated into color difference signals by demodulation circuits 7 and 8, respectively, and supplied to matrix circuit 2.

The burst gate circuit 4 extracts a color burst signal from the chromaticity signal in response to a gate pulse based on a horizontal pulse generated during the retrace period of horizontal deflection scanning, and supplies it to the color synchronization circuit 9. The color synchronization circuit 9 generates, based on the intermittent color burst signal inserted into the back porch of the horizontal blanking signal, a continuous wave subcarrier signal having a constant phase relationship with the color burst signal. It is something. For example, as shown in FIG.
An APC (automatic phase control) system that indirectly uses the J-crust signal is used. In FIG. 2, the color burst signal extracted by the burst foot circuit 4 is phase-compared with the oscillation output of a VCO (automatic voltage control) 21 in a phase comparator 20, and is output as a DC component signal proportional to the phase difference. Ru. This phase difference signal controls the oscillation frequency of the VCO 21 via a low pass filter (LPF) 22. As a result, the output of the VCO 21 is always locked to the phase of the color burst signal. A general PALD demodulation method is a method in which a demodulation operation is performed in synchronization with the subcarrier signal from the color synchronization circuit 9, that is, in synchronization with a subcarrier signal generated based only on the color burst signal.

In the PA LN 4i modulation method, the demodulation operation is performed in synchronization with the subcarrier signal obtained by further phase modulating the subcarrier signal from the color synchronization circuit 9 based on the chromaticity signal of the scanning line effective part of the color video signal. It is structured as follows.

That is, each output of the adder 5 and the subtracter 6 is
The subcarrier signal from the color synchronization circuit 9 is combined in an adder 14 via a doubler circuit 10.11 and a limiter circuit 12.13, and a phase modulator 15 modulates the phase of the subcarrier signal from the color synchronization circuit 9. In this way, a subcarrier signal having a phase generated by the chromaticity signal within the effective screen is obtained, and this subcarrier signal is supplied to the phase shift circuit 16. The subcarrier signal whose phase has been shifted by 90° in the phase shift circuit 16 is supplied to the BY demodulation circuit 7, and the subcarrier signal whose phase has been shifted by Oo or 180° is selected by the PAL switch 17 and is selectively converted to R- The signal is supplied to the Y demodulation circuit 8. The PAL switch 17 is switched and controlled by the output of a flip-flop (not shown), which is set by a flyback pulse generated during the horizontal blanking period, for example.

In the conventional PALN color demodulation circuit configured as described above, a constant reference phase of the subcarrier signal is generated using the chromaticity signal within the effective screen, so the DP (DP) of the color video 1 signal is This has the advantage that variations in color saturation (color depth) do not occur with respect to phase variations such as differential phase. In this case, when a signal with a poor S/N ratio is input, such as a VTR playback image or a color video signal obtained by demodulating an airborne transmission wave under a weak electric field, the output of the adder 74 input to the phase modulator 15 is Since the noise causes phase modulation, the desired performance cannot be obtained, and it is detected as unnecessary color noise on the reproduced image.

SUMMARY OF THE INVENTION The present invention aims to eliminate the drawbacks of the conventional ones as described above.
The object of the present invention is to provide a PALN color demodulation circuit which is capable of stably demodulating even PAL color video signals with poor S/N.

The PAL'N color demodulation circuit according to the present invention performs a demodulation operation in synchronization with a subcarrier signal based on a color burst signal extracted from a color video signal and a chromaticity signal of a scanning line effective part of the color video signal. A demodulation circuit that detects that the signal level of the chromaticity signal is small, and performs a demodulation operation in synchronization with a subcarrier signal generated only based on the color burst signal in accordance with this detection output. It is characterized by J.

Example Hereinafter, embodiments of the present invention will be described in detail based on the drawings.

FIG. 3 is a block diagram showing one embodiment of the present invention,
In the figure, parts equivalent to those in FIG. 1 are designated by the same reference numerals.

In the figure, noise limiters 30 and 31 for detecting the signal level of the chromaticity signal are installed before the doubler circuit 10111.
is provided.

