JPH08256350A - Chroma signal processing circuit - Google Patents

Abstract

PURPOSE: To improve color image quality for a very small signal by reducing color noise amplitude in the SECAM chroma demodulation processing especially. CONSTITUTION: A chroma signal is separated and given to an input 11. The chroma signal is given respectively to a chroma demodulation circuit 12 and a level detection circuit 13. A signal demodulated by the chroma demodulation circuit 12 is given to a gain control circuit 14, and a color difference signal is outputted from an output 15 by varying the gain with a control voltage from the level detection circuit 13. In the post-stages, the processing with a luminance signal (not shown) is made and the resulting signal is displayed on a screen. Thus, the color difference signal and the color noise are attenuated in response to an input signal level and since the higher color noise results in higher attenuation in the gain control circuit, the noise level appearing on the screen is reduced and the color image quality is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chroma signal processing circuit suitable mainly for use in a television receiver.

[0002]

2. Description of the Related Art There are three types of television color signal transmission currently being broadcast: PAL, NTSC and SECAM. These color signal transmission systems are widely adopted in each country, and it is possible to receive a plurality of systems by broadcasting from a neighboring country in a specific area. In this area, a multi-system television receiver capable of receiving these plural system signals is on sale.

First, a case of receiving the SECAM system will be described as an example with reference to FIG. 19 showing a conventional circuit. S
The ECAM method adopts an FM modulation method for transmitting color signals, and the other two methods are AM modulation. Therefore, it is necessary to separate the chroma signal from the SECAM system signal received by the tuner circuit (not shown) and perform FM demodulation. FIG. 19 shows a part of the FM demodulation process. Input terminal 19
The SECAM chroma signal input to 1 is first subjected to limiter amplification by the limiter 192 to remove noise, and then demodulated by the FM demodulation circuit 193. Output terminal 1 for demodulated color difference signal
It is output from 94, is combined with a luminance signal (not shown), and is displayed on the screen.

Here, the signal level and image quality of the incoming television signal will be described. The SECAM chroma signal input by the circuit of FIG.
It changes according to the output level. The limiter 192 does not change the output level even if the level changes a little. Therefore, even if there is a level change, the FM demodulation circuit 193
The input amplitude is stable, and the noise suppression effect unique to the FM system is also added to enable good image reception.

Recently, in order to reduce the cost related to signal processing, there is a tendency to remove the SECAM demodulation parts. This will be described with reference to FIG. As in the case of FIG. 19, the limiter 192 is used to limiter-amplify the input signal to eliminate the input level fluctuation. The output of the limiter 192 is passed through a PLL type FM demodulation circuit 201, and a color difference signal is output. The PLL system is suitable for incorporation because it does not require a tank circuit like the conventional quadrature detection, and has been widely used recently.

Since the PAL / NTSC system chroma demodulation circuit is an AM system, if the input level fluctuates, the demodulation output is likely to change accordingly. To avoid this, ACC
The (auto-chroma control) circuit stabilizes the amplitude of the chroma signal input to the demodulation circuit so that the image can be received well even if the level changes.

When the input level becomes lower enough to prevent the limiter from being applied, the output level of the limiter 192 lowers. Even if the input level changes, the FM modulation index does not change, so the demodulation output level does not change. In particular, FIG.
The demodulation circuit 201 using the PLL of 0 has a strong tendency.
However, C / N with a minute input signal that does not apply a limiter
Since the (carrier / noise) ratio is worse than at the standard level, the noise in the demodulation output becomes large. This noise is displayed on the screen as color noise, which is very unsightly and deteriorates the quality of the color screen. FIG. 21 shows the input dependence of demodulation and color noise level. Thus, as the input level becomes smaller, the color noise increases up to a level at which a killer detection circuit (not shown) determines that the state is black and white.

This is remarkable in the case of the PLL demodulation shown in FIG. 20, and the description will be continued with reference to FIGS. 22 and 23. PLL
The demodulation circuit 201 is a circuit for matching the frequency of the built-in VCO with the input signal, and the frequency limit at which the frequency can be matched is called the capture range. When the input signal becomes minute and the limiter is not applied, the demodulation sensitivity in the FM demodulation circuit is lowered and the capture range is narrowed. It suddenly drops near the VCO free-run frequency (fo) in the no-input state. As described above, in FM broadcasting, the modulation index does not change even when the input level is lowered, so the PLL tries to pull in this frequency deviation.

