JPH0757021B2 - Automatic gain control device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video signal automatic gain control device used in a magnetic recording / reproducing device, a television device or the like.

Conventional configuration and its problems Generally, an automatic gain control device for a video signal inserts a pulse of a certain level value in the horizontal blanking portion of the composite video signal, and the larger of the pulse and the video signal The gain of the composite video signal is controlled by detecting the sum of the signal and the magnitude of the sync signal. The pulse to be inserted into the horizontal blanking portion is generated based on the sync signal extracted from the composite video signal by the sync separation circuit. Therefore, when the video signal contains a noise pulse, the sync separation circuit malfunctions and extra pulses other than the sync signal appear in the output and normal level detection is not performed, resulting in abnormal gain of the composite video signal. There was an inconvenience that it was suppressed.

FIG. 1 is a block diagram showing an example of a conventional automatic gain control device. In FIG. 1, a composite video signal is applied to an input terminal 1 and passes through a gain control circuit 2 to an output terminal 3
And the potential at the tip of the sync signal is clamped by the clamp circuit 4, and then the sync separation circuit 5 and the addition circuit 6
Is supplied to. The output of the sync separation circuit 5 is applied to the delay pulse generation circuit 7, which generates a reference pulse of a predetermined level value at a predetermined phase delay time from the synchronization signal and supplies the reference pulse to the addition circuit 6. In the adder circuit 6, the reference pulse is added to the horizontal blanking portion of the video signal, the output is supplied to the peak detection circuit 8, and for example, a DC voltage according to the magnitude of the peak value is supplied to the value recording control circuit 2. The level of the composite video signal which is output to the output terminal 3 is automatically controlled.

In the automatic gain control device having the above-described configuration, when the composite video signal containing the noise pulse a as shown in FIG. 2A is supplied to the input terminal 1, the sync separation circuit 5 malfunctions and An output signal including an extra pulse b as shown in FIG. 2B is supplied to the delay pulse generation circuit 7. Therefore, an extra pulse appears in the output of the delay pulse generating circuit 7 as shown in FIG. 2C, and the output of the adding circuit 6 becomes as shown in FIG. 2D. When a signal as shown in FIG. 2D is applied to the peak detection circuit 8, V ₁ larger than the normal signal level V ₀ is detected and the gain of the gain control circuit 2 is suppressed abnormally. Generally, the peak detection circuit 8 has a discharge time constant set to be larger than the charge time constant, and therefore once V ₁ is detected, it takes a relatively long time to return to the same state, and the output terminal 3 outputs the voltage during that time. The level of the video signal to be reproduced continues to be smaller than the normal level.

The object of the present invention is to eliminate the above-mentioned drawbacks of the conventional example, and a stable gain control operation is possible even when the output of the sync separation circuit 5 includes an extra pulse other than the sync signal. The present invention provides an excellent automatic gain control device.

According to the present invention, the reference pulse supplied to the adder circuit is made smaller or smaller than the normal magnitude within a certain period from the leading edge of the output of the sync separation circuit, and the peak detection circuit is not supplied. The above object is realized by preventing the supply of an abnormally large signal.

Description of Embodiments FIG. 3 is a block diagram of an embodiment of an automatic gain control device according to the present invention. In FIG. 3, the composite video signal is applied to the input terminal 1, is taken out to the output terminal 3 through the gain control circuit 2, and the clamp circuit 4 clamps the potential at the tip of the sync signal. It is supplied to the adder circuit 6. The output of the sync separation circuit 5 is supplied to the gate circuit 9 and the mono-multi 10,
The mono-multi 10 outputs a pulse having a predetermined width which is triggered by the leading edge of the input signal, and the output causes the gate circuit 9 to be opened and closed. The output of the gate circuit 9 is supplied to the delay pulse generating circuit 7, which generates a reference pulse having a predetermined level value delayed by a predetermined time and is supplied to the adding circuit 6. In the adder circuit 6, the above-mentioned reference pulse is added to the horizontal blanking part of the composite video signal, and this output is supplied to the peak detection circuit 8 and, for example, a DC voltage according to the peak value is gain control circuit 2.
The level of the video signal supplied to and output to the output terminal 3 is automatically controlled.

In the automatic gain control device having the above-described configuration, when the composite video signal containing the noise pulse a as shown in FIG. 4A is supplied to the input terminal 1, the sync separation circuit 5 malfunctions and As shown in FIG. 4B, an output signal including an extra pulse b is supplied to the gate circuit 9 and the monomulti 10. As shown in FIG. 4E, a pulse having a predetermined width T ₁ which is triggered at the leading edge of the waveform shown in FIG. 4B is obtained at the output of the monomulti 10 and is supplied to the gate circuit 9. The gate circuit 9 operates so as to output the input as it is when the output of the mono-multi 10 is at the high level and not to output it when the output of the mono-multi 10 is at the low level.

