JPH031780A - Automatic gain control circuit - Google Patents

Abstract

PURPOSE:To solve a problem of AGC reply speed and gain fluctuation by stopping the operation of a peak detection circuit and a discharge circuit in a prescribed timing during the vertical blanking period. CONSTITUTION:A vertical oscillation circuit 7 is provided, which separates a vertical synchronizing signal from an input video signal and is oscillated in a prescribed timing for the vertical blanking period synchronously with the input video signal, a hold signal (e) is formed by the vertical oscillation waveform and stops a peak detection circuit 2 and a discharge circuit 4. Thus, the reply time of an AGC circuit depends on a peak detection capacitor and a discharge current of a discharge circuit and the AGC gain fluctuation in the vertical blanking period is improved and the problem of the AGC response time is solved.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an automatic gain adjustment circuit (AGC circuit "tfi") for video tape recorders, television receivers, etc.

Conventional & Techniques Figure 3 shows the conventional AGC circuit, and Figure 4 shows the image B shown in Figure 3. The operating waveform near the vertical synchronization signal of the signal is shown. 3rd
In the figure, the input video (a @a is input to the gain control circuit 1, the output signal of the gain control circuit 1 is input to the peak detection circuit 2, and the peak detection circuit W2
charges the capacitor 3 with a charge proportional to the maximum amplitude voltage of the video signal that is output from the output signal. The discharge circuit 4 is for discharging the capacitor 3. If the capacitor 3 is not discharged by the discharge circuit 4, the capacitor 3 will be charged to a voltage proportional to the maximum amplitude voltage of the video signal that is output from the output signal. There is no discharge 6.

Input image fg to gain control circuit 1! ia”3
When a is input, the gain control circuit 1 amplifies it by a constant R1j width of 8, and outputs an output signal S. The output signal S is simultaneously inputted to the peak detection circuit 2, and the video signal of the output signal S is peak-detected by the capacitor 3 and simultaneously converted into a DC voltage. The voltage of the capacitor 3 is compared with a reference voltage set inside the peak detection circuit 2, and the output signal C changes the amplification degree A of the gain control circuit 1 so that the difference between the two becomes smaller. That is, when the output amplitude of the gain control circuit 1 increases, the charging voltage d of the capacitor 3 increases, and as a result, the increase @ degree A of the gain control circuit 1 decreases, and conversely, the amplitude A of the gain control circuit 1 decreases. When the output J amplitude becomes small, the charging pressure d of the capacitor 3 decreases, and the amplification degree A of the gain control circuit 1 increases, so that the output amplitude of the gain control circuit 1 becomes constant. controlled.

Problem to be Solved by the Invention In FIG. 4, the input video signal a has a vertical synchronization signal period (
During the vertical blanking period), the video component disappears and only the synchronization signal remains. Therefore, the charging voltage d of the capacitor 3, which is converted into DC by the peak detection circuit 2 shown in FIG. 3, suddenly decreases, and the amplification degree A of the gain control circuit 1 also increases rapidly. As a result, even after the vertical blanking period ends, the amplification degree of the gain control circuit 1 remains high, so when a video component arrives, an output (n It becomes a problem.

As a method to alleviate such problems, there is a method of reducing the discharge current of the discharge circuit 4 shown in FIG. 3.

If the discharge current is made small, even if there is a vertical blanking period, the capacitor 3 in FIG. 3 can hold the peak value of the video signal. An ever-growing problem is alleviated.

However, in FIG. 3, if the discharge current of the discharge circuit 4 is made small, the discharge time of the capacitor 3 becomes very long when the input signal becomes small, and the response time of the AGC becomes long. A problem was occurring.

As described above, the conventional AGC circuit has a problem in that improving the gain fluctuation increases the AGC response time, and shortening the AGC response time increases the gain fluctuation after the vertical blanking period ends.

The present invention has been made to solve the above problems, and it is an object of the present invention to provide an excellent AGC circuit that does not have problems with AGC response speed and gain fluctuation.