However, when the capture range becomes narrower than the frequency deviation, the PLL cannot be pulled in and the lock is released. For example, when the 75% RY color bar signal is demodulated as shown in FIG. 23, if the lock is released at a part (dotted line part) of the 75% RY color bar signal, the center voltage is momentarily returned. The change in the color difference output due to this is very large, and as a result, color noise with a large amplitude appears on the screen, and the deterioration of the color image quality becomes serious.

[0010]

In the conventional chroma signal processing circuit described above, when the capture range becomes narrower than the frequency deviation, the PLL cannot be pulled in, the lock is released, and the center voltage returns to the center voltage for a moment. The change in the color difference output due to this is very large, and as a result, color noise with a large amplitude appears on the screen, and the deterioration of the color image quality becomes serious.

The present invention is particularly intended to reduce the color noise amplitude in the SECAM chroma demodulation process and improve the color image quality at the time of a minute signal.

[0012]

In order to solve the above problems, the present invention provides a level detection circuit for detecting the amplitude of a signal input to a chroma demodulation circuit, and changes the gain of the demodulation gain control circuit by the detection output. Let In particular, when the SECAM chroma demodulation is realized by the PLL system, the PLL demodulation loop gain is controlled by the level detection output.

[0013]

Since the level detection output changes according to the input level by the means described above, if the gain of the color difference signal near the input level where the color noise is generated is controlled using this, the color noise in the color difference output can be controlled. Can be reduced. Furthermore, SE
When the CAM chroma demodulation is configured by the PLL, the capture range can be expanded by increasing the loop gain at the minute level, and the generation of color noise due to the lock release can be reduced.

[0014]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a circuit configuration diagram for explaining one embodiment of the present invention. The chroma signal is separated from the video signal detected by the tuner circuit (not shown) and input to the input 11. The chroma signal is the chroma demodulation circuit 12
And to the level detection circuit 13, respectively. The signal demodulated by the chroma demodulation circuit 12 is input to the gain control circuit 14, the gain is changed by the control voltage from the level detection circuit 13, and the color difference signal is output from the output 15. In the latter stage, processing with a luminance signal (not shown) is performed and displayed on the screen as in the conventional case.

In this way, the color difference signal and the color noise can be attenuated according to the input signal level, and the larger the color noise, the larger the attenuation in the gain control circuit. The quality of color image quality can be improved.

Here, as described in the related art, when the ACC circuit is provided, it can be used. Therefore, another embodiment of the present invention will be described with reference to FIG. The ACC circuit is a variable gain amplifier A as shown in FIG.
It is composed of a CC amplifier 21 and an ACC detection circuit 22 for detecting the level of the output of the ACC amplifier.
Often provided in the signal path. ACC amplifier 21
Is the output level of a certain level ACC?
It is detected by the detection circuit 22, and the gain of the ACC amplifier 21 is adjusted to feed back so as to always have a constant level.

In this way, the ACC detection circuit 22 can perform the level detection of FIG. 1. Therefore, if the control voltage of the ACC detection circuit 22 is input as the control voltage of the gain control circuit without newly providing the level detection circuit. Good.

When the level detection circuit is the ACC circuit, the operation will be further described with reference to FIG. FIG.
FIG. 7A is a diagram showing settings around ACC. First, the output level of the ACC amplifier 21 monotonically increases as the input increases from very small. At this point, the ACC amplifier 21 is higher than the detection level set by the ACC detection circuit 22.
Output is small, and the gain of the ACC amplifier 21 is set to the maximum. When the input level further increases, the set level of the ACC detection circuit 22 is reached. The ACC loop starts to work from this level.

