JPS60109387A - Automatic gain controller - Google Patents

Abstract

PURPOSE:To prevent an abnormally large signal from being supplied to a peak detection circuit by making a reference pulse supplied to an adder smaller than the normal size for a certain period from a front edge of an output of a synchronizing separator circuit. CONSTITUTION:A complex video signal is added to an input terminal 1, taken out on an output terminal 3 through a gain control circuit 2, and supplied to a synchronous distribution circuit 5 and an adder 6 after a voltage of the tip of a synchronizing signal is clamped in a clamp circuit 4. An output of the circuit 5 is supplied to a gate circuit 9 and a monostable multivibrator 10, which outputs about 3mu second pulse triggered at the front edge of an input signal. When an output of the monostable multivibrator 10 is a high level, the circuit 9 outputs it directly, and when the output of the monostable multivibtator 10 is a low level, the circuit 9 will not output it. Therefore the signal whose excess pulse is removed can be obtained as an output of the circuit 9.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an automatic gain control device for video signals used in magnetic recording and reproducing devices, television devices, and the like.

Conventional configuration and its problems In general, an automatic gain control device for a video signal inserts a pulse of a certain level value into the horizontal blanking portion of a composite video signal, and then inserts a pulse of a certain level value into the horizontal blanking portion of a composite video signal, and then selects the larger of this pulse or the video signal. The gain of the composite video signal is controlled by detecting the sum of the magnitude of the synchronization signal and the magnitude of the synchronization signal. The pulses inserted into the horizontal blanking portion are generated based on a synchronization signal extracted from the composite video signal by a synchronization separation circuit.

Therefore, when noise pulses are included in the video signal, etc.
If the synchronization separation circuit malfunctions, extra pulses other than the synchronization signal appear in its output, and normal level detection is not performed, resulting in the inconvenience that the gain of the composite video signal is suppressed abnormally. .

FIG. 1 shows a block diagram of an example of a conventional automatic gain control device. In FIG. 1, a composite video signal is applied to input terminal 1 and passes through gain control circuit 2 to output terminal 3.
After the potential at the tip of the synchronization signal is clamped by the clamp circuit 4, the synchronization separation circuit 5 and the adder circuit 6
supplied to

The output of the synchronization separation circuit 5 is applied to a delay pulse generation circuit 7, which generates a reference pulse of a predetermined level value with a predetermined phase delay time from the synchronization signal, and supplies the generated 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, and this output is supplied to the peak detection circuit 8, which supplies, for example, a DC voltage to the gain control circuit 2 according to the magnitude of the peak value. The level of the composite video signal taken out to the output terminal 3 is automatically controlled.

In the automatic gain control device configured as described above, if a composite video signal containing a noise pulse a is supplied to the input terminal 1 as shown in FIG. 2A, for example, the synchronization separation circuit 6 may malfunction. An output signal including an extra pulse b as shown in FIG. 2B is supplied to the delayed pulse generation circuit 7. Therefore, an extra pulse appears in the output of the delayed pulse generating circuit 7 as shown in FIG. 2C, and the output of the adder circuit 6 becomes as shown in the second diagram.

When a signal as shown in the second diagram is applied to the peak detection circuit 8, the normal signal level ■. 1 is detected, and the gain of the gain control circuit 2 is abnormally suppressed. Generally, the peak detection circuit 8 is set larger than the discharge time constant or charge time constant, so once ■1 is detected, it takes a relatively long time to return to the original state, and during that time the output terminal 3
The level of the video signal outputted during the process continues to be lower than the normal level.

Purpose of the Invention The present invention eliminates the drawbacks of the conventional example described above, and enables stable gain control operation even when the output of the synchronization separation circuit 6 contains extra pulses other than the synchronization signal. This provides an excellent automatic gain control device.

Structure of the Invention The present invention detects the peak by making the reference pulse supplied to the adder circuit or the leading edge of the output of the synchronous separation circuit within a certain period d5, smaller than the normal magnitude, or not being supplied with fjL. The above object is achieved by preventing abnormally large signals from being supplied to the circuit.

DESCRIPTION OF THE EMBODIMENTS FIG. 3 shows a block diagram of an embodiment of the automatic gain system according to the present invention. In Fig. 3, a composite video signal is applied to the input terminal 1, passed through the gain control circuit 2 and taken out to the output terminal 3, and after the potential at the tip of the synchronization signal is clamped by the clamp circuit 4, the synchronization separation circuit 5 and an adder circuit 6. The output of the synchronization separation circuit 5 is supplied to the d-gate circuit 9 and also to the monomulti 10, which is triggered by the front of the input signal and outputs a predetermined width pulse, which opens and closes the gate circuit 9. do. The output of the gate circuit 9 is supplied to a delayed pulse generation circuit 7, which generates a reference pulse of a predetermined level value delayed by a predetermined time, and is supplied to the adder circuit 6. In the adder circuit 6, the reference pulse is added to the horizontal blanking portion of the composite video signal, and this output is supplied to the peak detection circuit 8. For example, a DC voltage corresponding to the peak value is supplied to the gain control circuit 2 and output. The level of the video signal taken out to the terminal 3 is automatically controlled.

