JPS58161476A - Automatic gain control device - Google Patents

Abstract

PURPOSE:To eliminate the effect of noise, by operating a clamp circuit and bringing a gain control circuit output into a clamp potential, when a synchronization separating circuit does not output an output signal for a prescribed time or over, in a magnetic video recording and reproducing device and a television device, etc. CONSTITUTION:When the output signal of the synchronization separating circuit 5 is lost, a differentiation circuit 10 is operated in a time corresponding to several horizontal scanning periods, the clamp circuit 3 is operated, resulting that the potential of noise is rapidly decreased to the clamping potential. Since the width of noise is wide, a peak detection circuit 6 detects the width determined with the time constant of a differentiation circuit 3 only, charges charged in a hold circuit 7 are limited. Noise cannot almost be detected by selecting properly the relation between the time constant of the circuit 10 and that of charging to the hold circuit from the peak detection circuit.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic gain control device for video signals used in devices that process video signals, such as magnetic recording and reproducing devices and television devices.

Generally, an automatic gain control device for a video signal inserts a pulse of a certain level value into the peak level value of the composite video signal or the blanking portion of the video signal, and calculates the sum of this pulse and the level value of the horizontal synchronizing signal. By detecting the gain, the gain is controlled so that the peak level of the composite video signal or the level of the synchronization signal included in the composite video signal is constant. Here, the pulses inserted into the blanking portion are generated based on the horizontal synchronization signal extracted from the composite video signal by the synchronization separation circuit. A conventional automatic gain control device will be described below with reference to FIGS. 1 and 2.

FIGS. 1 and 2 are diagrams of an example of a conventional automatic gain control circuit. In FIG. 1, an input signal such as a composite video signal applied to a terminal 1 is input to a gain control circuit 2. It passes through the clamp circuit 3 and is taken out from the output terminal 4, and
The signal is added to the sync separation circuit 6, and the sync signal is separated. The synchronization signal separated by the synchronization separation circuit 6 is returned to the clamp circuit 3, and the potential of the synchronization signal portion of the output signal of the gain control circuit 2 is clamped to a predetermined clamp potential.

This clamped composite video signal is sent to the peak detection circuit 6
．． - the peak detection circuit 6 detects the peak value of the input signal, the hold circuit 7 holds it with an appropriate time constant to control the gain of the gain control circuit 2, and as a result, the peak value of the input signal is An output signal with a constant peak value is output. FIG. 2 is an example of an automatic gain control device that outputs a synchronizing signal in a composite video signal while keeping the level constant.
Similar to the example in the figure, the input signal such as a composite video signal applied to the input terminal 1 has its gain controlled by the gain control circuit 2.
Clamp circuit 3. The synchronization signal portion is clamped by the synchronization separation circuit 6. The output of the synchronization separation circuit 6 is also applied to a delayed pulse generation circuit 8, which generates a pulse delayed by a predetermined time from the horizontal synchronization signal and having a predetermined level. This pulse is combined with a signal in which the synchronizing signal portion, which is the output of the clamp circuit 3, is clamped to a constant DC potential, and a mixing circuit 9
The pulse of the predetermined level is superimposed on the blanking portion of the composite video signal. The peak detection circuit 6 detects the level of the sum of the pulse and the synchronization signal, and the hold circuit A holds this with an appropriate time constant to control the gain of the gain control circuit 2 and output the synchronization signal from the output terminal 4. A video signal with a constant level of 5=-' is output. Here, the peak level of the pulse superimposed on the blanking portion of the composite video signal is usually set approximately equal to the white signal level of the composite video signal.

Therefore, if the synchronization signal part of the composite video signal shrinks relative to the video signal part, or if noise exceeding the white signal level is introduced, the peak detection circuit detects the peak value of the noise in the video signal instead of the above-mentioned pulses. You will be able to detect and control these levels. FIG. 3 is a diagram illustrating an input signal to an automatic gain circuit, and in an automatic gain control device configured as shown in FIGS. , when a noise larger than the peak level or the pulse level applied to the blanking portion is mixed in, this noise is applied to the peak detection circuit 6, and a voltage represented by vl in the same figure (IL) is applied to the hold circuit 7. , charges are charged at a rate corresponding to the charging time constant. This kind of noise often occurs when switching channels on a TV receiver.
Depending on the type of receiver, it may be twice the video signal level, several tens of times
Some have a width of -8600 hours. Also, Figure 3(b)
In this case, the pulse in the negative direction is clamped by the clamp circuit, so the peak detection circuit has a narrow width pulse in the negative direction.
A voltage v2 shown in is applied. After all, when such noise is applied to the input terminal, the voltage of the hold circuit becomes abnormally high, and the gain of the gain control circuit is abnormally suppressed. Generally, the discharging time constant (recovery time) of the hold circuit is the charging time constant (1 hour of recovery) from the peak detection circuit.
attack time), so once it is abnormally charged, it will take a long time to return to normal, and during that time the level of the video signal output to output terminal 4 will be normal. The situation continued to be lower than the level of . In particular, when integrating this automatic gain control device, if the filter constituting the hold circuit is a one-terminal filter to reduce the number of terminals, the discharge time constant must be made larger to improve rectification characteristics. Naturally, this trend will become stronger.