Even if the input level increases, the ACC detection circuit 22 lowers the gain of the ACC amplifier 21 so as to suppress it, so that the output level of the ACC amplifier 21 becomes constant. When the amplitude exceeds the level that can be suppressed by the ACC amplifier 21, it increases monotonically thereafter. At this time, the gain of the ACC amplifier 21 is kept at the minimum. ACC detection circuit 22
The control voltage of the ACC amplifier 2 is as shown by the dotted line in FIG.
The output of 1 changes when it becomes flat. When the standard input level is en, generally, the maximum gain and the cover range of the ACC amplifier 21 are set so that flat parts are formed above and below the standard input level en.

When the input level at which the limiter is not applied is es as shown in FIG. 18, the limiter gain is usually larger than the maximum gain of the ACC amplifier 21, and therefore AC
The input level es becomes smaller than that of the flat output portion of the C amplifier 21. To put it the other way around, the limiter is applied to the flattened portion of the ACC amplifier 21. The minute signal portion where the color noise starts to occur is between the standard input level en and the input level es, and it is desirable that the standard color difference output level can be obtained at the standard input level en level.

Therefore, if a necessary portion of the ACC amplifier control voltage between the input level es and the standard input level en is cut out by the control slice voltage and the gain control circuit 14 is controlled by the voltage, the color noise is generated. Therefore, the demodulation output level can be reduced. This state is shown in FIG. The gain is attenuated in the minute input direction from the intersection of the control slice voltage and the ACC control voltage. In this way, the level of the color difference signal output does not change abruptly, and the level can be sufficiently attenuated at a minute level where color noise is generated, and suitable control is possible.

Not only the color noise but also the color difference signal output is suppressed at a minute level, resulting in a change in color saturation.
As a result, this is preferable. At the input level where the color noise is noticeable, the demodulation circuit starts to lose lock as described above, and noise as shown in FIG. 20 appears. The color noise generated as a result of this, that is, large-amplitude noise of a color different from the original color, is often the primary color of red and blue in particular, and the trailing pattern is also noticeable. Good. However, if the gain is suddenly changed when the input level slightly changes, the screen becomes flicker, so it is necessary to smoothly reduce the gain as shown in FIG. 3B. In that sense, a smooth level detection voltage such as the ACC control voltage is required.

In the example of FIG. 3, it is assumed that the part where the output of the ACC amplifier 21 becomes flat is near the input level es. That is, the ACC amplifier 21 covers a level as small as color noise is generated. This level is empirically -4 with respect to the standard input level en.
The level is attenuated by about 0 dB, and the flattening start portion of the general ACC amplifier 21 is set in the vicinity of -10 dB to -20 dB.
It is necessary to widen the cover range of the amplifier 21.

For example, in the case of a television receiver that performs only SECAM chroma signal processing, the setting may be changed as described above, but in the case of a multi-system compatible system that also performs PAL / NTSC system processing, another method is preferable. Therefore, a second other embodiment of the present invention will be described with reference to FIG.

Although the PAL and NTSC systems are different in details, they are both AM systems, and since the present invention has the same effect, the details of the processing will not be touched upon. Usually, it is often used as a demodulation circuit or each color difference signal is output from the same circuit.
The following description will be given on the assumption that there are two color difference signal processes of PAL / NTSC (hereinafter abbreviated as PAL system).

In the PAL system, ACC has already been performed because of AM demodulation.
There is a circuit. It seems that if the signal level is flattened to a minute signal level, a stable demodulated color difference signal can be obtained even at a minute signal level. However, A for the ACC detection voltage
The gain change rate of the CC amplifier is correspondingly increased, and the gain is likely to change due to noise. As a result, the color difference output changes like a line flicker, and the flicker of the color saturation on the screen is likely to occur. In order to avoid this, PAL
The cover range of the ACC amplifier in the system is set not to be wider than necessary. In the case of SECAM, even if it is wide, there is a limiter in the subsequent stage, so that the amplitude change can be removed and the line flicker of the demodulated color difference signal does not occur. Even if the coverage of the ACC circuit is wide, SECAM
No problem if it is a chroma demodulation circuit.

