JPS61198991A - Secam decoder - Google Patents

Abstract

PURPOSE:To stabilize a clamping action without intricating the circuitry by providing a gain control amplifier in front of a clamp circuit and lowering the gain of the said amplifier for a period equal to the horizontal blanking period. CONSTITUTION:A composite video signal of SECAM system is separated into a brightness signal Y and an FM chrominance signal CFM by a Y/C separator 12. The signal CFM and the same signal delayed by one horizontal scanning period by a delay circuit 13 are applied to a switching circuit 14, from one terminal of circuit 14, an FM chrominance component difference signal CR-Y of each line is continuously obtained, and from another terminal, the same signal of the blue is also obtained. CR-Y undergoes a decoder 20 and is supplied to a low-pass-filter 21, and a decoded chrominance-component difference signal (R-Y) is obtained. The said signal (R-Y)P is supplied to a gain control amplifier 22 where its gain is throttled for the determined period above-mentioned. As a result, a horizontal blanking pulse H.BLK is supplied to a mono-multi 17, so that the AC gain of the periodic amplifier 22 is lowered than that in normal operating time for a period slightly shorter than the horizontal blanking period TB.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a decoder suitable for use with SECAM video signals, and particularly to improvements in its clamp circuit system.

[Summary of the invention]

The present invention relates to a decoder suitable for use with SECAM video signals, and in particular to improvements in its clamp circuit system.
Before clamping the FM demodulated color signal, the noise components contained in this color signal are sufficiently suppressed, and then the above-mentioned clamping operation is executed, thereby preventing fluctuations in the clamp level due to noise components. This achieves stable lamp operation.

[Conventional technology]

As is well known, the video signal of the SECAM system is one in which color difference signals are alternately inserted for each horizontal line, and the signal relationship particularly during the horizontal blanking period is shown in FIG. 3.

FIG. 3A shows the SECAM synchronization signal portion, where Y is a luminance signal, CFM is an FM color signal, and P is a horizontal synchronization pulse. An unmodulated subcarrier CB is inserted at the level between the horizontal synchronizing pulse PH and the luminance signal Y, and this subcarrier CB is selected to have a different frequency for each horizontal line.

This SECAM synchronization signal and horizontal planking pulse H-
The relationship with BLK is as shown in A and B in the same figure, and the relationship with FM color telegraph CFM included in the video signal is as shown in A to C in the same figure. Therefore, the unmodulated subcarrier CB portion is also FM modulated.

Now, in order to decode such a video signal, a decoder 10 as shown in FIG. 4 is used.

Terminal 1 is connected to a Y/C separated FM color telegraph CFM (see Figure 5A. In this example, of a pair of color signals, the red FM color difference (
i CR-Y) is supplied to the demodulator 2.
The color difference signal (RY) p shown in FIG. 5B is demodulated by being supplied to the low-pass filter 3 through the . Since this color difference signal (RY)P is pre-emphasized, the high frequency components are emphasized as shown in the figure.

This color difference signal (R-Y) P is supplied to the clamp circuit 4 so that the level of the non-modulated subcarrier section T+ (FIG. 5) is changed to the clamp pulse (pulse width of about 0.2 μsec) Pc (pulse width of about 0.2 μsec) (C ) and then supplied to the blanking circuit 5 for a horizontal blanking period T.
Noise S generated during B.

is removed. Noise SN occurs because the horizontal blanking section TB, which is not FM modulated, is also demodulated.

The fifth diagram shows the horizontal blanking pulse +1.BLK.

The color difference signal (R-Y) P from which noise has been removed is subjected to frequency correction in the de-emphasis circuit 6, and then supplied to the amplifier 7, where the gain is adjusted and output to the output terminal 8.
The well-known color difference signal RY shown in FIG. 5E is obtained.

[Problem that the invention seeks to solve]

By the way, as shown in FIG. 5, if the section in which the non-modulated subcarrier c8 is inserted in the horizontal blanking period TB is T1, and the other sections are T2, then the section T1
Since the subcarrier CB exists, noise does not occur from this interval T1.

However, since there is no signal in the other section T2, noise SN as shown occurs in this section T2.

As is well known, this noise SN has a relatively high level, and since the noise level is not constant from line to line, the demodulation level of the non-modulated subcarrier section T1 also varies greatly due to this noise SN.

The clamp operation is performed on this non-modulated subcarrier section T.

Therefore, the level of clamp point a is undefined for each line. For this reason, the clamp level is not constant and the clamp operation cannot be stabilized.

When the clamp level fluctuates, the levels of the demodulated red and blue color difference signals fluctuate up and down.

In addition, since the phase of the unmodulated subcarrier CB differs by 180' between the first, second and third lines, the demodulation level in section T1 is also affected by the sequence of this unmodulated subcarrier. .

In order to eliminate such drawbacks, a signal selected to have the same frequency as the unmodulated subcarrier is inserted into the interval T2 to suppress the noise SN generated in this interval T2. In this case, since the frequency of the non-modulated subcarrier differs from line to line, it is necessary to prepare two oscillators, which has the disadvantage of complicating the circuit configuration.

Therefore, the present invention solves these conventional problems and proposes a SECAM decoder that can stabilize the clamp operation without complicating the circuit configuration.

[Means for solving problems]

In order to solve the above-mentioned problems, in the present invention, as shown in FIG. The gain is controlled to be sufficiently small during a period TG (FIG. 2C) that is approximately equal to the blanking period T8.

[Effect]

According to this configuration, since the AC gain of the gain control amplifier 22.32 is sufficiently lowered during the period TG than in normal use, the noise level mixed into the period TC can be sufficiently suppressed (see Fig. 2D). ), the fluctuations in the demodulation level in the section T can be effectively reduced. As a result, the clamp level at the clamp point a is stabilized, and the clamp operation can be made even more accurate.

