JP2503023Y2 - AGC circuit for video intermediate frequency signal - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to an AGC circuit for a video intermediate frequency signal installed inside a ghost removing device having a ghost removing function, a television receiver, or the like.

[Prior Art] In television receivers and the like, the gain of the receiver is automatically adjusted according to the strength of the received radio wave in an amplification / detection system composed of a video intermediate frequency amplification circuit and a video detection circuit in the subsequent stage. An AGC (automatic gain control) function is added to control and always obtain a constant video detection output.

In this AGC, the peak value type that uses the peak value of the composite video signal including the synchronization signal, the average value type that uses the average value, and the horizontal flyback pulse (retrace pulse) is used to extract only the synchronization signal portion and its peak. There is a keyed type that uses a value, but the peak value type AGC (peak AGC) is mainly used.

In a television video signal amplification / detection system to which peak AGC is applied, as shown in FIG. 4, the video intermediate frequency signal output from a tuner (not shown) in the preceding stage is transmitted between the band pass filtering circuit (BPF) 31 and the video intermediate signal. The signal is supplied to the video detection circuit 33 through the frequency amplification circuit (VIF) 32, and the detected video signal output from the video detection circuit 33 is supplied to a video amplification circuit in the subsequent stage (not shown). The peak AGC error signal detection circuit 34 detects the peak value of the video signal after detection output from the video detection circuit 33, and outputs a value obtained by subtracting a predetermined control target value from this as a peak AGC error signal. The low frequency component of the peak AGC error signal is supplied to the gain control terminal of the video intermediate frequency amplification circuit 32 through the loop filter 35 to control its amplification gain. The control of the amplification gain is performed so that the peak value of the output of the video detection circuit 33 approaches the predetermined control target value held in the peak AGC error signal detection circuit 34.

In addition to the above, the amplification gain of the video intermediate frequency amplification circuit 32 may be controlled so that the peak value of the output of the video amplification circuit (not shown) approaches a predetermined control target value.

[Problems to be Solved by the Invention] When a ghost elimination function is added to a television receiver including the peak AGC circuit shown in FIG. 4, the following problems occur.

That is, as shown in FIG. 5, the waveforms of the equalizing pulse and the vertical synchronizing signal within the vertical blanking period are as shown in waveform (A) if there is no ghost. However, this signal waveform is shown as a waveform after video detection input to the peak AGC error signal detection circuit 34, and the polarity is inverted with respect to the video intermediate frequency signal under negative modulation. On the other hand, in the positive ghost component, as illustrated in the waveform (B), a signal having a smaller amplitude than the original signal (A) and having the same polarity appears with some delay. The actual waveform in which the positive ghost component (B) is superimposed on the original signal (A) is (C)
become that way. In this case, the control target value of the peak AGC is L
If it is set to, the spike-like level change β based on the AGC operation that attempts to bring the step-like level increasing portion α generated at the leading edge of the vertical synchronizing signal due to the ghost toward the control target value L Occurs.

Therefore, the distortion of the leading edge portion of the vertical synchronizing signal generated by the ghost and the peak AGC is difficult to detect a correct stepped ghost by the method of utilizing the leading edge portion for detecting the ghost, and the ghost is removed. It is one of the direct factors that deteriorate the function.

Further, in the method of detecting a ghost by using a predetermined ghost elimination reference signal (GCR) inserted at a predetermined position within the vertical blanking period, not at the leading edge portion of the vertical synchronization signal, the ghost detection is performed. As long as there is no problem with peak AGC. However, this method cannot eliminate the spike-like level change (waveform distortion) β caused by the ghost and the peak AGC, which is one of the factors that make the sync separation operation of the subsequent display system unstable.

Therefore, an object of the present invention is to provide an AGC circuit for a video intermediate frequency signal, which causes less waveform distortion.

[Means for Solving the Problems] Accordingly, the present invention provides the first object to achieve the above-mentioned object.
The present invention is an AGC circuit for a video intermediate frequency signal installed in a device for removing a ghost component from a television signal, and an AGC circuit for extracting an AGC error signal from a signal after video detection.
The error signal detection circuit and the AGC error signal are input, and the horizontal and vertical sync signals are separated from the signal after the video detection to determine the frequency interleave relationship to determine the keyed AG.
An AGC mode control unit for switching between C and peak AGC, and a loop filter for inputting an output signal of the AGC mode control unit and supplying a low frequency component thereof to a gain control terminal of a video intermediate frequency amplification circuit are provided.