Therefore, in the embodiment of FIG. 4, the ACC circuit which was one in FIG. 3 is divided into two, and the ACC amplifier 43 and the ACC that cover the vicinity of the standard input level en required in the PAL system are provided.
It is composed of a first ACC loop 41 including a detection circuit 44, an ACC amplifier 45 that covers the vicinity of a minute level, and a second ACC loop 42 including an ACC detection circuit 46. First
The output of the ACC loop 41 is the PAL chroma demodulation circuit 4
7 and outputs the output of the second ACC loop 42 to SECAM.
Output to the chroma demodulation circuit. The operation will be described with reference to FIG.

First, when the input chroma signal level is sufficiently low, the gains of the ACC amplifier 43 and the ACC amplifier 45 are maximum, and the output level of the ACC amplifier 43 monotonically increases. The ACC amplifier 43 is an A for small amplitude.
This is a CC circuit, and when it exceeds a predetermined value of the ACC detection circuit 46, the second ACC loop 42 starts to operate and flattens the output. The input level further increases, and the second ACC loop 42
Above the cover range, the output starts to increase again,
When the value exceeds the predetermined value of the CC detection circuit 44, it becomes flat again. At this time, the second ACC loop 42 has already reached the minimum gain, and only the gain of the first ACC loop 41 changes to stabilize the output level of the second ACC loop 42.

By sharing the cover range in this manner, it is possible to set the range in the first ACC loop 41 mainly focusing on the PAL system, and to set the range in the second ACC loop 42 mainly focusing on the SECAM color difference gain control. Figure 5
Since the control voltage of the ACC amplifiers 43 and 45 changes within the range as shown in (b), the control voltage from the ACC detection circuit 46 can be used as it is for gain control. The slice cutting operation of the control voltage described in FIG. 3 is basically unnecessary. By controlling the gain of the color difference signal in this way, the result is that the color noise and the color difference signal can be smoothly attenuated as shown in FIG.

The embodiment of FIG. 4 also shows the multi-system support, which will be described. When the television receiver is compatible with the three systems, the SECAM chroma demodulation circuit 12 may be gain-controlled by the control voltage of the second ACC loop 42. PAL system chroma demodulation circuit 47
If the output signal of the first ACC loop 41 is supplied to, the above-mentioned problem of line flicker does not occur. PAL
The control voltage of the second ACC loop 42 may be used in the demodulation processing of the system. In the example of FIG. 4, the control voltage application of the second ACC loop 42 is not limited to SECAM.

In the above description, the gain control circuit 14 is provided after the chroma demodulation circuit 12, and the gain is controlled by the level detection means such as ACC. However, it is possible to give the function to the demodulation circuit without actually providing an independent gain control circuit. This case is shown in FIGS. 6 to 9 and will be described as third to sixth embodiments of the present invention.

In the embodiment shown in FIG. 6, a demodulation gain control terminal 62 is provided in the FM demodulation circuit 61 of SECAM, and a control voltage for the level detection circuit is applied. The embodiment shown in FIG. 7 is PAL.
The demodulation gain control terminal 72 is provided in the AM demodulation circuit 71 of the system,
The control voltage of the level detection circuit is applied. FIG.
Is a PAL-based AM demodulation circuit 81 and a demodulation gain control terminal 8
Even if the gain control circuit 82 including 1 is provided, exactly the same effect can be obtained.

Further, in the case of a receiver including a PAL system and a SECAM system, it is possible to reduce the circuit scale by using both of them without separately providing a gain control circuit. FIG. 9 shows this case, and the demodulation circuits 91 and 9 of the SECAM and PAL systems
The color difference signal from 2 is switched by the switch 93 and input to the gain control circuit 94. The switching signal of the switch 93 can be generated, for example, as a result of a system discrimination circuit (not shown).

FIG. 10 shows a multi-color chroma signal processing circuit when the embodiments of FIGS. 4 and 9 are combined, and a seventh other embodiment of the present invention will be described. The demodulation circuit here means not only the demodulator but also all the various processes required for demodulation except the ACC circuit. The PAL demodulation circuit 47 may include a color control function in which the user controls the chroma signal level to change the color saturation of the screen. If the control from the level detection circuit is performed in parallel (dependent) with the user control, the gain control function can be easily installed with almost no increase in the number of elements. In the case of a multi-system compatible image receiver, the color control often comes after the switch 93. This is because one control circuit controls all system signals, but even in this case, control from level detection can be added to the color control.