〔Example〕

FIG. 1 is a system diagram showing an example of a 5ECA decoder 10 according to the present invention.

A SECAM type composite video signal is supplied to the terminal 11, and this is separated into a luminance signal Y and an FM color telegraph CFM by a Y/C separator 12, and the separated FM color telegraph CFM and this are transferred to an IH signal by a delay circuit 13. (H is one horizontal scanning period) The delayed FM color telegraph CFM is supplied to the switching circuit 14, and a red FM color difference signal CR-Y is continuously obtained for each line from one of its outputs, and a blue FM color difference signal CR-Y is obtained from the other output. Similarly, the FM color difference signal CB-Y is obtained for each line.

Since the red and blue channels are configured identically,
To explain the configuration and operation starting from the red channel, the red FM color difference signal CR-Y (FIG. 2A) is sent to the demodulator 20.
By being supplied to the low-pass filter 21 through
The color difference signal (RY) p shown in FIG. 2B is demodulated and supplied to the gain control amplifier 22, where its gain is narrowed down only for a predetermined period T2.

Therefore, a pulse forming circuit 16 that is synchronously controlled by the horizontal synchronizing pulse PH is provided, and the horizontal blanking pulse BLK (FIG. 2 F) obtained from this circuit is
7, in this example the horizontal blanking period T
A gain control pulse PG having a pulse width slightly shorter than B is formed and supplied to the gain control amplifier 22. As a result, only during the period Tc, the AC gain is sufficiently lowered than during normal operation. In this example, the AC gain is lowered by about 20 dB.

As a result of this gain control operation, only the noise SN generated in the section T2 is outputted from the gain control amplifier 22 with its level sufficiently attenuated, as shown in the second diagram. If the noise level is sufficiently suppressed in this way, the demodulation level will not be affected by this noise level, so the demodulation level in section T2 will be approximately constant for each line.

Although the phase of the unmodulated subcarrier changes for each predetermined horizontal line as described above, since the noise level is sufficiently suppressed, the fluctuation in the demodulation level in section T1 due to this phase change is also very small.

The demodulation level of the gain-controlled color difference signal (R-Y) p in section T1 is clamped by a clamp pulse PC (FIG. 2E) in a clamp circuit 23. In this case, the clamp level does not vary depending on each horizontal line for the reason mentioned above, so the clamp operation is stable.

The clamped color difference signal (R-Y) P is blanked by the blanking circuit 24 for a blanking period TB. Note that the blanking pulse BLK that has passed through the delay circuit 18 is supplied to the blanking circuit 24, but this is for compensating for a signal delay due to the clamping operation in the previous stage.

The color difference signal (RY) p from which noise has been removed by the blanking operation is subjected to high frequency correction in the de-emphasis circuit 25 and then guided to the output terminal 27 via the amplifier 26. A color difference signal RY shown in G is obtained.

The blue channel also has the same configuration and operation as described above, so a detailed explanation thereof will be omitted, but in FIG.
30 is a demodulator, 31 is a low-pass filter, and 32 is a gain control amplifier, the AC gain of which is controlled only during period TG by the above-mentioned gain control pulse PG.

33 is a clamp circuit to which the clamp pulse pc is supplied;
34 is a blanking circuit for noise removal, 35 is a de-emphasis circuit, 36 is an amplifier, and 37 is a blue color difference signal B.
-Y is the output terminal.

With this configuration, if the gain control pulse PG shown in FIG. 2C is supplied, fluctuations in the demodulation level in the section T due to noise are removed, so that a substantially constant demodulation level can be accurately clamped for each line. I can do it.

Gain control pulse PG pulse 'PFA T
c.

may be chosen equal to the horizontal blanking period TB.

〔Effect of the invention〕

As explained above, according to the present invention, the clamping operation is performed after suppressing the level of the noise S generated during the horizontal blanking period TB, so that the demodulation level in the section T1 is changed by the noise S. There is no variation from line to line, and stable clamping operation can be achieved at all times.

Furthermore, according to the present invention, the configuration for achieving the above-mentioned object is only provided with the gain control amplifier 22, 32 and the monomulti 17. Easy to configure,
The configuration can be simplified.

Therefore, the present invention is applicable to the above-mentioned SEC4M decoder and is extremely suitable.

[Brief explanation of drawings]

FIG. 1 is a system diagram showing an example of the SEC4M decoder according to the present invention, FIG. 2 is a waveform diagram for explaining its operation, and FIG.
The figure is a waveform diagram showing the relationship between SECAM video signals and horizontal blanking, etc., used to explain the present invention. Figure 4 is a system diagram showing an example of a conventional SEC4M decoder.
The figure is a waveform diagram for explaining the operation. 10 is SEC4M decoder, 20.30 is demodulator, 22
．． 32 is a gain control amplifier, 23.33 is a clamp circuit, 24.34 is a blanking circuit, 16 is a pulse forming circuit, 17 is a monomulti, CFM (CR-Y
, CB-Y) is an FM color signal, PC is a clamp pulse, and PG is a gain control pulse. Fig. 3 31” CAM decoder 1 whistle AFjl

Claims (1)

[Claims] After demodulating the SECAM type FM color signal, it is supplied to a gain control amplifier, and the gain of the gain control amplifier is sufficiently lowered for at least a period approximately equal to the horizontal blanking period, and this horizontal blanking is performed. A SECAM decoder that suppresses noise components included in a period, and clamps the level of a section in which an unmodulated subcarrier is inserted in a color signal in which the noise component has been suppressed.