JPH11164226A - Agc circuit device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize an excellent video output by compensating a leakage current of an automatic gain control AGC capacitor to apply a stable AGC voltage to a video intermediate frequency amplifier in a keyed AGC system. SOLUTION: The device is provided with a video intermediate frequency AGC amplifier 1 whose gain is controlled by a voltage of an AGC capacitor 5 and amplifies a video intermediate frequency signal, a video intermediate frequency detector 2 that detects a video signal from an output of the video intermediate frequency AGC amplifier 1, a vertical synchronizing signal detector 2 that extracts a synchronizing signal from the video signal, and an AGC detector 3 that compares the video signal with a reference signal for a vertical synchronizing signal period of the video signal based on the vertical synchronizing signal, provides an output of an AGC signal to set a gain of the video intermediate frequency AGC amplifier 1 to the AGC capacitor 5 and provides an output of a very small current to compensate a leakage current at the end of the AGC capacitor 5, and the gain of the video intermediate frequency AGC amplifier 1 is made stable by compensating fluctuation in the AGC voltage due to discharge of the AGC capacitor for a period other than the vertical synchronizing signal with the very small current.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to an AGC (Auto
More particularly, the present invention relates to a circuit configuration of a keyed system using a vertical synchronization period in an AGC detection circuit of a video intermediate frequency amplification stage of a television receiver.

[0002]

2. Description of the Related Art Generally, a video intermediate frequency amplifying stage includes a variable gain amplifying section, and the gain of the video intermediate frequency amplifying section is automatically adjusted based on a gain control voltage supplied according to a change in an average detection voltage of the intermediate frequency amplifying section. Is controlled.

A circuit for this purpose is an AGC circuit device. Conventionally, an image signal obtained by detecting the output of a video intermediate frequency amplifier with a video intermediate frequency detector is AGC detected by an AGC detector. Based on the average value of the video signal, A
GC voltage was generated.

Since the conventional AGC circuit device is configured as described above, the AGC operates normally when there is a disturbance in the video signal period except for the horizontal retrace period and the vertical retrace period of the video signal. There is a problem that it does not.

On the other hand, a keyed system in which the level in the vertical blanking period is synchronously detected and the gain of the intermediate frequency amplification stage is controlled based on the signal is considered. According to such a method, a stable AGC can be realized because there is no influence of disturbance during the video signal period.

FIG. 2 is a block diagram of a keyed AGC circuit device constructed based on such a concept. As shown in the figure, a video intermediate frequency signal from a tuner (not shown) is amplified by a video intermediate frequency AGC amplifier 1, and a video signal is extracted by a video intermediate frequency detector 2 at the next stage. This video signal is sent to a signal processing circuit (not shown), decomposed into color signals, and displayed on a picture tube.
On the other hand, from this video signal, the vertical synchronization detector 4
The vertical synchronization signal is separated.

The video signal from the video intermediate frequency detector 2 is averaged detected by the AGC detector 3, but based on the vertical synchronization signal sent from the vertical synchronization detector 4, Only the detection output is output,
The signal is held by the AGC capacitor 5 until the next vertical synchronizing signal. The gain of the video intermediate frequency AGC amplifier 1 is automatically controlled based on the AGC detection signal held in the AGC capacitor 5.

With such a configuration, the video intermediate frequency AGC amplifier 1 can perform a stable AGC operation without being affected by disturbance during the video signal period.

[0009]

However, the keyed type AGC circuit device as shown in FIG. 2 is configured so that the AGC voltage is held by the AGC capacitor 5.
To perform stable operation, the AGC capacitor 5
Must be held stable for at least one vertical period. However, in reality, A
The charge stored in the GC capacitor 5 is discharged, and the AGC voltage output from the capacitor changes. For this reason, there is a problem that the gain of the video intermediate frequency AGC amplifier 1 changes in one vertical period, and as a result, sag occurs.

The present invention solves the above-mentioned problems of the prior art, and provides an AGC circuit device capable of supplying a stable AGC voltage to a video intermediate frequency amplifier and realizing good video output in a keyed system. The purpose is to:

[0011]

To achieve the above object, the present invention provides an amplifying means for controlling the gain of an AGC capacitor and amplifying a video intermediate frequency signal;
A video detection unit that detects a video signal from an output of the amplification unit; a synchronization detection unit that extracts a synchronization signal from the video signal from the video detection unit; and a vertical synchronization signal from the synchronization detection unit. In a first period which is at least a part of a vertical blanking period, an image signal is compared with a reference signal, an AGC signal for setting a gain of the amplifying means is output to the AGC capacitor, and the first period is output. An AGC circuit device comprising: an AGC control unit that outputs a small current for compensating for a leak current at an AGC capacitor end in at least a part of a second period other than the above period.

[0012]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a partial circuit block diagram of an AGC circuit device according to an embodiment of the present invention. In particular, FIG. 1 shows the configuration of an AGC detector (AGC control circuit) in the basic configuration of FIG.

As shown in the figure, a video signal from the video intermediate frequency detector 2 is input to a comparator 8. on the other hand,
The reference side of the comparator 8 is supplied with a constant reference voltage from a reference voltage generator 11. The comparator 8 includes a switch 7
The constant current source 6 via the switch 9 and the constant current source 1 via the switch 9
0 are respectively connected. The switches 7 and 9 are controlled by a keyed control circuit 12 which generates a control signal synchronized with the vertical synchronization signal from the vertical synchronization detector 4. Further, the constant current source 6 and the reference voltage generator 11
Are also synchronously controlled by the keyed control circuit 12.

Next, the operation of the above configuration will be described.