This noise limiter 30.31 is suitable for S/N when the electric field is weak.
When a color video signal with poor quality is input, the signal below a predetermined limiting level is cut and the chromaticity signal is transmitted to the subsequent stage. In this case, in parts where the signal level of the chromaticity signal is low (light color), the chromaticity signal is also cut, but the output signals of the limiter circuits 12 and 13 are not present, and the phase modulator 15 The subcarrier signal from the color synchronization circuit 9 is directly derived from the output of the color synchronization circuit 9, and the demodulation automatically shifts to PΔLD demodulation, so it is possible to demodulate the chromaticity signal.Also, since the screen has a light color, it is possible to demodulate the chromaticity signal by PALo demodulation. No concentration changes, etc. are detected.

Next, the circuit operation of the present invention will be explained with reference to the waveform diagram of FIG. 4.

When the electric field is weak, etc., the noise signal (B
) is superimposed on the signal like (C) when the noise limiter 3
Entered as 0 or 31. This input signal (C) is set at a limiting level (+
V, -V) and below are cut, resulting in a waveform like (D).

This signal (D) is doubled by a doubling circuit 10.11, and noise on a large amplitude portion is cut off by a limiter circuit 12.13, thereby becoming a pulse signal (E).

Pulse signal (E
) are added by the adder 14 and supplied to the phase modulator 15 as a phase modulation signal.

Here, as is clear from FIG. 4, when a color burst signal with poor S/N is input, the phase modulation signal (E) is not generated, so the chromaticity signal is Phase modulation based on the color burst signal is not performed, and the demodulation is automatically shifted to synchronized demodulation with the subcarrier signal generated based only on the color burst signal, that is, PA Lo iflml. Note that during PALo demodulation, since the color density is low, fluctuations in color saturation due to jitter are difficult to detect and do not pose a particular problem. During input with good S/N, a phase modulation signal is always input to the phase modulator 15, and phase modulation based on the chromaticity signal is performed on the subcarrier signal from the color synchronization circuit 9, so that the color burst signal and color PALN demodulation is performed in synchronization with the subcarrier signal generated based on the frequency signal, that is, PALN demodulation.

Note that the noise limiter 30.31 may be inserted after the doubler circuit 10.11. Further, the noise limiters 30 and 31 may be other noise reduction circuits, etc.
For example, in cases where the noise frequency is limited, it is also possible to use a filter circuit.

FIG. 5 is a block diagram showing another embodiment of the present invention, in which parts equivalent to those in FIG. 1 are designated by the same reference numerals. In this embodiment, level detection circuits 50 and 51 generate a detection output when the signal level of the chromaticity signal is above a certain level, and turn on (closed) state in response to this detection output. A switch 52 is provided.

In this configuration, when a chromaticity signal with a certain color density or less is input, the phase modulation signal to the phase modulator 15 is cut by the switch 52, so that PALo demodulation is not performed as in the above embodiment. It will be automatically migrated.

Note that one switch 52 may be provided between the input of the doubler circuit 10.11 and the output of the limiter circuit 12.13, or may be provided so as to bypass the input and output terminals of the phase shifter aS. good.

L! L standing, LL As explained above, according to the present invention, it is possible to arbitrarily select between PA LN demodulation and PALo demodulation, and PALo demodulation is used for signals with poor S/N in weak electric fields. With this configuration, color noise will not be detected on the reproduced image, and a PALN color demodulation circuit that can stably demodulate even a PAL color video signal with a poor S/N ratio can be obtained.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a conventional example, FIG. 2 is a block diagram showing the configuration of the color synchronization circuit in FIG. 1, FIG. 3 is a block diagram showing an embodiment of the present invention, and FIG. FIG. 5 is a waveform diagram for explaining the operation of the circuit shown in FIG. 5, and FIG. 5 is a block diagram showing another embodiment of the present invention. Explanation of symbols of main parts゛ 1... Luminance/chromaticity separation circuit 3...
1H delay circuit 4...Burst gate circuit 7.8...Demodulation circuit 9...Color synchronization circuit 10.11...2 multiplier circuit 14... ... Adder 15 ... Phase modulator 30.31 ... Noise limiter 50.5
1...Level detection circuit 52...Switch circuit

Claims (1)

[Claims] A PAL_N color demodulation circuit performs a demodulation operation in synchronization with a subcarrier signal based on a color burst signal extracted from a PAL color video signal and a chromaticity signal of a scanning line effective portion of the color video signal, the circuit comprising: and detecting means for detecting that the signal level of the color burst signal is small, and performing a demodulation operation in synchronization with a subcarrier signal generated only based on the color burst signal in accordance with the detection output of the detection means. Features PAL_N color demodulation circuit.