JP2926802B2 - Video signal processing device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video signal processing device that performs A / D conversion on a clamped video signal to perform signal processing.

2. Description of the Related Art In recent years, in order to digitally process a video signal, video signal processing devices with A / D conversion are increasing.

Hereinafter, an example of the above-described conventional video signal processing device will be described with reference to the drawings.

FIG. 2 shows a circuit diagram of a conventional video signal processing device. In FIG. 2, Q1, Q2, Q3 and Q4 are transistors, R1, R2, R3 and R4 are resistors, VR1 and VR2 are variable resistors, C
1, C2 and C3 are capacitors. Reference numeral 1 denotes an A / D converter which A / D converts a clamped video signal and outputs a digital video signal. Reference numeral 2 denotes an analog switch whose on / off is controlled by a clamp pulse. A variable resistor VR1 for reference level adjustment is connected to the base of a first emitter follower circuit composed of a transistor Q1 and a resistor R1. The variable resistor VR1 is stabilized by a capacitor C1 and gives a reference level of the A / D converter 1. A composite video signal is input to a second emitter follower circuit composed of a transistor Q2 and a resistor R2, and connected to a third emitter follower circuit composed of a transistor Q3 and a resistor R3 via a capacitor C2. /
A clamped video signal is supplied to an analog input terminal of the D converter 1. 4th transistor Q4 and resistor R4
The variable resistor VR2 for adjusting the clamp level is connected to the base of the emitter follower circuit, and is stabilized by the capacitor C3 to apply the clamp level to the base of the third emitter follower circuit via the analog switch 2.

Regarding the video signal processing device configured as described above,
The operation will be described below.

First, the DC component of the composite video signal input to the second emitter follower circuit is cut off by the capacitor C2, and when the analog switch 2 is turned on by the clamp pulse, the video signal is adjusted by the variable resistor VR2 and the fourth Is fixed to the clamp level given by the emitter follower circuit of Therefore, when the clamp pulse is given during the horizontal synchronization period of the video signal, the video signal in which the peak value of the horizontal synchronization signal is fixed to the clamp level and the DC component is reproduced is input to the third emitter follower circuit. At this time, the clamped video signal output from the third emitter follower circuit is output from the A / D converter 1 with reference to the reference level given by the first emitter follower circuit.
D converted. Therefore, the variable resistor VR2 must be adjusted so that even a part of the clamped video signal does not have a part lower in voltage than the reference level.

Problems to be Solved by the Invention However, the above-described configuration requires two adjustments, that is, the adjustment of the reference level of the A / D converter and the adjustment of the clamp level. There has been a problem that a correct digital video signal cannot be obtained if a part of the signal has a voltage lower than the reference level.

The present invention has been made in view of the above problems, and provides an image signal processing apparatus that reduces the number of adjustment points and does not cause an adjustment error.

Means for Solving the Problems In order to solve the above problems, a video signal processing device according to the present invention comprises a clamp device for fixing a required point of a video signal to a predetermined potential (clamp level), An A / D converter for A / D conversion and a reference voltage (reference level) adjusting means for the A / D converter are provided, and the clamp level is interlocked with the voltage obtained by the reference level adjusting means. It is like that.

According to the present invention, when the reference level is adjusted by the above-described configuration, the clamp level is determined in accordance with the reference level. Therefore, the means for adjusting the clamp level is omitted, and an adjustment error such as a video signal having a lower potential than the reference level is prevented. It can be eliminated.

Example Regarding a video signal processing device according to an embodiment of the present invention,
This will be described with reference to the drawings.

FIG. 1 is a circuit diagram of a video signal processing device according to an embodiment of the present invention. In Fig. 1, Q1, Q2, Q
3, Q4 is a transistor, R1, R2, R3, R4 are resistors, VR1 is a variable resistor, C1, C2, C3 are capacitors, 1 is an A / D converter,
Reference numeral 2 denotes an analog switch, which operates in the same manner as the switch having the same number in FIG. The difference from FIG. 1 is that the transistors forming the first emitter follower circuit
R5 is added as the emitter resistance of Q1, and the connection point between the resistances R1 and R5 is connected to the base of the fourth emitter follower circuit (transistor Q4).

Regarding the video signal processing device configured as described above,
The operation will be described below with reference to FIG. First variable resistance
If VR1 is adjusted and the emitter voltage of the transistor Q1, which is the output of the first emitter follower circuit, that is, the reference level is Vf, the base voltage (Vb4) of the fourth emitter follower circuit is VB4 = Vf + (Vcc −Vf) ÷ (R1 + R5) × R5. Here, assuming that the voltages between the base and the emitter of the transistors Q4 and Q3 are Vbe4 and Vbe3, respectively,
The video signal clamped at (Vb4 + Vbe4) is provided to the base of Q3. Therefore, the video signal clamped by (Vb4 + Vbe4-Vbe3) is supplied to the analog input terminal of the A / D converter 1. At this time, if it is determined that the resistance R5 satisfies (Vb4 + Vbe4−Vbe3)> Vf, that is, (Vcc−Vf) ÷ (R1 + R5) × R5 + Vbe4−Vbe3> 0, the reference level of the A / D converter 1 is adjusted. Accordingly, it is possible to provide a clamp level such that the video signal does not become lower than the reference level.

As described above, according to the present embodiment, the emitter resistance of the emitter follower circuit that supplies the reference level is R1.
By using two resistors R5 and connecting the resistors R1 and R5 to the input of the emitter follower circuit that supplies the clamp level, if the reference level is adjusted, the clamp level is determined accordingly. The adjustment means is omitted, and it is possible to eliminate an adjustment error such that the video signal has a lower potential than the reference level.

In the above embodiment, the input of the variable resistor VR1 is a first emitter follower circuit. However, the first emitter follower circuit may be a variable output shunt regulator having a more stable output voltage.

As described above, according to the present invention, a clamp device for fixing a required point of a video signal to a predetermined potential (clamp level) and an A / D converter for A / D converting the clamped video signal And the reference voltage (reference level) of the above A / D converter
Since the clamp level is linked with the voltage obtained by the reference level adjusting means, the clamp level is determined according to the adjustment of the reference level. It is possible to eliminate adjustment errors such as omission and a potential of the video signal being lower than the reference level.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a video signal processing device according to an embodiment of the present invention, and FIG. 2 is a circuit diagram of a conventional video signal processing device. 1 A / D converter, 2 Analog switch, VR1 Variable resistor, Q1-Q4 Transistor, R1-R5 Resistor
C1 to C3 ... capacitors.

Claims (1)

(57) [Claims] A clamp device for fixing a required point of a video signal to a predetermined clamp potential;
A / D converter for A / D conversion, and a reference voltage adjusting means for the A / D converter, the reference voltage obtained from the adjusting means is applied to the base of a transistor constituting an emitter follower circuit, An emitter resistor of the transistor is formed by connecting a first resistor and a second resistor in series, and a potential at a connection point between the first resistor and the second resistor is supplied to the clamp device, and the A / D conversion is performed. The resistance value of the first resistor and the second resistor are set so that the reference voltage of the heater becomes higher than the clamp voltage.
A video signal processing device for setting a resistance value of a resistor.