JPS62122465A - Clamp circuit - Google Patents

Abstract

PURPOSE:To decrease the time controlling a video signal level to a reference level by switching the time constant in response to the fluctuation of the video signal level. CONSTITUTION:A video signal inputted to an input terminal 1 is converted into a digital signal (b) by an analog/digital converter 8 together with an offset voltage (a). The digital signal (b) inputted to a discrimination circuit 2 is discriminated for the level and when the level is larger than the reference level, a voltage (c) is applied to lower the level of the offset voltage (a) and when the level is smaller than the reference level, a voltage (d) increasing the level of the offset voltage (a) is applied and outputted from the conversion circuit 4. On the other hand, the digital signal (b) inputted to a fluctuation discrimination circuit 3 is discriminated for the fluctuation by fluctuation discrimination circuits 13-15, and when the fluctuation is larger than the 1st threshold value, a short time constant signal (e) is fed from a time constant circuit 16 to the offset voltage (a) and when smaller than the 2nd threshold value, a long time constant signal (g) is fed from a time constant circuit 18.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a clamp circuit, and particularly to a clamp circuit that controls a video signal in a predetermined section of a synchronized portion of a video signal to a predetermined reference level. Regarding circuits.

[Conventional technology]

Conventionally, this type of clamp circuit digitally determines the magnitude of the video signal level in a predetermined section of the synchronized portion of the video signal and a predetermined reference level, and calculates the result with a certain time constant. The video signal level was controlled by converting it into an analog offset voltage and applying this analog offset voltage to the video signal.

[Problem that the invention seeks to solve]

The conventional clamp circuit described above controls the video signal level by converting the judgment result into an analog offset voltage with a certain time constant, so if the amount of variation in the video signal level is large, the video This has the disadvantage that it takes a long time to control the signal level to the reference level.

SUMMARY OF THE INVENTION In view of the above, an object of the present invention is to provide a clamp circuit that can shorten the time it takes to control a video signal level.

[Means for solving problems]

The clamp circuit of the present invention digitally determines the magnitude of the video signal level and the reference level so that the video signal level in a predetermined section of the synchronous part of the video signal becomes a predetermined reference level. The clamp circuit controls the video signal level by converting the result into an analog offset voltage having a time constant and applying it to the video signal. a fluctuation determination circuit that digitally determines the magnitude of the difference from the threshold;
If the variation determination circuit determines that the amount of variation in the video signal level is large, the time constant becomes small; and if the variation determination circuit determines that the amount of variation in the video signal level is small, the time constant becomes large. and a time constant switching circuit that switches the time constant and applies it to the analog offset voltage so that .

〔Example〕

Next, the present invention will be explained with reference to the M plane.

The figure is a block diagram of one embodiment of the present invention. The clamp circuit of this embodiment digitally determines the magnitude of the video signal No. 13 level in a predetermined section of the synchronized portion of the video signal inputted to the input terminal l and a predetermined reference level. A circuit 2, a fluctuation determination circuit 3 that digitally determines the magnitude of the amount of variation in the video signal level and a plurality of predetermined threshold values, and a conversion circuit 4 that converts the determination result in the determination circuit 2 into an analog offset voltage.
, an analog offset voltage with a time constant outputted from the time constant switching port 1IPi5 and the time constant switching circuit 5, which switches a plurality of time constants according to the judgment result of the fluctuation judgment circuit 3, is added to the video signal to generate the video signal. It is composed of 11?11 circuits 6 that control the level.

A first determination circuit 2 outputs a logic level "0" when the digital code of the video signal level in a predetermined section of the horizontal synchronization portion of the video signal is greater than the digital code of a predetermined reference level. It consists of a circuit 9 and a second judgment circuit IO which outputs a logic level "O" when the digital code is smaller than a predetermined reference level.

The conversion circuit 4 includes a first switch 11 that creates a voltage C for lowering the offset voltage when the video signal level is higher than the reference level, and a voltage for raising the offset voltage a when the video signal level is lower than the reference level. d
and a second switch 12 that makes the first determination circuit 9
or the first switch 11 or the second switch 1 depending on the logic level “0” output of the second determination circuit 10.
2 is closed to control the level of offset voltage a.

The fluctuation determination circuit 3 determines logic level "0" when the digital code of the video signal level in a predetermined section of the horizontal synchronization portion of the video signal is larger than a threshold of a predetermined fluctuation amount (7) ml. The first output
fluctuation determination circuit 13, and a second threshold value (first threshold value) for the amount of fluctuation.
a second fluctuation determination circuit 14 that outputs a logic level "0" when the threshold value is larger than a second threshold value);
If the amount of variation is less than or equal to the second threshold, the logic level “
and a third fluctuation determination circuit 15 that outputs "0".