The present invention was devised to correct the above-mentioned problems, and FIGS. 4 and 6 show block diagrams of embodiments thereof.

FIG. 4 shows an embodiment of the present invention, in which an input signal such as a composite video signal applied to an input terminal 1 has its gain controlled by a gain control circuit 2, passes through a cluster 71 circuit 3, and is output from an output terminal 4. The synchronizing signal is taken out and applied to the synchronizing separation circuit 6 to separate the synchronizing signal, and this synchronizing signal is returned to the clamp circuit 3 via the differentiating circuit 10 to determine the potential of the synchronizing signal portion of the output signal of the gain control circuit 2. Clamp to the clamp potential. When the horizontal synchronizing signal is input, the differentiating circuit 1o outputs the horizontal synchronizing signal almost as is, operates the clamp circuit during the period when the synchronizing signal is output, and keeps the synchronizing signal portion of the composite video signal at a predetermined potential. Clamp. Furthermore, when the output of the synchronous separation circuit 5 disappears, the output of the differentiating circuit 10 also disappears, but after a certain period of time according to the time constant, the differentiating circuit 1o applies a certain potential to its output terminal to operate the clamp circuit 3. The clamp circuit is activated by generating a potential that exceeds the limit. Now, in the case of the conventional example shown in Fig. 1, Fig. 3 (1
When a video signal containing noise as shown in L) is input,
No output signal is output to the output of the synchronous separation circuit 5 during the noise period. In addition, in the case of Figure 3(b), even if the noise disappears and the video signal is input again, the negative tip of the noise is clamped to the clamp potential, so the noise is reduced as shown in Figure 3(C). Even the subsequent video signal has a high DC potential, and when such a signal is input to the synchronization separation circuit, no synchronization signal is output for a while.

Therefore, the noise in Figure 3 (2L) or the noise in Figure 3 (C
) is applied to the peak detection circuit 6. However, in the embodiment of the present invention shown in FIG. The differentiating circuit 1o operates in the corresponding time, causing the clamp circuit 3 to operate, and as a result, the potential of the noise part is rapidly lowered to the clamp potential, and even if the noise width is wide, it is determined by the time prime number of the differentiating circuit 3. Since only the width is detected by the peak detection circuit 6, the hold circuit 7 is charged and 9
・The amount of charge that can be generated will be limited. Now, this differential circuit 10
By appropriately selecting the relationship between the time constant of , and the charging time constant of charging from the peak detection circuit to the hold circuit, noise is hardly detected. This is an example in which the present invention is applied to a method of an automatic gain control device, in which an input signal such as a composite video signal applied to an input terminal 1 has its gain controlled by a gain control circuit 2, and passes through a clamp circuit 3. The synchronizing signal is extracted from the output terminal 4 and applied to the synchronizing separation circuit 6 to separate the synchronizing signal. Clamping the potential of the portion to a predetermined potential is similar to that shown in FIG. Also, in this case, the differential circuit 1
The operation and effect of 0 are also the same as those explained in conjunction with FIG.

