JPH06318869A - Video signal use a/d conversion circuit and agc circuit - Google Patents

Abstract

PURPOSE:To obtain the video signal use A/D conversion circuit operated at a theoretical S/N limit value at all times and the video signal use AGC circuit in which an output level is made completely constant without presence of noise increase or nonlinear distortion. CONSTITUTION:A synchronizing signal amplitude detection circuit 5 for a video signal input 1 in the video signal use A/D conversion circuit detects an amplitude of a synchronizing signal of the video signal input 1 and the detected signal is amplified properly and the result is impressed to a reference voltage terminal 4 of the A/D conversion circuit. Furthermore, a D/A conversion circuit 9 converts a quantization signal output 3 from the video signal use A/D conversion into an analog video signal in a video signal use AGC circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an A / D conversion circuit and an AGC circuit for converting a video signal into a quantized code.

[0002]

2. Description of the Related Art FIG. 2 is a block diagram of an A / D conversion circuit. The reference voltage 4 is connected to the terminal Vref of the A / D conversion circuit 2, and the magnitude of the reference voltage 4 defines the dynamic range when the video signal input 1 is converted into the quantized signal 4. In many A / D conversion circuits, this dynamic range matches the magnitude of the reference voltage.

Further, quantization noise is generated according to the reference voltage and the number of bits of A / D conversion, which causes a theoretical S / N (signal-to-noise ratio) limit value of the A / D conversion circuit. Stipulated.

Japanese Patent Laid-Open No. 63-299689 discloses an A / D conversion circuit for video signals, which is provided with an AGC circuit (automatic gain adjustment circuit) in front of the A / D conversion circuit to suppress fluctuations in the level of the video signal. However, the video signal input is A
It is disclosed that the D / D conversion circuit is kept within the dynamic range.

FIG. 3 is a block diagram of an example of the above conventional AGC circuit for video signals.

The video signal input 1 is input to the variable gain amplifying circuit 7, and the synchronizing signal amplitude detecting circuit 5 detects the synchronizing signal level in the video signal from the output 8 of the variable gain amplifying circuit 7 and supplies it to the variable gain amplifying circuit 7. Feed back and control its gain. As a result, the video signal output 8 is kept substantially constant regardless of the level fluctuation of the input 1.

Further, Japanese Laid-Open Patent Publication No. 295286/1992 discloses that the synchronization signal level detection circuit is provided on the output side of the A / D conversion circuit to correct the A / D conversion output value.

[0008]

In the video signal A / D conversion circuit, it is necessary to match the maximum value of the video signal input with the above dynamic range in order to increase the S / N ratio. Therefore, when the reference voltage of the A / D conversion circuit matches the dynamic range, it is necessary to match the maximum value of the video signal input with the reference voltage.

Therefore, in the prior art, the above AGC is used.
The circuit stabilizes the maximum level of the video signal input to the A / D conversion circuit, and makes the magnitude close to the magnitude of the reference voltage, for example.

However, the AGC circuit generally has a large variation in characteristics, and its output fluctuates in proportion to the size of the video signal input. Therefore, the maximum level of the video signal input is set to the reference voltage. However, the maximum level must be set to a value lower than the reference voltage, so that the A / D conversion circuit cannot operate at its best S / N. there were.

Further, since the AGC circuit performs the gain control operation by utilizing the non-linearity, distortion occurs, and since the video signal requires a high band characteristic, the circuit configuration becomes complicated and the cost becomes high. Was also a problem.

An object of the present invention is to provide a video signal A / D conversion circuit which can solve the above problems.

[0013]

In order to solve the above problems, the amplitude of a synchronizing signal of a video signal input is detected, and
If necessary, a voltage obtained by appropriately amplifying this is applied to the reference voltage terminal to raise the S / N ratio of the video signal A / D conversion circuit to a theoretical limit value.

Further, the amplitude of the synchronizing signal of the video signal input is detected, and if necessary, this is appropriately amplified and used as the reference voltage of the A / D conversion circuit, and the quantized signal output of the A / D conversion circuit is used as D / A. By converting, an AGC circuit for a video signal without distortion is obtained.

[0015]

The S / N ratio of the video signal A / D conversion circuit is raised to the theoretical limit value by using the amplitude signal of the synchronizing signal or a signal obtained by appropriately amplifying the amplitude signal as the reference voltage.

The amplitude signal of the synchronization signal, or
A signal obtained by appropriately amplifying this is used as a reference voltage of the A / D conversion circuit, and the quantized signal output of the A / D conversion circuit is D / A converted to obtain a video signal AGC circuit without distortion.