Here, since the width of the extra panel b actually included in the output of the sync separation circuit 5 is a relatively narrow pulse of 1 μsec or less, the width T ₁ of the output pulse of the monomulti 10 is set to the horizontal sync signal. If it is set to be slightly narrower than the width of 1 and wider than 1 μsec, for example, 3 μsec, a signal from which an extra pulse is removed is obtained at the output of the gate circuit 9 as shown in FIG. It is supplied to the generating circuit 7 and is shown in FIG.
As described above, a reference pulse having a predetermined level value delayed by a predetermined time T ₂ is generated and supplied to the adder circuit 6. Delay time
T ₂ is set to, for example, about 2 μsec so that it is shorter than the width of the back porch in the horizontal blanking area. In the adder circuit 6, a reference pulse as shown in FIG. 4G is added to the horizontal blanking part of the composite video signal, and a signal as shown in FIG. 4H is obtained at the output thereof, that is, at the input of the peak detection circuit 8. Since the normal signal level V ₀ is detected, the gain of the gain control circuit 2 is kept stable.

In the above example, the gate circuit 9 is used as a means for generating the reference pulse (Fig. 4F) from the output of the sync separation circuit 5 (Fig. 4B) and the output of the monomulti 10 (Fig. 4E). However, the same effect can be obtained by using a multiplier instead.

In the above example, the delay pulse generating circuit 7 operates so as to delay the input signal as it is for a predetermined time. The effect of is obtained.

Another embodiment is shown in FIGS. 5 and 6. 5 and 6
In the figure, the same parts as those in FIG. 3 are indicated by the same numbers.

5 is different from FIG. 3 in that the output of the sync separation circuit 5 is connected to the delay pulse generation circuit 7 to generate a reference pulse of a predetermined level value at a predetermined delay time to cause the gate circuit 9 to generate the reference pulse. The relatively narrow extra width contained in the reference pulse is supplied by opening and closing the gate circuit 9 with the output of the monomulti 10, which is a pulse of a predetermined width which is supplied and triggered at the leading edge of the output of the sync separation circuit 5. It can be easily inferred that the same effect as that of the example shown in FIG. 3 can be obtained by only supplying the signal to the adder circuit after removing the pulse.

Next, in FIG. 6, the only difference from FIG. 5 is that the mono-multi 10 is triggered at the leading edge of the output of the delay pulse generation circuit 7, which is different from the example shown in FIG. It can be easily analogized that the completely same effect, that is, the completely same effect as the example shown in FIG. 3 is obtained.

As described above, according to the present invention, the sync separation circuit 5 malfunctions due to noise pulses or the like contained in the composite video signal with a relatively simple structure, and an extra pulse other than the sync signal is generated at the output thereof. Even in such a case, it is possible to realize an excellent automatic gain control device capable of performing a stable gain control operation without the output signal being abnormally suppressed.

[Brief description of drawings]

FIG. 1 is a block diagram showing an example of a conventional automatic gain control device, FIG. 2 is a waveform diagram for explaining the operation, and FIG. 3 is a block diagram showing an embodiment of the automatic gain control device according to the present invention. FIG. 4 is a waveform diagram for explaining the operation, and FIGS. 5 and 6 are block diagrams showing another embodiment of the automatic gain control device according to the present invention. 1 ... Input terminal, 2 ... Gain control circuit, 3 ... Output terminal, 4 ... Clamp circuit, 5 ... Sync separation circuit, 6 ...
Adder circuit, 7 ... Delayed pulse generation circuit, 8 ... Peak detection circuit, 9 ... Gate circuit, 10 ... Mono-multi.

Claims (3)

[Claims] 1. A gain control circuit for controlling the gain of a composite video signal, a sync separation circuit for extracting a sync signal from the composite video signal, and only from a leading edge of the sync separation circuit output signal to a first time. A mono-multi circuit that generates an output signal, a gate circuit that prevents the sync separation circuit output signal from being output during a period in which the mono-multi circuit generates an output, and a trailing edge of the output signal of the gate circuit is a second circuit. A delay pulse generation circuit for generating a reference pulse delayed by time, an addition circuit for adding the reference pulse to the output signal of the gain control circuit so that the reference signal has a polarity opposite to that of the synchronizing signal, and a magnitude of the output signal of the addition circuit. And a peak detection circuit for controlling the gain of the gain control circuit according to the height. 2. A gain control circuit for controlling the gain of a composite video signal, a sync separation circuit for extracting a sync signal from the composite video signal, and only from a leading edge of the sync separation circuit output signal to a first time. A mono-multi circuit for generating an output signal, a delay pulse generation circuit for generating a signal obtained by delaying a trailing edge of the output signal of the sync separation circuit by a second time, and a delay period during which the mono-multi circuit generates an output. A pulse generating circuit, a gate circuit for generating a reference pulse so that the output signal is not output, an adder circuit for adding the reference pulse to the output signal of the gain control circuit so that the reference signal has a polarity opposite to that of the synchronizing signal, and the adder circuit. And a peak detection circuit for controlling the gain of the gain control circuit according to the magnitude of the output signal of the automatic gain control device. 3. A gain control circuit for controlling the gain of a composite video signal, a sync separation circuit for extracting a sync signal from said composite video signal, and a signal obtained by delaying the trailing edge of said sync separation circuit output signal by a second time. A delayed pulse generation circuit for generating
A mono-multi circuit that generates an output signal only from a leading edge of the delay pulse generation circuit output signal to a first time, and a period in which the mono-multi circuit generates an output, the delay pulse generation circuit output signal is not output. A gate circuit for generating a reference pulse in this way, an adder circuit for adding the reference pulse to the output signal of the gain control circuit so that the reference signal has a polarity opposite to that of the synchronizing signal, and a signal depending on the magnitude of the output signal of the adder circuit. And a peak detection circuit for controlling the gain of the gain control circuit.