Now, in FIG. 5, consider the case where the pulse delay circuit 8 is constituted by a monostable multivibrator that is triggered by the leading edge of the horizontal synchronization signal which is the output signal of the synchronization separation circuit 6. Here, the width of the output signal of the monostable multi-by 1° break is longer than the width of the horizontal sync signal in the composite video signal, starting from the leading edge of the horizontal sync signal and ending at least on the blanking portion. As a pulse delay circuit, there is another method of delaying the rear end of the horizontal synchronization signal, but if the synchronization signal of the input video signal is shortened under some conditions, the synchronization separation circuit 6 malfunctions, and the blanking part is extracted. In this case, this method has the drawback that the delay pulse is superimposed on the video signal, which lowers the gain of the gain control circuit and further shrinks the synchronization signal portion, making the malfunction of the synchronization separation circuit even worse. A method is used in which a monostable multivibrator is triggered at the leading edge of a horizontal synchronization signal. In this case, even if the sync separation circuit extracts the blanking portion, no pulse will be superimposed on the subsequent video signal. However, the drawback in this case is that if random noise is input as an input signal, such as when a channel with no television broadcast is received, the monostable multivibrator will generate output randomly. In other words, when a normal video signal is input, it is the synchronization signal part or the blanking part that is mixed with the monostable multivibrator output signal, but not the video signal. , in the case of the above-mentioned random noise, the output of the monostable multivibrator and the noise will be mixed. Now, when noise is mixed, the output level of the mixer circuit 9 becomes extremely high, and as a result, the potential of the hold circuit 7 becomes high, reducing the gain of the gain control circuit 2, so that normal operation is restored from this state. The disadvantage is that the response is very slow when a signal is input. Now, if the monostable multivibrator is configured to be triggered by the output of the differentiating circuit 1o, the synchronization separation circuit 6 will randomly generate an output signal when the above-mentioned noise is input.1. The width of the output signal generally becomes narrower, and even if the number of output signals is large, the average value becomes low. Therefore, at the output of the differentiating circuit 1o, the lowest level DC potential of the output signal becomes high, and at the same time the clamp circuit 3 is always operated, the setting is made so that the monostable, constant multivibrator is not triggered.
61476" (4) Since the clamp circuit is always operating, the noise at the output of the clamp circuit 3 is attenuated to a level determined by the ratio of the output impedance of the gain control circuit 2 and the impedance of the clamp circuit 3, and The average potential also becomes a clamp potential, the peak potential of noise decreases, and the potential of the hold circuit 7 decreases, so the response when switching from such a state to a normal state becomes very fast.

As described above, according to the present invention, the gain of the automatic gain control device can be prevented from being abnormally lowered even when the DC potential is high and wide noise is added, such as when switching channels in television broadcasting. You can. Further, even if a pulse is generated in the negative direction and pushes the DC potential at the output of the clamp circuit, the clamp circuit can be quickly restored and the influence of noise can be almost eliminated. Generally, the sync separation circuit has the effect of stabilizing the clamp operation, but when the output signal of the sync separation circuit is used for other purposes, such as servo circuits and chroma signal processing circuits, the differential effect becomes stronger. Then, if the level of the synchronization signal shrinks under some conditions, some kind of noise gets mixed in, or the frequency characteristics of the transmission system change, the operation of the synchronization separation circuit becomes unstable. It is not possible to provide the effects described in the embodiment.

In addition, the present invention uses a monostable multivibrator as a pulse delay circuit to control the level of a synchronizing signal at a constant level. It is also effective for the response when switching to , but such an effect cannot be provided in the synchronous separation circuit.

[Brief explanation of drawings]

FIGS. 1 and 2 are block diagrams showing conventional automatic gain control devices, and FIGS. 3 (IL), (b), (C
) is a waveform diagram explaining the input signal of the automatic gain control device, FIG. 4 is a block diagram of the automatic gain control device in one embodiment of the present invention, and FIG. 6 is a block diagram of another embodiment of the present invention. . , 1...Input terminal, 2...Gain control circuit, 3...Clamp circuit, 4...Output terminal, 6...Synchronization separation Circuit, 6...Peak detection circuit, 7...Bold circuit, 8...
・Pulse delay circuit, 9...Mixing circuit, 1o...
... Differential circuit. Name of agent: Patent attorney Toshio Nakao (1st person)
m1 6 Figure 2 Figure 3

Claims (2)

[Claims] (1) a gain control circuit that controls the gain of a composite video signal according to a control signal; a sync separation circuit that extracts a horizontal synchronization signal from the composite video signal; and a differentiation circuit that differentiates the output signal of the sync separation circuit; A clamp circuit that is controlled by the output signal of the differentiating circuit and clamps the potential of the horizontal synchronizing signal portion included in the output composite video signal of the gain control circuit to a preset clamp potential; a peak detection circuit that detects the peak potential of the clamped composite video signal; and a peak detection circuit that holds the potential detected by the peak detection circuit with a predetermined time constant;
and a hold circuit that controls the gain of the gain control circuit, and operates the clamp circuit when the synchronization separation circuit does not output an output signal for a predetermined period of time or more, so that the output of the gain control circuit is at the clamp potential. An automatic gain control device characterized in that: (2) A monostable multivibrator and a mixing circuit are provided between the clamp circuit and the peak detection circuit, and the monostable multivibrator is coupled to the output of the differentiating circuit, and each time the output of the differentiating circuit operates the clamp circuit. triggering the monostable multivibrator, the mixing circuit mixes the output of the clamp circuit and the output of the monostable multivibrator at a constant ratio, and the peak detection circuit detects the peak voltage of the output of the mixing circuit. The automatic gain control device according to claim 1, characterized in that the automatic gain control device is configured to: