Television – Image signal processing circuitry specific to television – Gray scale transformation
Reexamination Certificate
1998-01-16
2001-02-27
Hsia, Sherrie (Department: 2614)
Television
Image signal processing circuitry specific to television
Gray scale transformation
C348S572000
Reexamination Certificate
active
06195133
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an automatic gain control (AGC) circuit, and more particularly, to a digital AGC circuit which performs AGC by controlling a TOP voltage of an A/D converter.
2. Description of Related Art
An AGC circuit automatically controls a gain to give a constant output value at any time despite changes in input value. A conventional AGC circuit is made using an analog device. While, the conventional analog AGC circuit has no specific problem when used in an analog system, it is inconvenient to use in a digital system. Further, an AGC circuit which can perform a more accurate automatic gain control is needed. Meanwhile, an accurate AGC circuit, using mostly digital circuits, has been disclosed in a Korean patent No. 95-10063.
SUMMARY OF THE INVENTION
The present invention provides a digital automatic gain control (AGC) circuit in which all circuits except for a low pass filter are digital circuits, and which performs an accurate AGC by controlling a TOP voltage of an A/D converter.
The digital AGC circuit of the present invention comprises an A/D converter, an AGC gate pulse generator and a TOP voltage controller. The A/D converter converts a video signal clamped to a reference level into digital data having a voltage between a predetermined reference voltage and a TOP voltage. The AGC gate pulse generator generates an AGC gate pulse for detecting a pedestal level region of the video signal, according to vertical and horizontal synchronous signals. The TOP voltage controller controls the TOP voltage of the A/D converter on the basis of a sample value extracted from the output of the A/D converter and a sync tip value of a standard signal, while the AGC gate pulse generated by the AGC gate pulse generator is enabled.
The TOP voltage controller comprises an AGC comparator, a PWM pulse generator and a low pass filter. The AGC comparator compares a mean value (M) of a predetermined number of samples extracted from the digital data output by the A/D converter with the sync tip value (K) of the standard signal, while the AGC gate pulse is enabled. The pulse width modulation (PWM) pulse generator generates a PWM pulse signal corresponding to the output of the AGC comparator. The low pass filter integrates the PWM pulse signal output by the PWM pulse generator and transforms the result into a direct current voltage to be used as the TOP voltage of the A/D converter.
The AGC comparator comprises a sample mean value unit, a 1/K value setting unit and a multiplier. The sample mean value unit calculates the mean value (M) of N samples extracted from the output of the A/D converter, while the AGC gate pulse is enabled. The 1/K value setting unit obtains a sync tip value (K) of a standard signal and outputs the reciprocal value (1/K) of the sync tip value (K). The multiplier multiplies the mean value (M) output by the sample mean value unit by the value (1/K) set by the 1/K value setting unit and outputs the product (M/K) to the PWM pulse generator.
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Hsia Sherrie
Samsung Elctronics Co., Ltd.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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