JPH04207371A - Automatic gain control device - Google Patents

Abstract

PURPOSE:To obtain stable gain control by mutually comparing only synchronizing signal parts not to be changed by the contents of video signals to be compared out of the video signals. CONSTITUTION:A video signal outputted from a video signal detector 10 is supplied to a waveform equalizer 20 and a 1st clamp circuit 80, the video signal inputted to the equalizer 20 is equalized at its waveform and inputted to the 2nd clamp circuit 90 through a voltage control type amplifier 30. Only the synchronizing signal levels out of video signals to be compared are mutually compared by using the output of a comparator for comparing the outputs of two detection circuits 40, 50 for respectively detecting the synchronizing signal levels of signals obtained by clamping an input signal and an output signal at the same voltage. Thereby even when there exists a change in amplitude due to the contents of video signals or a time difference between compared signals, stable gain control can be executed.

Description

[Detailed Description of the Invention] E-Industrial Application Field] The present invention relates to an automatic gain control circuit, and in particular, when a waveform equalization operation is performed on an input signal, the signal level changes before and after the operation. This invention relates to an automatic gain control circuit that prevents this from occurring.

(Prior Art) Automatic gain control circuits that maintain a constant level of an output signal regardless of fluctuations in the level of an input signal are commonly used in electronic equipment, such as the high-frequency amplification stage of a tuner or It is used in intermediate frequency amplification stages, etc. In conventional automatic gain control circuits, if there is a distortion component in the input signal, the output level will be different from the original level. When waveform equalization is performed to remove distortion components contained in the output signal, the signal level after waveform equalization changes depending on the distortion components.This is called ghosting in the television receiver. Confirm the explanation of the case where wave effects are created using a removal device.

When a video signal, which is an input signal to a television receiver, has transmission distortion, a ghost appears on the reproduced image. A method is known in which this ghost is removed by automatically controlling the tap coefficients of the transvaal filter.
The figure shows the approximate east of the mountain.

The video detector 10 has an automatic gain control circuit 101 inside, and operates so that the video signal output is constant and becomes a bell based on the signal before waveform equalization.

The removal device 102 includes a lance bar filter 103. Adder 104. Comparator 105. Basic signal generator 106. The video signal from the video detector 10 is transmitted to the comparator 10.
5, the reference signal generated by the reference signal generator 106 is compared with the reference signal to detect an error. Since this error is a ghost component, the tap coefficient control circuit is configured to obtain No. 43 using the transparser and +1 filter 3 to eliminate this error]
07 controls the taps of the transmission versano 1 filter 103. Transbar Lee-jl Filter 1 () 3
An adder adds the correction signal for eliminating the error obtained through the above and the image a51, f, -1 of the image detector 10.
By adding W by 04 (11-) to remove the strike component, video No. 45 is obtained. As described above, the video signal output of the video detector 10 has the above waveform, etc. Based on the signal before conversion, that is, the video signal containing ghosts, the video output I2 is controlled to a constant value, so that the output signal of the adder 04 is It changes depending on the level (j, 1 to 11 of the goals 1 to 1. Automatic gain control circuit 115 consisting of a classical circuit 111, a comparator 112, and a low-pass filter 113
is considered. The video output signal from the adder 1 ( ) 4 is input to a voltage controlled amplifier 114 . Here, the gain-controlled video signal and the video signal output of the video detector 10 are (1st 41, clamp circuit Ill, 109
Then, the comparator 112 compares the amplitudes between the two, and outputs an output according to the difference in amplitude. □7 This output is 1] - Accordingly, the video signal whose waveform has been equalized by the ghost removal device 102 is inputted as a control signal of the 1viTL control type amplifier 114 according to the bus filter 113. The gain is controlled to be at the same level as the video signal output level of the video detector 10.

[Problem to be solved by the invention]

The above conventional technology directly compares the levels of video signals, so changes in the video signal level due to changes in video content,
When removing front ghosts etc. using a waveform equalizer, consideration must be given to the time difference that occurs between the output of the video detector and the output of the waveform equalizer, that is, the two comparison signals for performing gain control. First, when performing gain control, there is a problem that the operation becomes unstable due to these effects.

The present invention aims to solve the above problems and perform stable gain control. 2. [Means for Solving the Problems] In order to achieve the above objects, a comparative We focused on and compared only the synchronization signal part that does not change depending on the video content of the video signals, and decided to perform automatic gain control. It was designed to look like this.

