JPS60145729A - Signal clamping method - Google Patents

Abstract

PURPOSE:To correct automatically offset to clamp a signal constantly by comparing the digital output signal of an A/D converter with a reference digital level and subtracting the difference signal level from the digital output signal of the A/D converter. CONSTITUTION:An analog input signal is converted to a digital signal by an A/D converter 1 and this digital signal is supplied to a comparator 2 and an adder/subtractor 3. The reference digital level by which signals should by clamped is set to the comparator 2, and this reference digital level and the level in the blanking period of the digital output signal of the output of the A/D converter 1 are compared with each other, and the difference signal level is outputted to the adder/subtractor 3. The adder/subtractor 3 subtracts the difference signal level from the comparator 2 from the digital output signal from the A/D converter 1, thus obtaining a digital clampled output signal.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention automatically corrects offsets in analog-to-digital converters (hereinafter referred to as A/D converters), and in particular digitally clamps digital television signals. This invention relates to a signal clamping method suitable for

Conventional configuration and its problems When converting a television signal from an analog signal to a digital signal, the analog television signal is clamped by an analog clamp circuit and then converted into an A/D converter by an A/D converter.
For example, if there is a method of keyed clamping using an analog clamp circuit or a reference analog level, the reference analog level and the reference level of the A/D converter are easily affected by temperature changes and aging. However, it was difficult to keep the clamp level constant.

In addition, a digital signal that compares the digital output signal of the A/D converter with a reference digital level that is stable against temperature changes, etc., and controls the clamp level of the clamp circuit placed before the A/D converter accordingly. There is a clamp method, but in this case, the clamp level is the digital signal level of the A/D converter output.
Since feedback is not performed unless the output changes by more than 1 LSB, an error of at least 1 LSB occurs, and since the method detects the output error and then feeds it back, there is a drawback that correction is delayed.

OBJECTS OF THE INVENTION The present invention eliminates the drawbacks of the prior art, and aims to provide a method for clamping signals stably and accurately without being affected by temperature changes or aging.

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention compares the digital output signal of an A/D converter with a reference digital level, and subtracts the difference signal level from the digital output signal of the A/D converter. The offset of the /D converter is automatically corrected, and a digital signal whose signal is always clamped to a constant value is obtained.

DESCRIPTION OF EMBODIMENTS The configuration of an embodiment of the present invention will be described below with reference to the drawings.

In FIG. 1, 1 is an A/D converter that converts an analog input signal such as a television signal into a digital signal; 2
3 is a comparator that compares the level of the input digital signal with a reference digital level, and 3 is an adder. The analog input signal is converted into a digital signal by the A/D converter 1,
The digital output signal is supplied to a comparator 2 and an adder/subtractor 3.

A reference digital level, which is a level for signal clamping, is set in the comparator 2, and this reference digital level is compared with the blanking period of the digital output signal output from the A/D converter 1, and the difference signal level is calculated. is output to the adder/subtractor 3. The adder/subtractor 3 adds or subtracts the difference signal level from the comparator 2 from the digital output signal from the A/D converter 1 to obtain a digitally clamped output signal. For example, when the digital output signal from the A/D converter 1 is higher than the reference digital level, the adder/subtractor 3 calculates the difference signal level from the comparator 2 from the digital output signal; When the digital output signal is lower than the reference digital level, the difference signal level is added to the digital output signal to clamp the signal.

FIG. 2 is a diagram showing another embodiment of the present invention.

In the figure, 1 is an A/D converter, 2 is a comparator, 3 is an adder/subtractor,
Reference numeral 4 denotes an averager that integrates and averages the difference signal level from the comparator 2. There is a method of signal clamping the back porch (BACK-PORCH) in the television signal, but in this case, when comparing the level with the reference digital level in comparator 2, the level comparison is performed by sampling the back porch of the digital output signal. However, since there is a burst signal in the bank pouch, the correct difference signal level cannot be obtained unless a plurality of samples are sampled and averaged. Also, in order to reduce the influence of noise, it is preferable to perform a plurality of samplings and average them. This averager may also average the difference signal level of several lines before.

Furthermore, if cranking with a certain degree of precision is performed by an analog clamp circuit located before the A/D converter, the difference signal level from comparator 2 is small, so
The analog clamp may be used to shift the difference signal level to either positive or negative, and the adder/subtractor 3 may perform either addition or subtraction. Further, if the difference signal level is small, a read-only memory in which a program for addition and subtraction is written can be used in the adder/subtractor 3.

Effects of the Invention The present invention has the above configuration, and calculates the error voltage by comparing the output of the A/D converter with the reference digital level, and adds or subtracts this error voltage from the output of the A/D converter. Since it is corrected, it has the advantage that it is not affected by temperature changes and that there is no time delay even when it is corrected. Furthermore, depending on the dynamic range of the input signal used, an analog clamp circuit is not necessarily required before the A/D converter, and even if an analog clamp is required, it has the advantage that its precision does not have to be sufficiently high.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an embodiment of a circuit implementing the signal clamping method of the present invention, and FIG. 2 is a block diagram of a second embodiment of the present invention. 1... A/D converter, 2... Comparator, 3...
...Adder/subtractor, 4...Averaging device.

Claims (2)

[Claims] (1) The output signal of an analog-digital converter that converts an analog signal into a digital signal is compared in level with a reference digital signal corresponding to the clamp level setting value, and the difference signal level is converted into the output signal of the analog-digital converter. A signal clamping method characterized by subtracting from. (2) A signal according to claim 1, characterized in that the difference signal level is averaged to obtain an averaged difference signal level, and the averaged difference signal level is subtracted from the output signal of an analog-to-digital converter. Clamp method.