[0017]

1 is a block diagram of an embodiment of an A / D conversion circuit for video signals according to the present invention. In the present invention, the dynamic range of the A / D conversion circuit 2 is made to coincide with the sync signal level of the video signal input 1 or the level obtained by appropriately amplifying the sync signal level, and the S of the A / D conversion circuit for video signals is adjusted.
Always keep / N at its theoretical limit. In FIG. 1, the video signal input 1 is input to the A / D conversion circuit 2 and simultaneously applied to the synchronization signal detection circuit 5. The sync signal detection circuit 5 detects and outputs the amplitude of the sync signal in the video signal input 1, and the DC amplification circuit 6 appropriately DC-amplifies the sync signal amplitude to generate the reference voltage 4 of the A / D conversion circuit 2. To do.

Since the dynamic range of the A / D conversion circuit 2 is defined by the reference voltage 4 as described above, when the amplitude signal of the synchronizing signal is used as the reference voltage 4, the dynamic range of the A / D conversion circuit 2 becomes the above. Follow the sync signal amplitude.

As a result, the A / D conversion circuit 2 is limited to the theoretical limit of S / N regardless of the fluctuation of the video signal input 1.
Can be operated with.

FIG. 4 is a block diagram of an AGC circuit for video signals according to the present invention.

In FIG. 4, video signal inputs 1 to A
The part up to the quantized signal output 3 of the / D conversion circuit 2 is the same as the circuit configuration shown in FIG. Therefore, the S / N best quantized signal output 3 is obtained regardless of the size of the video signal input 1. Also, no distortion occurs.

Further, in the conventional AGC circuit shown in FIG. 3, since the internal noise of the variable gain amplifier circuit 7 is added to the noise of the video signal input 1, there is a problem that the S / N of the video signal output 8 is deteriorated. there were.

In the present invention, however, the reference voltage 4 of the A / D conversion circuit 2 is taken out through the low-pass filter having a sufficiently narrow band in the sync signal detection circuit 5, so that the reference voltage 4
Noise is not a problem. That is, the quantized signal output 3
S / N of is not deteriorated more than the S / N of the video signal input 1.

The quantization noise of the A / D conversion circuit 2 can be arbitrarily reduced by appropriately increasing the number of quantization bits.

The D / A conversion circuit 9 outputs the quantized signal output 3
Is converted into an analog video signal output 81. Since the size of the video signal output 81 is defined by the reference DC voltage in the D / A conversion circuit 9, it is independent of the size of the video signal input 1.

Therefore, the sync signal level of the video signal output 81 is set to a predetermined level regardless of the size of the video signal input 1.

From the above, it is possible to obtain an ideal video signal AGC circuit in which the S / N is the best, the distortion is not generated and the noise is not increased, and the output is completely unrelated to the fluctuation of the video signal input 1.

[0028]

According to the present invention, it is possible to provide an A / D conversion circuit for a video signal which can output a quantized signal having the best theoretical S / N value regardless of the level fluctuation of the video signal input. .

Further, the variable gain amplifying circuit for the video frequency band can be omitted and the influence of the internal noise and the non-linear distortion of the variable gain amplifying circuit can be neglected.
A circuit can be provided.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of an A / D conversion circuit for video signals according to the present invention.

FIG. 2 is a block diagram of a conventional A / D conversion circuit for video signals.

FIG. 3 is a block diagram of a conventional AGC circuit for video signals.

FIG. 4 is a block diagram of an AGC circuit for video signals according to the present invention.

[Explanation of symbols]

 1 Video signal input 2 A / D conversion circuit 3 Quantized signal 4 Reference voltage 5 Synch signal amplitude detection circuit 6 DC amplification circuit 7 Variable gain amplification circuit 8, 81 Video signal output 9 D / A conversion circuit

Claims (2)

[Claims] 1. A video signal A / D conversion circuit for defining a dynamic range when an analog signal is converted into a digital signal by a reference voltage, comprising means for detecting a sync signal amplitude of a video signal input, An A / D conversion circuit for a video signal, wherein the output of the sync signal amplitude detection means is appropriately amplified and applied to a reference voltage terminal of the A / D conversion circuit, if necessary. 2. A video signal AGC circuit, means for detecting the amplitude of a synchronizing signal included in a video signal input, means for appropriately amplifying the output of the synchronizing signal amplitude detecting means, and said synchronizing signal amplitude. An A / D conversion circuit for digitally converting the video signal input using the output of the detection means or a signal obtained by appropriately amplifying the detection signal as a reference voltage, and a D for analog conversion of the quantized signal output of the A / D conversion circuit.
An AGC circuit for a video signal, comprising an A / A conversion circuit.