Particularly when the SECAM demodulation is performed by the PLL system, there is color noise caused by the narrowing of the capture range as described with reference to FIGS. 22 and 23. There is another way to improve this, so I will describe it. FIG. 11 shows another eighth embodiment of the present invention. The SECAM chroma signal is input from the input terminal 111, and is input to the limiter circuit 112 and the level detection circuit 113, respectively. The output of the limiter circuit 112 is FM demodulated by the PLLFM demodulation circuit 114, and a color difference signal is obtained from the output terminal 115. Here, the FM demodulation circuit 11
Reference numeral 4 denotes a circuit capable of controlling the loop gain, and controls the loop gain by the detection output of the level detection circuit 113.

In this way, the level detection circuit 113
When it is detected that the level is minute, the loop gain of the FM demodulation circuit 114 can be increased and the capture range can be expanded. As shown in FIG. 12, the range at the minute level expands in both the upper and lower directions, and the color noise as shown in FIG. 23 can be reduced. If the same input level is used, FIG. 12 is wider, and more FM signals having the same frequency deviation can be captured. The level detection circuit is shown in FIG. 2 and FIG.
As described above, it can be realized by using the ACC circuit. If the ACC cover range and the color noise generation level are optimally set, the same effect can be obtained by either method.

An example in which this is specifically realized will be described below. FIG. 13 shows the inside of the PLL type FM demodulation circuit 114. The PLL is mainly composed of two blocks, that is, a VCO 114a and a phase comparison circuit 114b.
The oscillation signal of the CO 114a and the S input to the input 114c
Match the frequencies of the ECAM chroma signals. The VCO control voltage when they match is the demodulation signal itself, and the color difference signal is demodulated from the output 114e.

Here, as a specific example of the loop gain control,
For example, the bias current source 114f of the phase comparison circuit 114b
Is controlled by the level detection output, the comparison sensitivity can be controlled, and as a result, the loop gain can be changed. For example, when the level becomes extremely small, increasing the current of the current source 114f increases the gain and widens the capture range.

FIG. 14 shows another specific example of the loop gain control. The variable resistance circuit 114g is provided in the internal configuration of the PLL type FM demodulation circuit 114 as described above. Phase comparison circuit 1
If 14b is a current output type, loop gain control is possible by varying its load resistance. If the resistance value of the variable resistance circuit 114g is increased when the input becomes a minute level, the loop gain becomes large and the capture range becomes large. The output characteristic (open loop) of the phase comparison circuit 114 at this time is shown in FIG. Phase comparison circuit 114b
If the resistance value changes with respect to the output current of, the slope of the output voltage changes.

However, in reality, it is not necessary that the loop gain is large, and there is an optimum value. If the loop gain is too large, the demodulation waveform will cause ringing, or the demodulation band will be longer than necessary (it is easy to operate as noise = large demodulation noise). Therefore, it is desirable that the gain be increased only for a signal having a large frequency deviation.

An example at this time will be described with reference to FIG. 16 as another specific example of the loop gain control. FIG.
Shows the variable resistance circuit 114g of FIG. 14 with an offset. The output current of the phase comparison circuit 114b is passed through the resistors R1 and R2. The resistor R1 is a fixed voltage source 114
It is biased by h and biases the resistor R2 through the switching detection circuit 114i. The switching detection voltage is controlled by the level detection output, and the starting voltage of the switch circuit 161 is changed. When the output current of the phase comparison circuit 114b is zero, the switch detection circuit 114i turns on the switch circuit 161, and the load of the phase comparison circuit 114b is a parallel resistance of the resistors R1 and R2. When the output current increases in the positive direction, for example, the comparison output voltage increases. When the comparison output voltage exceeds a predetermined voltage (controlled by the level detection circuit) set by the switching detection circuit 114i, the switch circuit 161 is turned off and the resistor R2 is disconnected from the resistor R1. The load resistance at this time is only the resistance R1, and the load resistance is increased and the gain is increased as compared with the case of zero. If the switching detection voltage is controlled so as to become smaller as the level becomes minute, the voltage switched by the load resistance becomes closer to zero (free run = fo) as shown by the arrow in FIG. 18, and the capture range expansion start point becomes closer to zero. .