The time constant switching circuit 5 includes a first time constant circuit 16, a second time constant circuit 17, a third time constant circuit 18, and switches 19 for operating these time constant circuits 16 to 18, respectively.
~21. In the time constant circuits 16 to 18, the switches 19 to 21 are closed only when the outputs of the fluctuation determination circuits 13 to 15 are at the logic level "O", respectively, and output the time constant No. 13 e - g. When the amount of variation in the video signal level is large, the time constant is set to a small time constant, and when the amount of variation is small, the time constant is set to a large time constant.
The time constant of −g is chosen such that e<(g.

The control circuit 6 has one input terminal connected to the input terminal l,
It consists of a video amplifier 7 whose other input terminal is grounded, and an 8-bit analog-to-digital converter 8 connected to the video amplifier 7.

Next, the operation of the clamp circuit of this embodiment configured as described above will be explained.

The video signal input to the input terminal 1 is amplified by the video amplifier 7 together with the offset voltage a, and is input to the analog-to-digital converter 8.
It is converted into a bit digital signal. This 8-bit digital signal is input manually to a judgment circuit 2 and a fluctuation judgment circuit 3.

The level of the digital signal input to the judgment circuit 2 is judged by each judgment circuit 9 and 10. If the level is higher than the reference level, the output of the judgment circuit 9 becomes logic level "0" and the switch 11 is turned on. A voltage C which is closed and lowers the level of the offset voltage a is added to the offset voltage a and outputted from the conversion circuit 4.

Further, when the digital signal is smaller than the reference level, the output of the judgment circuit 10 becomes logic level "0", the switch 12 is closed, and a voltage d is applied to the off-cent voltage a to raise the off-seno'vNN2O level. The signal is then output from the conversion circuit 4.

Furthermore, when the level of the digital signal S is equal to the reference level, the logic level of the outputs of the judgment circuit 9 and the judgment circuit 0 are both "L", and the switches 11 and 12 are opened to open the off-cent voltage a of the reference level. is output.

On the other hand, the digital signal input to the fluctuation determination circuit 3 is
Each fluctuation determination circuit 13 to 15 determines the amount of fluctuation, and if the amount of fluctuation is larger than the first threshold, the logic level of the fluctuation determination circuit 13 becomes 0'', and the switch 19 of the time constant switching circuit 5 is closed, and a short time constant signal e is applied from the time constant circuit 16 to the offset voltage a.

Further, when the amount of variation in the digital signal manually input to the variation determination circuit 3 is smaller than the first threshold value and larger than the second threshold value, the logic level of the variation determination circuit 14 is set to "0".
°, the switch 20 of the time constant switching circuit 5 is closed, and a relatively short time constant signal f is applied from the time constant circuit 17 to the offset voltage a.

Further, when the amount of variation in the digital signal inputted to the variation determination circuit 3 is smaller than the second threshold, the logic level of the variation determination circuit 15 becomes "O", and the switch 21 of the time constant switching circuit 5 is turned off. When closed, a long time constant signal g is applied to the offset voltage a from the time constant circuit 18 at 710.

In this way, the larger the amount of variation in the video signal level, the shorter the time constant signal added to the offset voltage.
The time required to control the level of the video signal to the reference level can be shortened.

In the above embodiment, the variation determination circuit has three types of threshold values, but it goes without saying that the present invention can be applied even if there are four or more types of threshold values.

[Effects of the Invention] As explained below, the present invention has the effect of shortening the time for controlling the video signal level to the reference level by switching the time constant according to the amount of variation in the video signal level. be.

[Brief explanation of drawings]

The figure is a block diagram showing a clamp circuit according to an embodiment of the present invention. In the figure, ■... Input terminal, 2... Judgment circuit, 3... Fluctuation judgment circuit, 4... Conversion circuit, 5... Time constant switching circuit. , 6...Control circuit, 7...Video amplifier, 8...Analog-digital converter, 9, IO...
Judgment circuit, 11,, 12... Switch, 13-15... Fluctuation judgment circuit, 16-I8... Time constant circuit, 19-21... Switch, a... Offset voltage, b ...8-bit digital signal, c, d...voltage, e-g...time constant signal.

Claims (1)

[Claims] Digitally determines the magnitude of the video signal level and the reference level so that the video signal level in a predetermined section of the synchronized portion of the video signal becomes a predetermined reference level. In a clamp circuit that controls the video signal level by converting the result into an analog offset voltage having a time constant and applying it to the video signal, the amount of variation in the video signal level and a plurality of predetermined thresholds are a fluctuation determination circuit that digitally determines the magnitude of the video signal level; and when the fluctuation determination circuit determines that the amount of fluctuation in the video signal level is large, the amount of fluctuation in the video signal level is adjusted so that the time constant becomes small. A clamp circuit comprising: a time constant switching circuit that switches the time constant so that the time constant becomes large when it is determined that the time constant is small, and applies the time constant to the analog offset voltage.