JPH0225314B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an AGC method for a video signal that keeps the output level of a video signal containing disturbance constant.

FIG. 1 shows a system diagram of the conventional AGC method for video signals, and FIG. 2 shows waveforms showing its operating status. In Figure 1, IN is a video input terminal, 1 is a synchronous separation circuit, 2 is a clamp circuit, 3 is a variable gain amplifier, 4 is a switch circuit, 5 is a detection circuit, and 6 is a detection circuit.
Error amplifier for AGC, OUT is the video output terminal. Also, in FIG. 2, denotes an input video signal waveform, denotes a synchronization separation waveform, and denotes a delay waveform for clamping.

In Fig. 1, the input signal from the video input terminal IN is separated into a synchronization signal and an image signal (waveforms in Fig. 2) by the synchronization separation circuit 1, and this separated synchronization signal is delayed and output as shown in Fig. 2. Create a waveform. At this time, the clamp circuit 2 clamps the waveform of the input video signal with the waveform shown in FIG. 2, and fixes the pedestal signal section to the voltage of _VCL shown in FIG. The switch circuit 4 switches the input video signal (waveform shown in Figure 2) that has passed through the variable gain amplifier 3 with the signal with the waveform obtained by the sync separation circuit 1, resulting in the V _P signal shown in Figure 2. Remove the sync signal section. The detection circuit 5 detects the signal and detects the value of V _P shown in FIG. That is, here, since the signal of the pedestal section is fixed by the clamp circuit 2, the value of (V _CL - V _P ) is detected, and then compared with a value set in advance by the error amplifier 6, this (V _CL - The variable gain amplifier 3 is controlled so that the value of V _P ) is kept constant.
The value of V _CL −V _P ) is kept constant, so that even if the signal from the video input terminal IN fluctuates, the output signal from the video output terminal OUT does not change.

However, in reality, due to disturbances such as ghost interference, a signal with a delayed signal added to it, such as the waveform shown in Figure 2, may be input, and this can be processed using the same process described above.
When AGC processing is performed, an error signal of the magnitude Ve shown in the waveform of Figure 2 is generated, and the variable gain amplifier 3 is controlled with an excessive control signal, assuming that the input signal is smaller than the actual one, so the video output terminal There were problems such as the OUT signal being larger than the specified signal, making it impossible to use it for AGC circuits for video signals in areas where there is a lot of ghost interference.

The present invention was made to eliminate these drawbacks, and by averaging the AGC reference voltage, the AGC
This reduces the impact on Hereinafter, this will be explained in detail using figures.

FIG. 3 is a system diagram showing one embodiment of the present invention, and FIG. 4 is a waveform diagram showing its operating status.
In FIG. 3, 7 is a reference waveform detection circuit, 8 is a low-pass filter, 9 is a sample-and-hold circuit, and the others are the same as those shown in FIG. 1 with the same symbols. Also, in Fig. 4, the waveform of a part of the input video signal is a clamp waveform, the symbol is a switching waveform, the symbol is a waveform showing an example of an interference signal (delayed ghost), and the symbol is a waveform when the waveform is added to the waveform and ghost interference is included. This is the waveform of a portion of the input video signal.

In Fig. 3, when there is an input signal from the video input terminal IN, first, the sync signal and the image signal are separated by the sync separator circuit 1 as explained in the prior art section, and the waveforms are shown in Fig. 4. A clamp pulse is generated, and the clamp circuit 2 clamps the signal from the video input terminal IN with the clamp pulse from the sync separation circuit 1 created above, and the average value of the peak value of the sync signal is determined as V _CL shown in Figure 4. Fix the voltage to .
On the other hand, the reference waveform detection circuit 7 detects a section with no image from the synchronization signal obtained by the synchronization separation circuit 1, and generates a switching pulse shown in the waveform of FIG. Next, the switch circuit 4 switches the video signal that has passed through the variable gain amplifier 3 using the waveform control signal obtained by the reference waveform detection circuit 7, and converts the video signal into a waveform that is less susceptible to ghosting.
The amplified video signal is passed during HIGH sections A and B, and the pedestal portion is detected.

Here, if there is ghost interference as shown in the waveform of FIG. 4, the input video signal becomes as shown in the waveform (=+) of FIG. 4. Considering the operation at this time, the switch circuit 4 switches the signal shown in FIG. 4 with the output waveform from the reference waveform detection circuit 7, and the output waveform becomes It is possible to obtain a reference signal that is not available.

Next, the input video signal due to the influence of the ghost waveform delayed further than shown above becomes as follows.

In this operation, the output waveform of the switch circuit 4 corresponding to the above description is as follows. Further, this signal is sampled by the next sample-and-hold circuit 9 to form a waveform as shown in (in the figure, . . . indicates a hold period), and the influence of ghost interference is shown only in the shaded portion shown in .

Since this signal is further smoothed by the low-pass filter 8, it is possible to obtain a reference number that is less affected by ghost interference, so that a more accurate reference voltage V _a −V _CL can be determined. Next, the AGC error amplifier 6 controls the variable gain amplifier 3 so that the voltage (V _A - V _CL ) remains constant at a preset value, so the magnitude of the synchronizing signal component is kept constant. , video output terminal
For the OUT output signal, even if the signal from the video input terminal IN changes, the output signal level will not change because the sampled waveform will not be affected even if the signal includes ghost interference such as a waveform.

As detailed above, according to the present invention, since the average value of the synchronization signal peak and the average value of the pedestal section are compared, it is possible to configure an AGC circuit with little output level fluctuation even in response to various types of interference, and to eliminate ghost interference. This means that it can be used in AGC circuits for video signals in areas with a lot of noise, and can be expected to have excellent effects.

[Brief explanation of drawings]

Figure 1 is a system diagram of the conventional AGC method for video signals.
FIG. 2 is a waveform diagram showing its operating state, FIG. 3 is a system diagram showing an embodiment of the present invention, and FIG. 4 is a waveform diagram showing its operating state. IN...Video input terminal, 1...Sync separation circuit, 2...
Clamp circuit, 3... variable gain amplifier, 4... switch circuit, 5... detection circuit, 6... error amplifier for AGC,
7...Reference waveform detection circuit, 8...Low pass filter,
9...Sample hold circuit, OUT...Video output terminal.

Claims (1)

[Scope of Claims] 1. In an AGC method for video signals, means for detecting a portion where there is no image signal from a synchronization signal obtained from a series of input signals to create a switching pulse; and the synchronization signal. means for detecting a pedestal signal by switching the input signal clamped by the switching pulse using the switching pulse; and means for averaging the pedestal signal and extracting the value thereof, the average value of the synchronization signal and the pedestal signal being Detects the voltage difference between the voltage difference and the voltage difference, compares the voltage difference with a preset value to detect fluctuations in the input signal, and controls the voltage difference to maintain a constant value to control the output signal. A video signal characterized by being configured so that the level is always kept constant.
AGC method.