Means for Solving the Problems In order to solve the problems of the conventional example described above, the AGC circuit of the present invention separates a vertical dosing signal from an input video signal, and synchronizes it with the input video signal during its vertical blanking period. A vertical oscillation circuit that oscillates at a predetermined timing is provided, a hold signal is generated from this vertical oscillation waveform, and the hold signal is configured to stop the peak detection circuit and the discharge circuit.

Effects With the above configuration, the response time of the AGC circuit can be determined by the peak detection capacitor and the discharge current of the discharge circuit, and the vertical blanking period can improve AGC gain fluctuations, while also solving the problem of AGC response time. can also be solved.

Example Hereinafter, one embodiment of Komei Ki will be described based on the drawings.

FIG. 1 is a block diagram of an automatic gain adjustment circuit according to an embodiment of the present invention, and FIG. 2 is an operational waveform of FIG. 1.

In FIG. 801, an input video signal a is input to a gain control circuit 1 and a vertical synchronization signal separation circuit 5.

The vertical synchronizing signal molecule l@M 5 detects the vertical synchronizing signal at a predetermined timing Vs (according to e in FIG. 2) within the vertical blanking period by the terminal 6 connected to it, and outputs the detected signal. is input to the vertical oscillation circuit 7. Circuit 7 starts vertically! , i This timing Vs changes the capacitor +J8 from the M i state to the charging state. The vertical oscillation circuit 7 charges and discharges the capacitor 8, and the voltage of the capacitor 8 becomes the voltage VH set inside the vertical oscillation circuit 7.
When reaching J, the vertical oscillation circuit 7 changes from a charging state to a discharging state. Therefore, the vertical oscillation circuit 7 performs oscillation in synchronization with the input video signal a, and outputs a signal e having a charging period t2 and a discharging period t1, as shown in e of FIG.

Here, t1+t2=vertical synchronization of the input video signal This output signal @e is a signal for stopping the peak detection circuit 2 and the discharge circuit 4, and during the period t2 shown in FIG. 2, the peak detection in FIG. Circuit 2 and discharge circuit 4
stops working.

Therefore, as shown in d of Fig. 2, even if the input video signal a enters the vertical blanking period, the voltage of the capacitor 3 in Fig. 1 does not drop significantly, and as shown in b of Fig. 2, Even after the vertical blanking period ends, the amplitude of the video signal does not increase at any time.

Effect of light As described above, according to the present invention, the operation of the peak detection circuit and the discharge circuit is stopped at a predetermined timing within the vertical blanking period, so that the AGC response, which has been a problem with conventional AGC circuits, is improved. An excellent AGC circuit is obtained that solves the problems of speed and gain variation.

[Brief explanation of drawings]

Figure 1 is a 70-step diagram of the AGC circuit of one embodiment of this Komei.
Figure 2 is an operating waveform diagram of the AGC circuit, and Figure 3 is the conventional AGC circuit.
The block diagram of the GC circuit, Figure m4, is an operating waveform diagram of a conventional AGC circuit. 1... Gain control circuit, 2... Peak detection circuit, 3... Capacitor, 4... Discharge circuit, 5...
- Vertical synchronization signal separation circuit, 7... Vertical oscillation circuit. Agent Yoshi Morimoto Buddhist painting 1st drawing 3rd drawing/

Claims (1)

[Claims] 1. A gain control circuit and a vertical periodic signal separation circuit into which a video signal is input, and detecting the voltage of a capacitor charged by the output signal of the gain control circuit, and outputting a signal to control the amplification degree of the gain control circuit. a discharge circuit that discharges the voltage of the capacitor; and a vertical oscillation circuit that receives the vertical synchronization signal separated by the vertical synchronization signal separation circuit and oscillates at a predetermined timing within the vertical blanking period. An automatic gain adjustment circuit configured to stop the operation of the peak detection circuit and the discharge circuit by a signal that oscillates at a predetermined timing during a vertical blanking period of the vertical oscillation circuit.