By doing so, the loop gain increases only for a signal having a deep frequency deviation, so that an appropriate loop gain value can be held for a signal near the center frequency that originally has a capture capability.

Since there is a concrete example suitable for realizing the system of FIG. 16, it will be described with reference to FIG. The switching detection and switch circuit is realized by the voltage follower connection buffer of the transistors Q1 to Q4 and the current source 171. The current I1 of the current source 171 is controlled by the output of the level detection circuit. When a current is applied from the outside to the base terminal of the transistor Q1 which is the output of the buffer circuit, the current is I
If it is greater than 1, the transistors Q1 and Q2 switch and the buffer output resistance becomes very large (ideally infinite).

Therefore, when the output voltage of the phase comparison circuit 114b becomes larger than the product of the current I1 of the current source 171 and the resistor R2, the resistor R2 is apparently turned off. If the output voltage of the phase comparison circuit 114b is within the above product voltage, the buffer does not switch, so the resistor R2
Is turned on. Current I1 in the level detection circuit
Is controlled and I1 is made smaller as the level becomes smaller, loop gain control becomes possible.

If the control from the level detection circuit is used for both loop gain and color difference signal gain control, the effect of reducing color noise can be further increased. In a model capable of receiving both the PAL system and the SECAM system even with a multi-system compatible receiver, the PAL system already has an ACC circuit, so that FIG.
The method using both FIG. 7 and FIG.

Recently, PAL type BPF and SEC
In many cases, an AM bell filter is built in, and widening the ACC system cover range is very effective in effectively utilizing the small dynamic range of the built-in filter.

4 and 1 in which two ACC circuits are connected in cascade.
In the case of 0, ACC2 is added to the gain control shown in FIG.
Alternatively, the control voltage of the first ACC circuit 41 can be used for the loop gain control of the SECAM demodulation. Explaining in the direction in which the input signal shifts from a large level to a small level, an improvement can be made by a loop gain before the color noise starts to be generated, and the color noise that is still generated can be suppressed by suppressing the color difference amplitude. As compared with the method in which only the control voltage of the second ACC circuit 42 is used, good results can be obtained with respect to the generation of color noise. As compared with the method using only the control voltage of, good results can be obtained for the occurrence of color noise.

Although the ACC circuit closest to the demodulation in the processing system is taken as an example of the level detection circuit, it is sufficient if the input level can be detected. Therefore, the A of the intermediate frequency (IF) processing circuit is used.
The GC voltage may be used.

[0049]

As described above, by using the circuit of the present invention, it is possible to reduce the color noise amplitude especially in the chroma signal demodulation processing of the SECAM system and improve the color image quality at the time of a minute signal.

[Brief description of drawings]

FIG. 1 is a block diagram for explaining an embodiment of the present invention.

FIG. 2 is a block diagram for explaining another embodiment of the present invention.

FIG. 3 is a characteristic diagram for explaining the operation of FIG.

FIG. 4 is a block diagram for explaining a second other embodiment of the present invention.

FIG. 5 is a characteristic diagram for explaining the operation of FIG.

FIG. 6 is a block diagram for explaining a third other embodiment of the present invention.

FIG. 7 is a block diagram for explaining a fourth other embodiment of the present invention.

FIG. 8 is a block diagram for explaining a fifth other embodiment of the present invention.

FIG. 9 is a block diagram for explaining a sixth other embodiment of the present invention.

FIG. 10 is a block diagram for explaining a seventh other embodiment of the present invention.

FIG. 11 is a block diagram for explaining an eighth other embodiment of the present invention.

12 is a characteristic diagram for explaining the effect of FIG.

FIG. 13 is a block diagram illustrating a specific example of loop gain control.

FIG. 14 is a block diagram for explaining another specific example of loop gain control.

15 is an output characteristic diagram of the phase comparison circuit of FIG.

FIG. 16 is a block diagram for explaining another specific example of loop gain control.

17 is a specific block diagram for implementing the system of FIG.

FIG. 18 is a characteristic diagram for explaining the operation of FIG.