Furthermore, in the second aspect of the present invention, the AGC mode control unit has a relationship between a horizontal and vertical sync separation circuit that separates a horizontal and vertical sync signal from the signal after the video detection and a frequency interleave relationship from the horizontal and vertical sync signal. It is provided with a horizontal and vertical sync coincidence / non-coincidence determination circuit for judging, and a mode switching circuit for controlling the AGC error signal by an output signal from the horizontal / vertical sync coincidence / non-coincidence determination circuit and the vertical sync separation circuit. It is a thing.

[Embodiment] Next, an embodiment of the present invention will be described with reference to FIGS.

FIG. 1 is a block diagram showing the configuration of an image amplification / detection system including an AGC circuit for an image intermediate frequency signal according to an embodiment of the present invention, where 1 is a band pass filtering circuit (BPF) and 2 is an image intermediate signal. Frequency amplification circuit (VIF), 3 is video detection circuit, 4 is A
GC error signal detection circuit, 5 is AGC mode control unit,
6 is a loop filter.

FIG. 2 is a block diagram showing an example of the configuration of the AGC mode control unit 5 of FIG. 1, 21 is a horizontal sync signal separation circuit for separating the horizontal sync signal from the video signal after video detection, and 22 is a vertical sync signal. Vertical sync signal separation circuit that separates signals, 23 is a horizontal sync match / mismatch circuit that detects whether the frequency interleave relationship is maintained from the separated horizontal sync signal, and 24 is the frequency interleave relationship is maintained from the vertical sync signal A vertical sync coincidence / disagreement circuit for detecting whether or not there is a gate, 25 is a gate circuit, and 26 is a mode switching circuit.

In the figure, a video intermediate frequency signal output from a tuner (not shown) in the preceding stage is supplied to a video detection circuit 3 through a band pass filtering circuit 1 and a video intermediate frequency amplifying circuit 2.
The detected video signal (composite video signal on which the synchronization signal is superimposed) is supplied to a ghost removing circuit (not shown) in the subsequent stage, and also to the AGC error signal detection circuit 4 and the AGC mode control unit 5.

The AGC error signal detection circuit 4 detects the peak value of the video signal and supplies a value obtained by subtracting a predetermined control target value from this peak value to the AGC mode control unit 5 in the subsequent stage as a peak AGC error signal.

The AGC mode control unit 5 determines the frequency interleaving relationship from the video signal after detection, and if the relationship is maintained, it is in the keyed AGC mode, and if the relationship is not maintained, it is horizontal except the horizontal blanking period. During the scanning period, the AGC error signal is controlled so as to be in the normal peak AGC mode in which the AGC operation is not performed, and is supplied to the loop filter 6 in the subsequent stage.

The loop filter 6 supplies the low frequency component of the AGC error signal to the gain control terminal of the video intermediate frequency amplifier circuit 2 to control this amplification gain. The control of the amplification gain is performed so that the peak value of the output of the video detection circuit 3 approaches the predetermined control target value held in the AGC error signal detection circuit 4.

As a result, when the frequency interleaving relationship is maintained, the actual waveform (C) in which the positive ghost component is superimposed on the original signal shown in FIG. 5 by the keyed AGC is the same as the intermediate frequency amplifier circuit and the video detection circuit. Output from
It is supplied to a ghost removing circuit (not shown) in the subsequent stage, and it becomes possible to accurately detect or erase the ghost. This is a keying pulse in which only the horizontal sync signal is extracted from the signal after video detection, and the AGC voltage proportional to its amplitude is obtained. As shown in FIG. 3D, the horizontal scanning period other than the horizontal blanking period. Is because AGC operation is not performed.

3A shows the equalizing pulse and the vertical synchronizing signal in the video signal, and FIG. 3C shows the waveform showing the signal output from the vertical synchronizing separating circuit 22 in the AGC mode control unit 5. .

