IF demodulator circuit with variable IF gain control

Details IF demodulator circuit with variable IF gain control IF demodulator circuit with variable IF gain control

1997-01-22

1998-11-17

Flynn, Nathan J.

Television

Receiver circuitry

Demodulator

348727, H04N 544

Patent

active

058383956

ABSTRACT:
An IF demodulator circuit with IF gain control at a variable time constant, which receives an IF carrier including synchronizing pulses, a black porch and picture information with a positively modulated picture signal, in which the IF picture signal is amplified to a nominal amplitude value by the gain control, and which includes an IF demodulator (3) which demodulates the IF picture signal and supplies a picture signal from the output, while, for a gain control which is independent of DC offsets that may occur in the picture signal, the demodulator circuit is constructed in such a way that a black level detector (8) for generating a black level signal used as a threshold value signal in conformity with the amplitude value of the black porch of the picture signal supplied by the amplitude demodulator (3) is provided, that a DC offset circuit (4) is provided, which raises the DC component of the picture signal and/or lowers the DC component of the threshold value signal in such a way that both signals are offset by a predetermined value relative to each other, that a comparator circuit (7) is provided which receives the two signals which are offset relative to each other, in which signals the comparator circuit compares the voltage value of the synchronizing pulses occurring in the picture signal with the voltage value of the threshold value signal, and that the comparator circuit (7) supplies a comparison signal in dependence upon this comparison, in dependence upon which comparison signal the time constant of the gain control is changed.

REFERENCES:
patent: 4500922 (1985-02-01), Van Roessel
patent: 4885546 (1989-12-01), Araki
patent: 5272531 (1993-12-01), McGinn
Datasheet #TDA 9815--Multistandard/MAC VIF-PLL With QSS-IF And Dual FM-PLL/AM Demodulator, Mar. 1994.

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