In the automatic gain control device having the configuration as described above, if a composite video signal containing a noise pulse a as shown in FIG. 4A is supplied to the input terminal 1, the synchronization separation circuit 5 may malfunction. Then, 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 with a predetermined width T1 triggered by the leading edge of the waveform shown in FIG. 4B is obtained from the output of the monomulti 10 and is supplied to the gate circuit 9. The gate circuit 9 is
When the output of the monomulti 10 is at a high level, the input is output as is, and when the output of the monomulti 10 is at a low level, the input is not output.

Here, since the width of the extra pulse b actually included in the output of the synchronization separation circuit 6 is mostly a relatively narrow pulse of 1 μ♡C or less, the width T of the output pulse of the monomulti 10 is
1 is the width of the horizontal synchronizing signal: for example, 3μ5tIC, so that it is somewhat narrower and wider than 1μ, the extra pulses are removed from the output of the gate circuit 9 as shown in FIG. 4F. 7j signal? 1-1 delay time T2, which is supplied to the delayed pulse generation circuit 7 to generate a reference pulse of a predetermined level value delayed by a predetermined time T2 as shown in FIG. is set to approximately 21, for example, so that it is shorter than the width of the bank pouch in the horizontal blanking portion. In the adder circuit 6, a reference pulse as shown in FIG. 4G is added to the horizontal blanking portion of the composite video signal, and the reference pulse as shown in FIG.

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 synchronization 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 to delay the input signal by a predetermined amount of time, but instead, only the trailing edge of the input signal is delayed by a predetermined amount of time. The same effect can be obtained.

Other embodiments are shown in FIGS. 5 and 6. Figures 5 and 6
In the figure, parts similar to those in FIG. 3 are designated by the same numbers.

The difference between FIG. 5 and FIG. 3 is that the output of the synchronization separation circuit 5 is connected to a delay pulse generation circuit 7, and a reference pulse of a predetermined level value is generated at a predetermined delay time to generate a gate circuit 9. monomulti 10 which is a pulse of a predetermined width triggered by the leading edge of the output of the sync separator circuit 5.
The only difference is that the relatively narrow extra pulse contained in the reference pulse is removed by opening and closing the gate circuit 9 using the output of the reference pulse, and then the pulse is supplied to the adder circuit.
It can be easily inferred that the same effect as the example shown in FIG. 3 can be obtained.

Next, the parts in Figure 6 that are different from Figure 11 are as follows:
The monomulti 10 is triggered by the leading edge of the output of the delayed pulse generating circuit 7, and the effect is exactly the same as the example shown in FIG. 5, that is, the same as the example shown in FIG. 3. It can be easily inferred that the effect can be obtained.

Effects of the Invention As described above, according to the present invention, the synchronization separation circuit 5 malfunctions due to noise pulses included in the composite video signal, and extra pulses other than the synchronization signal are sent to the output with a relatively simple configuration. It is possible to realize an excellent automatic gain control device that can perform stable gain control operations without abnormally suppressing the output signal even in cases where this occurs.

[Brief explanation 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 its operation, and FIG. 3 is a block diagram showing an example of an automatic gain control device according to the present invention. , FIG. 4 is a waveform diagram for explaining its operation, and FIGS. 6 and 6 are block diagrams showing other embodiments 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..synchronous separation circuit, 6..-addition circuit, 7..delay pulse generation circuit, 8
・ ・Peak detection circuit, 9... Gate circuit, 10 ・
-Mono multi. Name of agent: Patent attorney Toshio Nakao and one other person
1 Figure 2 Figure 3riA Figure 4 Figure 5

Claims (1)

[Scope of Claims] 1) a gain control circuit that controls the gain of a composite video signal; a sync separation circuit that extracts a synchronization signal from the composite video signal; first and second delay circuits having a second delay time; when the convergence of the sync separation circuit output signal is wider than the first delay time, a delayed pulse generation circuit that generates a reference pulse delayed by a delay time; a mixing circuit that mixes the reference pulse with the gain control circuit output signal so that the polarity is opposite to that of the synchronization signal; An automatic gain control device comprising: a peak detection circuit that controls the gain of the gain control circuit accordingly. 2) a first delay circuit in which the delay pulse generation circuit generates an output signal for a first delay time from the front of the synchronous separation circuit output signal; and a period in which the first delay circuit generates an output;
a gate circuit that makes the magnitude of the synchronous separation circuit output signal smaller than a normal magnitude and outputs it or prevents it from being output; and a reference that delays the trailing edge of the gate circuit output signal by a second delay time. 2. The automatic gain control device according to claim 1, further comprising a second delay circuit that generates a pulse. 3) A delay pulse generation circuit includes a first delay circuit that generates an output signal for a first delay time from the leading edge of the synchronous separation circuit output signal, and a second delay time for the trailing edge of the synchronous separation circuit output signal. a second delay circuit that generates a signal delayed by
During the period in which the first delay circuit generates an output, the second delay circuit output signal is output with a magnitude smaller than a normal magnitude or is not output, thereby generating a reference pulse. The automatic gain control device according to claim 1, comprising a gate circuit. 4) The delay pulse generation circuit includes a second delay circuit that generates a signal obtained by delaying the trailing edge of the output signal of the sync separation circuit by a second delay time; a first delay circuit that generates an output signal for a delay time of
and a gate circuit that generates a reference pulse by making the magnitude of the delay circuit output signal smaller than the normal magnitude and outputting it, or by not outputting it. Gain control device.