FIG. 19 is a block diagram for explaining conventional SECAM chroma signal processing.

20 is a block diagram when the SECAM process of FIG. 19 is performed by a PLL-FM demodulation circuit.

FIG. 21 is a characteristic diagram showing signal levels in SECAM chroma signal processing.

22 is a characteristic diagram for explaining the operation of the FM demodulation of FIG.

23 is a waveform chart used to explain the problem of FIG. 20. FIG.

[Explanation of symbols]

12 ... Chroma demodulation circuit, 13 ... Level detection circuit, 14 ...
Gain control circuits 21, 43, 45 ... ACC hump, 2
2, 44, 46 ... ACC detection circuit, 41 ... First ACC loop, 42 ... Second ACC loop, 47 ... PAL system chroma demodulation circuit, 61, 91 ... FM demodulation circuit, 71, 81,
92 ... AM demodulation circuit, 82, 94 ... Gain control circuit, 11
4 ... PLLFM demodulation circuit.

Claims (12)

[Claims] 1. A chroma demodulation circuit for inputting a chroma signal and outputting a color difference signal from its output, a gain control circuit for varying the gain of the color difference signal, a detection circuit for detecting the level of the chroma signal, A chroma signal processing circuit comprising: a control means for controlling the gain of the gain control circuit based on the detection output of the detection circuit. 2. The level detection circuit is an ACC circuit,
2. The chroma signal according to claim 1, wherein the chroma signal is input to the ACC circuit, the output of the ACC circuit is input to a chroma demodulation circuit, and the gain of the gain control circuit is controlled by an ACC control voltage. Processing circuit. 3. The ACC circuit comprises two cascaded A's.
3. The chroma signal processing circuit according to claim 2, wherein the chroma signal processing circuit comprises a CC circuit, wherein the ACC control ranges do not overlap and the gain is controlled by either one of the ACC control voltages. 4. The demodulation circuit is compatible with a plurality of systems, and two ACC circuits are cascade-connected in order of large amplitude and small amplitude, and chroma demodulation is performed using the first stage ACC output depending on the system. The chroma signal processing circuit according to claim 3, wherein 5. The chroma signal processing circuit according to claim 1, wherein the level detection circuit is an AGC circuit of an intermediate frequency amplifier circuit section, and the gain is controlled by an AGC control voltage. 6. A chroma demodulation circuit comprising a plurality of chroma demodulation circuits, comprising a switching circuit for switching demodulated signals, wherein the output of the switching circuit is input to the gain control circuit and also used as the gain control circuit. The chroma signal processing circuit according to claim 4. 7. The chroma signal processing circuit according to claim 1, wherein the chroma demodulation circuit has a demodulation gain control function, the gain control is performed by the demodulation circuit, and an independent gain control circuit is eliminated. 8. A SECAM chroma demodulation circuit including a PLL type FM demodulation circuit for inputting a SECAM television system chroma signal and outputting a color difference signal from the output, and means for controlling a PLL loop gain of the demodulation circuit, A chroma signal processing circuit comprising: a level detection circuit for detecting the level of the input chroma signal; and control means for controlling the loop gain based on a detection output of the detection circuit. 9. The level detection circuit may be a single or two A circuits.
An ACC circuit in which a CC circuit is connected in cascade,
9. The chroma signal processing according to claim 8, wherein the SECAM chroma signal is input to the circuit, the output of the ACC circuit is input to the chroma demodulation circuit, and the loop gain is controlled by at least one of the ACC control voltages. circuit. 10. The chroma signal processing circuit according to claim 8, wherein the PLL type FM demodulation circuit comprises a VCO and a phase comparison circuit, and the loop gain is controlled by the phase comparison sensitivity. 11. The chroma signal processing circuit according to claim 8, wherein the load circuit of the phase comparison circuit is a variable resistance circuit, and the loop gain is controlled by this. 12. The variable resistance circuit comprises two fixed resistances and a switch circuit, and comprises switching means for disconnecting one resistance from the load circuit according to a voltage generated in the resistance, and the switching voltage is detected by a detection output. The chroma signal processing circuit according to claim 11, wherein the loop gain is controlled by being changed.