(Operation) In this invention, the comparison signal for performing gain control is
By clamping the input signal and the output signal to the same voltage and comparing the outputs of the two detection circuits and the rain detection circuit that respectively detect the synchronous No. 18 level of both signals, the comparison can be made by using the output of the comparator. Only the synchronization signal level of the desired video signal is compared. Therefore, stable gain control can be performed even when the amplitude changes depending on the video content or when there is a time difference between comparison signals.

〔Example〕

Hereinafter, embodiments of the present invention will be explained with reference to FIG. The video signal output from the video detector 10 is supplied to the waveform equalizer 20, and after color signal components and noise components are removed by a low-pass filter consisting of a resistor 71 and a capacitor 72, the video signal output is sent to the first classical circuit. 80. 1st
video detector 1 input terminal 75
A bus that turns on the h-run raster 81 only during the back porch period of 0 video signal output is human-powered. For this reason, synchronization 1. A signal is input to the signal detecting circuit 40 when the voltage 77 reaches the potential of the reference voltage 77. Condensing snow 4] 1, ζ charging is performed through the resistor 4:3 only during the period when the synchronization signal is input, and during the video period, it is discharged through the resistor 42. At this time, by taking a sufficiently long discharging time constant compared to the charging time constant, the potential of the emitter of the I/Rashi S and the emitter 43 (J, becomes almost equal to the voltage of the peak value of the synchronizing signal. Here, input The synchronization signal of the video signal v,1 is VREI + vll[-VFts = Vs+
(1) It is expressed as [ability]. Here V81. is the potential of the reference voltage 77, and V, □, is the base voltage of the transistor.
This is fIH-1 of the super community.

The video signal input to the waveform fixture 20 is subjected to waveform equalization, and then passed through a voltage controlled amplifier 30 to a resistor 7;
After color signal components and noise components are removed by a low-pass filter consisting of 74 and 74, the second Kuhn nib circuit 901 is input manually. The second multi-buff input terminal 76 is connected to the output of the voltage-controlled amplifier 30.
By inputting the bass so that the l-ran register 91 turns on/on only during the bassoi 7 vocal period of Noi@'q, the first The knob lamp circuit 90 of No. 2 and the synchronization/bell detection circuit 50 are activated (I L, and the output signal of the voltage control amplifier can be synchronized and output the bell vbl!. .2 VREl+ vsc %'Ell= Vll
(2) becomes. Here, 1,1 is the voltage at the emitter of the transistor 51. V R1 in formulas (1) and (2),
□l+■l11 is equal to 5, so V, -4+l and V
Using the comparator l(3l) means comparing the synchronization signal level of the video signal output of the video detector lO and the video signal output of the voltage-controlled amplifier 30.To do this, the synchronization level detection circuit 40 The outputs of
By controlling the gain by supplying it to the control terminal of 0, even if the amount of ghost components changes and the output 1z level of the waveform equalizer 20 changes, the output signal of the video detector 10 will not be the same as Beno 1. , the output signal level of the 1ltll eaves type amplifier 30/
l is the same as 1. Bell (can be played).

[Effects of the Invention] As explained above, the present invention is configured to control the gain based only on the synchronization signal level, so it is not affected by changes in the input video content or the waveform. It is possible to stably perform gain control without being affected by the time difference between comparison signals caused by equalization, and to make the video detection output and the signal level after waveform equalization the same level.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a pro-riff diagram showing a conventional example. 30・Voltage controlled amplifier, 40-・First synchronous level detection circuit, j) 0 Second synchronous 17 bell detection circuit 6(]・Comparator, 80・First Classical circuit, !10 Second Classical Circuit -,,,,,,+m, ,,,('7''', 1 hanging 1
Prisoner/θ 20 30

Claims (1)

[Claims] 1. A video detector that supplies a video detection signal, and a waveform equalizer that takes the video signal from this detector as an input signal and outputs a video signal from which waveform distortion, such as a ghost signal, is removed from this input signal. an amplifying means having an input terminal, an output terminal, and a gain control terminal, the input terminal of which is supplied with the video signal from the waveform equalizer, and the output terminal obtaining a gain-controlled output signal; The signal from the video detector and the output signal from the amplification means are input as comparison signals, and the reference levels of both comparison signals are made the same, and only the synchronization signal period of both comparison signals is compared. and a feedback means for controlling the gain by supplying the voltage from the comparing means to a gain control terminal of the amplifying means. control circuit.