JP2982273B2 - A / D conversion circuit for video signal - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an A / D converter for video signals, and more particularly to an A / D converter for video signals that performs A / D conversion by controlling a pedestal level of a video signal to be constant. The present invention relates to a conversion circuit.

[Conventional technology]

FIG. 2 is a block diagram of a conventional video signal A / D conversion circuit. In order to set the pedestal level of the video signal input from the input terminal 1 to a constant voltage and perform A / D conversion by the A / D converter 25, the pedestal level of the input video signal is controlled in accordance with the control signal Sc to make the pedestal level constant. An amplifier circuit 21 for outputting a video signal; a low-pass filter 22 for removing aliasing noise from a digital circuit; and a buffer amplifier circuit
23, and a clamp control circuit 24 that generates a control signal Sc. The clamp control circuit 24 receives the signal from the buffer amplifier circuit 23 and the clamp pulse Pc supplied from the input terminal 3, samples and holds the pedestal level of the video signal, obtains a DC voltage, and amplifies the signal. To perform a so-called feedback clamping operation.

[Problems to be solved by the invention]

Since the pedestal level of the video signal is A / D converted at a constant voltage, a DC current flows through the low-pass filter according to the average gradient of the video signal. For this reason, the coil constituting the low-pass filter must be free from distortion due to DC magnetization, and it has been difficult to reduce the size. It is possible to reduce the level of the video signal passing through the low-pass filter by providing an amplifier circuit on the output side of the low-pass filter to reduce distortion caused by DC magnetization. However, there is a disadvantage that noise from the device is easily mixed.

An object of the present invention is to provide a video signal A / D conversion circuit capable of reducing the DC current flowing through a low-pass filter and reducing the size of a coil.

[Means for solving the problem]

An A / D conversion circuit for a video signal of the present invention controls an pedestal level of an input video signal in accordance with a control signal to output a video signal having a constant pedestal level. A low-pass filter for removing components, a terminating resistor connected to an output terminal of the low-pass filter, and an A / D converter for receiving a video signal having a constant pedestal level output by the low-pass filter and converting the video signal into digital data. A clamp control circuit for detecting the pedestal level of the video signal input to the A / D converter and generating the control signal, wherein one end of the A / D converter is an output of the low-pass filter. By supplying the control signal to the other end of the terminating resistor connected to the end, the direct current flowing through the low-pass filter is reduced.

〔Example〕

Next, the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing one embodiment of the present invention. The video signal supplied to the input terminal 1 is amplified by the amplifier circuit 11, supplied to the A / D converter 15 via the low-pass filter 12 and the buffer amplifier circuit 13, converted into digital data, and output from the output terminal 2. Is done.

On the other hand, the clamp control circuit 14 receives the clamp pulse Pc supplied to the input terminal 3 and the video signal from the buffer amplifier circuit 13, and supplies a control signal Sc corresponding to the pedestal level of the video signal to the amplifier circuit 11. Then, a so-called feedback clamp operation for controlling the pedestal level of the input video signal to be constant is performed. The control signal Sc output from the clamp control circuit 14 is applied to the terminating resistor 17 of the low-pass filter 12 via the buffer amplifier circuit 16. Therefore, the low-pass filter
The DC potential difference between the input terminal and the output terminal of 12 is canceled, and the DC component flowing through the low-pass filter 12 is reduced.

In the present embodiment, the buffer amplifier circuit 13 is provided before the A / D converter 15, but may not be provided if the input impedance of the A / D converter 15 is high. Also, the buffer amplifier circuit 16 is unnecessary if the output impedance of the clamp control circuit 14 is sufficiently low in the video signal band and distortion is not caused by the video signal.

〔The invention's effect〕

As described above, according to the present invention, the control signal output by the clamp control circuit performing the feedback clamp operation is
By reducing the DC current flowing through the low-pass filter by supplying it to the terminating resistor on the output side of the low-pass filter, the coil constituting the low-pass filter can use a small coil without causing distortion due to DC magnetization. Can be reduced in size. In addition, since the fluctuation of the delay time of the low-pass filter due to the direct current is also low, the DP characteristics can be improved.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a block diagram showing an example of a conventional video signal A / D conversion circuit. 11,21 …… Amplifier circuit, 12,22 …… Low-pass filter, 13,16,23
…… Buffer amplifier circuit, 14,24 …… Clamp control circuit, 15,25
…… A / D converter, 17 …… Terminal resistor.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H03M 1/00-1/88 H04N 7/13

Claims (1)

(57) [Claims] An amplifier circuit for controlling a pedestal level of an input video signal in accordance with a control signal to output a video signal having a constant pedestal level, and a low-pass filter receiving an output of the amplifier circuit and removing a noise component. A terminating resistor connected to an output terminal of the low-pass filter, an A / D converter that receives a video signal having a constant pedestal level output from the low-pass filter and converts the video signal into digital data, and an input to the A / D converter. A clamp control circuit for detecting the pedestal level of the video signal to be generated and generating the control signal, wherein the terminating resistor has one end connected to the output end of the low-pass filter. A / D for a video signal, characterized in that a DC current flowing through the low-pass filter is reduced by supplying the control signal to the other end of the A / D.
Conversion circuit.