JP2885403B2 - Digital clamp circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION [Field of Industrial Application] The present invention relates to a digital clamp circuit for reproducing a DC level that should have a constant value among input signals into a DC level of a predetermined value.

2. Description of the Related Art A digital clamp circuit is a circuit for accurately reproducing a DC level of a video signal. This digital clamp circuit is used to perform a so-called back porch period corresponding to, for example, a bottom portion of a synchronizing signal, a period immediately after the synchronizing signal, and a period immediately before the start of a screen, or a video signal immediately before the synchronizing signal. Focusing on the fact that the so-called front porch period or the like corresponding to immediately after the end of the screen is a period having a constant DC level value, the input value in this period is clamped to a predetermined threshold value to be reproduced. is there. Therefore, the input value of the DC level is compared with a predetermined threshold value, and when the input value is large, the overall level is reduced according to the magnitude, and when the input value is small, Feedback control to the amplifier is performed so as to raise the overall level accordingly. In general, the feedback amount is determined not by using a signal of only one sample but by using a plurality of samples so that the feedback control does not become unstable due to the influence of noise or the like.

In an example of a conventional circuit of this type, for example, a comparison process of accumulating A / D-converted input signals for a plurality of sample periods corresponding to a front porch and comparing the accumulation result with a predetermined threshold value The effect of noise was eliminated by the circuit.
In another example, in order to reduce the size of the accumulation circuit,
A comparison processing circuit that integrates the result of subtraction between the A / D converted input signal and the threshold value has been used.

[Problems to be Solved by the Invention] As described above, the conventional circuit accumulates the sample value itself and calculates the difference between the sample value and the threshold value, or integrates the difference value between the sample value and the threshold value. Since the processing is performed, the number of bits of the integration circuit is required to be large, and there is a disadvantage that the circuit scale becomes large.

An object of the present invention is to provide a digital clamp circuit having a small circuit scale.

[Means for Solving the Problems] To achieve the above object, according to the present invention, as the comparison processing circuit of the digital clamp circuit, a data value of a DC level portion extracted by a gate signal is set to a predetermined threshold. A means is provided for outputting data of a sample number whose data value is larger than a threshold value and a sample number smaller than the threshold value, and this output information is input to the feedback amount determination circuit.

[Operation] The present invention accumulates the number of samples whose digital sample value of a signal to be clamped is larger than a threshold value and the number of samples whose digital sample value is smaller than the threshold value, that is, discriminates the magnitude with the threshold value. Features. The difference is that, in the prior art, for example, the value of a sample expressed in digital is accumulated and subtracted from a threshold value.

It takes about 8 bits per sample to represent the value of the subtraction result digitally as in the prior art. On the other hand, when only magnitude discrimination is used as output information as in the present invention,
It is represented by one bit per sample.

In the present invention, this works to simplify the processing of the feedback amount determination circuit in the next stage and to reduce the circuit scale.

Embodiment FIG. 1 shows an embodiment of the present invention, wherein 1 is a video input terminal, 2 is a differential amplifier circuit, 3 is an A / D converter circuit, 4 is a gate circuit, and 5 is a gate signal generating circuit. , 6 is a comparison circuit, 7 is a threshold setting terminal, 100 is a feedback amount determination circuit,
8 is an up / down counter, 9 is a D / A conversion circuit, and 10 is a video data output terminal.

A video signal input from a video input terminal 1 is set to a predetermined level in a differential amplifier circuit 2 and then converted from an analog signal to a digital signal in an A / D conversion circuit 3. The output of the A / D conversion circuit 3 passes only for a predetermined period to be clamped in the gate circuit 4. The gate signal representing the predetermined period is generated in gate signal generating circuit 5 as a signal representing a period of a predetermined number of samples in the front porch, for example, based on the horizontal synchronizing signal included in the input video signal. The data passing through the gate circuit 4 is compared in the comparator circuit 6 with the threshold value set at the threshold value setting terminal 7, and a signal L indicating that the data is larger than the threshold value and a signal L smaller than the threshold value. Circuit for determining the amount of feedback signal S for each sample
Output to 100. FIG. 1 shows a feedback amount determining circuit 100.
The up-down counter 8 inputs, for example, the signal L as a clock for down and the signal S as a clock for up. Therefore, if the signal L is generated more, the counter output becomes smaller, and if the signal S is generated more, the counter output becomes larger. The output of the up / down counter 8 is converted into an analog signal in the D / A conversion circuit 9 and fed back to the differential amplifier circuit 2.

