JPH0714217B2 - Automatic gain control device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic gain control device such as an ACC (Automatic chroma control) circuit for controlling a gain of a color signal to be constant in a television, a VTR or the like.

2. Description of the Related Art In recent years, there has been an increasing demand for digital processing of video signals, and it is required to digitally realize an automatic gain control device such as an ACC circuit which has been conventionally analog processed. A conventional automatic gain control device will be described below with reference to FIG. This example is an apparatus for controlling the level of an input color signal to be constant. The color signal is converted into a digital signal by the A / D converter 12. The burst signal level of the converted color signal is detected by the burst level detector 14, compared with the reference level by the level comparator 15, and the difference is integrated by the integrator 16 to control the variable gain amplifier 13. As a result, the output level of the variable gain amplifier 13 is kept constant. This is a digital replacement of the analog ACC circuit, and an example of analog is shown in Television Engineering Handbook pp989.

However, in the above configuration, when the level of the color signal input to the A / D converter is smaller than the dynamic range of the input of the A / D converter, the output of the variable gain amplifier 13 is Although it is kept constant as described above, there is a drawback that the quantization noise increases because the number of bits assigned to the color signal decreases.

Means for Solving the Problems In order to solve the above problems, an automatic gain control device of the present invention includes a first variable gain amplifier for varying the amplitude of an input signal and an A / D conversion for A / D converting its output. , A second variable gain amplifier for varying the amplitude of the output of the A / D converter, and an amplitude detector for detecting the amplitude of the output of the second variable gain amplifier,
A first comparator that compares the output with a first reference value, a first integrator that integrates the output and controls the first variable gain amplifier, an output of the first integrator and a second The second comparator for comparing the reference values of 1 and the control circuit for controlling the first variable gain amplifier according to the output thereof.

Effect The present invention can obtain the automatic gain control device which does not increase the quantization noise of the signal by effectively utilizing the dynamic range of the A / D converter with the above-mentioned configuration.

Embodiments An automatic gain control device of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of the first embodiment of the present invention. In this embodiment, the input signal is a color signal including a burst signal. The color signal is amplified by the variable gain amplifier 1 and converted into a digital signal by the A / D converter 2. The converted color signal is amplified by the variable gain amplifier 3. The variable gain amplifier 3 can be realized by, for example, a multiplier. The burst level detector 4 detects the level of the burst signal output from the variable gain amplifier 3, and the level is compared with the reference level a by the level comparator 5. The integrator 6 integrates the output of the level comparator 5 to control the variable gain amplifier 3. As a result, the burst level of the output of the variable gain amplifier 3 becomes equal to the reference level a. In the above operation, when the input level of the variable gain amplifier 3 is lowered, that is, when the A / D input level is lowered, the gain of the variable gain amplifier 3 is increased in order to keep the output level constant. If the output of the integrator 6 when the optimum color signal for the dynamic range is input to the A / D converter 2 is x, the level of the color signal input to the A / D converter 2 has decreased. When the output of the integrator 6 becomes larger than x,
On the contrary, when it increases, it becomes smaller than x. Therefore, if the output of the integrator 6 is x, it means that the optimum color signal for the dynamic range is input to the A / D converter 2. Therefore, if the output of the integrator 6 is compared with the reference level b by the level comparator 7, and the reference level b is taken as x above, the control circuit 8 controls the variable gain amplifier 1 so that the compared output becomes zero. , A / D converter 2 can be supplied with a color signal most suitable for its dynamic range. The control circuit 8 comprises a D / A converter 9 for D / A converting the output of the level comparator 7 and an integrator 10 for integrating the output.

In this embodiment, there are two feedback loops, a digital control loop that feeds back from the burst level detector 4 to the variable gain amplifier 3 and an analog feedback loop that feeds back from the level comparator 7 to the variable gain amplifier 1. By making the response of the analog loop slower than that of the digital loop, the two loops do not interfere with each other and malfunction.

Next, a second embodiment of the present invention will be described with reference to FIG.

In this embodiment, the control circuit 8 of the first embodiment is replaced with a control circuit 20, and all other components are the same.
The same numbers are given to the same components. Control circuit 20 is PW
It is composed of an M circuit 22 and an integrator 21. The PWM circuit 22 modulates the output of the level comparator 7 into a PWM signal and outputs it. This signal is usually converted into an analog voltage by filtering with a low pass filter, but in the present embodiment, the integrator 21 also functions as this low pass filter. By using this embodiment, the same effect as that of the first embodiment can be realized without using the D / A converter 9.

In the above description, the case where the input is a color signal has been described, but the effect of the present invention is not limited to this, and if the level detecting means is used instead of the burst level detector 4 according to the input signal. It can be applied to any signal.

EFFECTS OF THE INVENTION As described above, by using the automatic gain control device of the present invention, the output signal level can be kept constant while effectively utilizing the dynamic range of the A / D converter.

Further, the automatic gain control device of the present invention can control two independent loops of digital and analog with one amplitude detector (burst level detector 4 in the embodiment).

Further, in the automatic gain control device of the present invention, by using the PWM circuit for the control circuit, the D / A converter becomes unnecessary, and the circuit scale can be reduced.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a first embodiment of the present invention, FIG. 2 is a block diagram showing a configuration of a second embodiment of the present invention, and FIG. 3 is a configuration of a conventional example. FIG. 1,3 ... Variable gain amplifier, 4 ... Burst level detector, 6 ... Integrator, 8 ... Control circuit, 9 ... D / A converter, 20 ... Control circuit, 22 ... PWM circuit.

Claims (5)

[Claims] 1. A first variable gain amplifier for varying the amplitude of an input signal, an A / D converter for A / D converting the output of the first variable gain amplifier, and an output of the A / D converter. Of the second variable gain amplifier for varying the amplitude of the second variable gain amplifier, an amplitude detector for detecting the amplitude of the output of the second variable gain amplifier, and a first reference value for comparing the output of the amplitude detector with the first reference value. A comparator, a first integrator that integrates the output of the first comparator and controls the second variable gain amplifier, and a comparator that compares the output of the first integrator with a second reference value An automatic gain control device comprising: a second comparator; and a control circuit for controlling the first variable gain amplifier according to the output of the second comparator. 2. The automatic gain control device according to claim 1, wherein the input signal is a color signal to which a burst signal is added, and the amplitude detector detects the amplitude of the burst signal. 3. The control circuit comprises a D / A converter for D / A converting the output of the second comparator, and a second integrator for integrating the output. The automatic gain control device described. 4. The control circuit comprises a PWM circuit for PWM (pulse width modulation) the output of the second comparator and a third circuit for integrating the output.
2. The automatic gain control device according to claim 1, wherein the automatic gain control device comprises an integrator. 5. The automatic gain control apparatus according to claim 1, wherein the response speed for controlling the first variable gain amplifier is slower than the response speed for controlling the second variable gain amplifier. .