In the vertical blanking period, the video intermediate frequency detector 2
The video signal from does not contain video signal components,
Only the horizontal synchronization signal including the equalization pulse is included. During this time, when the vertical synchronization signal is input from the vertical synchronization detector 4, the comparator 8 compares the video signal from the video intermediate frequency detector 2 with the reference voltage from the reference voltage generator 11, and calculates the error. A voltage is output, and the voltage is output to the AGC capacitor 5 and held. This operation is controlled by the keyed control circuit 12 to which the vertical synchronization signal from the vertical synchronization detector 4 is input. Basically, the reference voltage generator 11 and the constant current source 6 are activated and the switch is activated. It is implemented by closing 7,9.

That is, the comparator 8 compares the activated voltage from the reference voltage generator 11 with the video signal in the vertical blanking period from the video intermediate frequency detector 2, and according to the error voltage, The AGC voltage is set to the AGC capacitor 5,
In this case, a charging current for determining the voltage of the AGC capacitor 5 is supplied from the constant current source 6 through the switch 7, and a discharging current is released from the switch 9 through the constant current source 10.

As described above, the keyed AGC operation is performed during the vertical synchronization period, and the AGC voltage is set in the AGC capacitor 5, so that the video intermediate frequency AGC amplifier 1
Subsequent gains are determined.

When the vertical synchronizing period has passed, the switch 9 is turned off, and the level of the reference voltage generator 11 is changed so that the output of the comparator 8 always becomes an output for charging the AGC capacitor 5.

On the other hand, the constant current source 6 is switched to a constant current value sufficient to compensate for the leakage current of the AGC capacitor 5, so that a very small charging current is supplied from the comparator 8 to the AGC capacitor 5. Is supplied, the leakage current causes AG until the next vertical retrace period.
Even if the C capacitor 5 is discharged, a current corresponding to the current is supplied and the voltage is kept constant.

As a result, the gain of the video intermediate frequency AGC amplifier 1 is kept constant.

Through the above operation, the discharge of the AGC capacitor 5 during one vertical period is caused by the AGC detector 3
Is compensated by the minute current supplied from the AGC capacitor 5 and a stable AGC output is output from the AGC capacitor 5.
Since it is provided to the amplifier 1, the gain of the video intermediate frequency AGC amplifier 1 is kept stable for one vertical period, and as a result,
The occurrence of sag due to gain fluctuation can be prevented.

In the above embodiment, the AGC operation of the AGC detector 3 is described as being performed while the vertical synchronizing signal is being input. However, the present invention is not limited to this. , May be performed during all or part of the vertical flyback period. The timing signal for this can be arbitrarily generated by the vertical synchronization detector 4 or the keyed control circuit 12 by a known configuration.

Further, in the above-described embodiment, a configuration in which a minute current is supplied to the AGC capacitor 5 in all periods other than the vertical synchronization period in order to compensate for a leak current of the AGC capacitor 5 has been described. , AGC detector 3 may be implemented during all or part of the period other than when keyed AGC operation is performed. Needless to say, the timing signal for this can be arbitrarily generated by the vertical synchronization detector 4 or the keyed control circuit 12 using a known configuration.

According to the above embodiment, it is possible to improve the sag and prevent disturbance during the picture period. Further, although peak detection can be adopted as an AGC detection method in SECAM positive modulation, it is particularly effective when performing peak detection with a CCIR signal of a SECAM signal.

[0026]

As described above, the AGC circuit device of the present invention detects the video signal during the vertical synchronization period, and sets the AGC voltage in order to determine the gain of the video intermediate frequency amplifier. In this configuration, the leakage current at the capacitor end holding the AGC voltage is compensated by supplying a very small current for one vertical period. Therefore, in addition to the effect of preventing disturbance in the video signal period by the keyed AGC, There is an effect that generation of sag due to voltage fluctuation is suppressed, and a stable video signal can be obtained based on a stable AGC operation.

[Brief description of the drawings]

FIG. 1 is a partial circuit block diagram of an AGC circuit device according to an embodiment of the present invention.

FIG. 2 is a block diagram of an AGC circuit device on which the present invention is based.

[Explanation of symbols]

 Reference Signs List 1 video intermediate frequency AGC amplifier 2 video intermediate frequency detector 3 AGC detector 4 vertical synchronization detector 5 AGC capacitor 6, 10 constant current source 7, 9 switch 11 reference voltage generator 12 keyed control circuit

Claims (5)

[Claims] An amplifier whose gain is controlled by a voltage of an AGC capacitor to amplify a video intermediate frequency signal, a video detector for detecting a video signal from an output of the amplifier, and a video signal from the video detector. A vertical synchronization detector that extracts a vertical synchronization signal; and, based on the vertical synchronization signal from the vertical synchronization detector, a video signal and a reference signal during a first period that is at least a part of a vertical blanking period of the video signal. In comparison, an AGC signal for setting the gain of the amplifier is output to the AGC capacitor, and the leakage current at the AGC capacitor end is reduced during at least a part of the second period other than the first period. An AGC circuit device comprising: an AGC control circuit that outputs a small current to be compensated. 2. The AGC circuit device according to claim 1, wherein said first period is a period during which a vertical synchronization signal is output, and said second period is a period during which a vertical synchronization signal is not output. . 3. The first period is a vertical retrace period,
2. The AGC circuit device according to claim 1, wherein the second period is a period other than a vertical blanking period. 4. The method according to claim 1, wherein the reference signal is switched between the first period and the second period, and an AGC signal based on an error between the video signal and the reference signal is output from the AGC control circuit during the first period. 4. The AGC circuit device according to claim 1, wherein a signal for charging said AGC capacitor is forcibly output during said second period. 5. The method according to claim 1, wherein a signal output to the AGC capacitor is switched to a small current sufficient to compensate for a leak current of the AGC capacitor during the second period. Any AGC circuit device.