If the frequency interleaving relationship is not maintained, the peak AGC mode is automatically switched to prevent the disadvantage of the keyed AGC that occurs in such a state.
In other words, switching to peak AGC makes accurate detection and erasure of ghosts difficult, but more than that,
When the frequency interleave relationship is broken, the position at which the output signal from the vertical sync separation circuit 22 appears changes, so that the AGC operation is performed during the horizontal scanning period other than the horizontal blanking period. It is possible to prevent vertical sticking caused by instability and instability of the position where the vertical sync signal from the vertical sync separation circuit 22 appears. The peak AGC is controlled by the synchronizing signal from the vertical synchronizing signal separation circuit 22 so that the operation does not stop and the waveform distortion shown in FIG. 5D does not occur during the vertical blanking period.

The AGC mode control unit 5 determines whether the frequency interleaving relationship is maintained or not by detecting the video signal after video detection by the horizontal and vertical sync separation circuits 21 and 22. And the horizontal and vertical sync signals are judged by the horizontal sync match / mismatch circuit 23 and the vertical sync match / mismatch circuit 24 in the subsequent stage depending on whether both horizontal and vertical sync appear in a certain cycle. .

The AGC mode control circuit 5 is composed of a digital circuit and performs vertical sync separation by digital, but this vertical sync separation circuit can be shared with a conventional vertical sync separation circuit.

As described above, the AG that controls the amplification gain of the video intermediate frequency amplification circuit so that the output of the video detection circuit approaches the predetermined target value.
The case of the C circuit is shown. However, the present invention can also be applied to an AGC circuit configured to control the amplification gain of the video intermediate frequency amplification circuit so that the output of the video amplification circuit installed after the video detection circuit approaches a predetermined target value. .

[Effects of the Invention] As described above, the present invention determines the relationship of frequency interleaving from the video signal after video detection, and determines the keying.
Since the AGC mode control unit that switches to AGC or peak AGC is installed, it is possible to accurately detect or eliminate the ghost.

[Brief description of drawings]

FIG. 1 shows a video intermediate frequency signal according to an embodiment of the present invention.
FIG. 2 is a block diagram showing the configuration of an image amplification / detection system including an AGC circuit, FIG. 2 is a block diagram showing an example of the configuration of the AGC mode control unit of FIG. 1, and FIG. 3 is a waveform explaining the operation of FIG. 4 and 5 are block diagrams showing the structure of a conventional video amplification / detection system including an AGC circuit for a video intermediate frequency signal, and FIG. 5 is a waveform diagram for explaining the problems of the conventional technology. 1,31 …… Band pass filter circuit, 2,32 …… Video intermediate frequency amplification circuit, 3,33 …… Video detection circuit, 4,34 …… AGC error signal detection circuit, 5 …… AGC mode control section, 6 , 35 ... Loop filter, 21 ... Horizontal sync signal separation circuit, 22 ... Vertical sync signal separation circuit, 23 ... Horizontal sync match / mismatch circuit, 24 ... Vertical sync match / mismatch circuit, 25 ... Gate circuit , 26 …… Mode switching circuit.

Claims (2)

(57) [Scope of utility model registration request] 1. An AGC circuit for a video intermediate frequency signal installed in a device for removing a ghost component from a television signal, the AGC error signal detecting circuit extracting an AGC error signal from a signal after video detection, and the AGC. An AGC mode control unit for inputting an error signal and separating the horizontal and vertical synchronization signals from the signal after the video detection to judge the relationship of frequency interleaving and switching to the keyed AGC and peak AGC, and an output signal of the AGC mode control unit And a low-frequency component that supplies the low frequency component to the gain control terminal of the video intermediate frequency amplifier circuit, and an AGC circuit for the video intermediate frequency signal. 2. The AGC mode control unit includes a horizontal / vertical sync separation circuit for separating a horizontal / vertical sync signal from the signal after the video detection, and a horizontal / horizontal for judging a frequency interleave relationship from the horizontal / vertical sync signal. And a vertical sync match / mismatch discrimination circuit, and a mode switching circuit for controlling the AGC error signal by output signals from the horizontal and vertical sync match / mismatch discrimination circuits and the vertical sync separation circuit. An AGC circuit for a video intermediate frequency signal according to claim 1.