In FIG. 1, the comparison circuit 6 and the feedback amount determination circuit 100 show the features of the present invention.

In the above description, the up-down counter 8 has been described as a counter. However, it is apparent that the same function can be realized even if the up-down counter 8 is realized not as a counter but as an accumulation circuit. Further, a comparison circuit is set between the up / down counter 8 and the D / A conversion circuit 9 so that the result of comparing whether the output of the up / down counter 8 is larger or smaller than a predetermined threshold reference value is subjected to D / A conversion. In this case, it is apparent that the D / A conversion circuit 9 can be constituted by 1-bit D / A.

FIG. 2 shows another embodiment of the feedback amount determination circuit 100, wherein 101 is a D / A conversion circuit, and 102 is an integration circuit. D
The / A conversion circuit 101 converts the signal supplied from the comparison circuit 6 into an analog signal, and outputs the result to an integration circuit 102.
Integrate analogously.

[Effect of the Invention] As described above, the feedback amount determination circuit 100
Since the number of samples whose input signal is larger than the threshold value and the number of samples smaller than the threshold value are integrated as a signal to be processed by the above-mentioned method, it is possible to use a 1-bit input signal per sample. The signal is simplified as compared with the case where about 8 bits are required.

Along with this, a complicated configuration of an adder and a flip-flop circuit was required to perform the accumulation operation in the feedback amount determination circuit with the conventional input signal, whereas in the case of the present invention, the counter can be realized by a counter. As a result, the circuit scale can be reduced.

Further, as a feedback amount determining circuit, by providing a means for D / A converting the output of the comparison processing circuit, and a means for integrating the D / A converted DC level in an analog manner,
The circuit scale could be further reduced.

[Brief description of the drawings]

FIG. 1 shows an embodiment of the present invention, and FIG. 2 shows another embodiment of the feedback amount determining circuit. DESCRIPTION OF SYMBOLS 1 ... Video input terminal 2 ... Differential amplifier circuit 3 ... A / D conversion circuit 4 ... Gate circuit 5 ... Gate signal generation circuit 6 ... Comparison circuit 7 ... Threshold setting terminal 8 ... Up / down counter 9, 101 D / A conversion circuit 100 Feedback amount determination circuit 102 Integration circuit 10 Video data output terminal

Claims (2)

(57) [Claims] An amplifying circuit for amplifying an input signal including a repetitive waveform such as a synchronization signal having a constant DC level portion, an A / D conversion circuit for A / D converting the output, and an A / D conversion circuit. A gate circuit that opens a gate only at a DC level portion of the repetitive waveform of the repeated data and allows data to pass therethrough; a gate signal generation circuit that generates a gate signal therefor; a data extracted from the gate circuit; A comparison processing circuit that performs a required comparison process between the two based on the input of the threshold signal, a feedback amount determination circuit that determines the amount of feedback to the amplifier circuit based on the output information thereof, and performs a feedback corresponding thereto. A digital clamp having a feedback circuit for clamping the DC level of the repetitive waveform of the input signal to the predetermined threshold level In the path, the comparison processing circuit compares the data value of the DC level portion extracted by the gate signal with a predetermined threshold value, and determines the number of samples and the number of samples whose data value is larger than the threshold value. Comparing means for comparing the cumulative value of an up / down counter operated by the output of the comparison processing circuit with a predetermined threshold reference value, and a means for comparing the feedback amount determining circuit. A 1-bit D / A converter for D / A-converting the result of whether the data value is larger or smaller than the threshold reference value. 2. An amplifying circuit for amplifying an input signal including a repetitive waveform such as a synchronization signal having a constant DC level portion, an A / D converting circuit for A / D converting the output, and an A / D converting circuit. A gate circuit that opens a gate only at a DC level portion of the repetitive waveform of the repeated data and allows data to pass therethrough; a gate signal generation circuit that generates a gate signal therefor; a data extracted from the gate circuit; A comparison processing circuit that performs a required comparison process between the two based on the input of the threshold signal, a feedback amount determination circuit that determines the amount of feedback to the amplifier circuit based on the output information thereof, and performs a feedback corresponding thereto. A digital clamp having a feedback circuit for clamping the DC level of the repetitive waveform of the input signal to the predetermined threshold level In the path, the comparison processing circuit compares the data value of the DC level portion extracted by the gate signal with a predetermined threshold value, and determines the number of samples and the number of samples whose data value is larger than the threshold value. Means for outputting the data of the comparison processing circuit, and means for D / A converting the output of the comparison processing circuit, and means for integrating the D / A converted DC level in an analog manner. Characteristic digital clamp circuit.