JPH05176253A - Agc circuit - Google Patents

Abstract

PURPOSE:To provide an AGC circuit applying AGC accurately to a video signal of AM positive modulation by using the AGC circuit applying head type AGC to an AM negative modulation video signal. CONSTITUTION:The AGC circuit consists of an amplifier 1 amplifying an AM positive modulation video signal, a comparator 3 receiving a detection signal obtained by detecting an output of the amplifier 1 at its noninverting terminal and applying a voltage corresponding to the output to the amplifier 1 as an automatic gain control voltage and reference voltage generating circuits TR, R3-R6, C connecting to an inverting terminal of the comparator 3 and generating a reference voltage to a pedestal portion of the video signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AGC circuit for AGC (automatic gain control) of an AM (amplitude modulation) positive modulation video signal.

[0002]

2. Description of the Related Art FIG. 4 shows a conventional AGC circuit for performing AGC-based negative-modulation video signals as a lead value type AGC. That is, in the television receiver, the AM negative modulation video signal S as shown in FIG.
1 is input to the intermediate frequency amplifier 1, amplified, and output to the detector 2. This detector 2 receives the A input from the amplifier 1.
The M negatively modulated video signal is detected to output a detection signal S2 as shown in FIG. 6, and a part of the detection signal S2 is supplied to the non-inverting terminal of the comparator 3. This detection signal S2
The DC level of fluctuates depending on the electric field strength, and when the electric field strength is strong, the DC level decreases. The inverting terminal of the comparator 3 is supplied with a reference voltage V1 as shown in FIG. 6, which is obtained by dividing the power supply voltage V ₀ by the resistors R1 and R2. The comparator 3 uses the reference voltage V as the leading end level of the horizontal synchronizing signal of the detection signal S2 input from the detector 2, that is, the leading value level.
1 and the difference is passed through the low-pass filter 4 to A
The AGC is performed by feeding back to the amplifier 1 as a GC voltage. As described above, the AGC voltage is changed by observing the fluctuation of the leading value level of the horizontal synchronizing signal which does not vary depending on the signal level but changes only by the electric field strength. By increasing the electric field, the leading value level of the horizontal synchronizing signal decreases. , When the electric field is reduced, the opposite is true.

[0003]

However, when an AGC circuit of AM positive modulation as shown in FIG.
The detection signal S4 of the positively modulated video signal is, as shown in FIG.
The white level of the video signal becomes the leading value, and the AGC voltage fluctuates depending on the signal level, so that there is a drawback that correct AGC cannot be performed.

The present invention has been made in view of the above-mentioned circumstances, and is an AGC for performing a leading-value type AGC for an AM negative modulation video signal.
An object of the present invention is to provide an AGC circuit capable of accurately AGCing an AM positive modulation video signal by using the circuit.

[0005]

In order to solve the above problems, the present invention provides a pedestal of a video signal to an inverting terminal of a comparator to which a signal obtained by detecting an AM positive modulation video signal is supplied to a non-inverting terminal. By connecting a reference voltage generating circuit for generating a reference voltage to a part, an AGC circuit for performing AGC for an AM negative modulation video signal is used,
This is so that the AGC of a positively modulated video signal can be accurately performed.

[0006]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 relates to an embodiment of the present invention and shows an AGC circuit for AGC of an AM positive modulation video signal. That is, in the television receiver, the AM positive modulation video signal S3 as shown in FIG. 2 is input to the intermediate frequency amplifier 1, amplified, and output to the detector 2. The detector 2 detects the AM positive modulation video signal input from the amplifier 1 and outputs a detection signal S4 as shown in FIG. 3 and also supplies the detection signal S4 to the non-inverting terminal of the comparator 3. The inverting terminal of the comparator 3 is grounded via the resistor R3 and is connected to the power supply voltage V ₀ via the resistor R4. The inverting terminal of the comparator 3 is connected to the power supply voltage V ₀ via the variable resistor R5 and also connected to the collector of the transistor TR, and the emitter of the transistor TR is grounded. The base of the transistor TR is connected to a terminal T via a speed-up capacitor C and a resistor R6 in parallel.
Connected to. This transistor TR and resistors R3 to R6
And the capacitor C constitutes a reference voltage generating circuit,
When the pulse signal S5 generated at the burst timing is input to the terminal T, the inverting terminal of the comparator 3 is supplied with the reference voltage S6 having a pulse waveform. Since the reference voltage S6 is based on the burst timing, as shown in FIG. 3, the reference voltage is generated only in the pedestal portion of the detection signal S4 and becomes 0V in the other detection signal S4 portions, and the reference voltage S6 is constantly detected. It shows a value lower than the voltage of S4. This comparator 3 is a detector 2
The pedestal portion of the detection signal S4 input from the above is compared with the reference voltage S6, and the difference is fed back to the amplifier 1 as the AGC voltage through the low pass filter 4 to perform the AGC. In this way, the AGC voltage is changed by observing the fluctuation of the pedestal portion of the detection signal S4 that does not change with the signal level but changes only with the electric field strength. By increasing the electric field, the level of the pedestal portion of the detection signal S4 decreases. , When the electric field is reduced, the opposite is true. Therefore, the AGC voltage is obtained in proportion to the DC level of the sync head value of the sync cycle, and can be configured by using the conventional AGC circuit that performs AGC for the negative-modulation video signal of the head value type.

As described above, the AGC circuit for the AM positive modulation video signal can be easily realized by using the conventional leading value type AGC circuit as it is. This makes it possible to realize sharing of the AGC circuit by a multi-system such as an AM negative modulation system and an AM positive modulation system.

[0009]

As described above, according to the present invention, the reference voltage for generating the reference voltage at the pedestal portion of the video signal is supplied to the inverting terminal of the comparator which detects and supplies the AM positive modulation video signal to the non-inverting terminal. An AGC that performs AGC for an AM negative modulation video signal by connecting a voltage generation circuit
A circuit can be used to accurately AGC an AM positive modulation video signal.

[Brief description of drawings]

FIG. 1 is a structural explanatory view showing an embodiment of the present invention.

FIG. 2 is a waveform diagram showing an example of an AM positive modulation video signal.

FIG. 3 is a characteristic diagram showing an example of the operation of the comparator according to the present invention.

FIG. 4 is a configuration explanatory diagram showing a conventional AGC circuit.

FIG. 5 is a waveform diagram showing an example of an AM negative modulation video signal.

FIG. 6 is a characteristic diagram showing an example of an operation of a comparator related to a conventional AGC circuit.

[Explanation of symbols]

1 ... Amplifier, 2 ... Detector, 3 ... Comparator, 4 ... Low-pass filter, TR ... Transistor, R1-R6 ... Resistor, C ... Capacitor.

Claims (1)

[Claims] 1. An amplifier for amplifying an AM positive modulation video signal, and a detection signal obtained by detecting the output of the amplifier is supplied to a non-inverting terminal, and a voltage corresponding to the output is supplied to the amplifier as an automatic gain control voltage. And a reference voltage generation circuit connected to the inverting terminal of the comparator to generate a reference voltage in the pedestal portion of the video signal.