Narrow-band quadrature demodulator for recovering analog video and

Abstract

A direct broadcast satellite sends subscribers signals received from an earth-station transmitter and typically having a 10 MHz bandwidth, 6 MHz for an analog video station and 4 MHz for up to 24 digital audio stations. When a subscriber selects one of the audio stations, an FM demodulator recovers the 10 MHz signal; a variable gain amplifier amplifies the received signal; and an A/D converter digitizes the amplified signal at (e.g.) 24.576 MHz. A digital synthesizer produces trigonometric functions (sine, cosine) at (e.g.) 24.576 MHz, and mixers downconvert the signals to baseband. The sampling frequency is then reduced to 256 KHz, twice the digital audio signal baud rate. In response to the 256 KHz signals, a first servo varies a VCO frequency to obtain a regulation by the A/D converter of the digitizing frequency at 24.576 MHz and another servo regulates the frequency of the trigonometric functions. A microcomputer provides a fixed coarsely amplified gain to the 256 KHz signals dependent upon the relative amplitudes of the video and audio signals. The amplitudes of the coarsely amplified signals are detected, filtered and converted to an analog voltage to precisely regulate the gain of the variable gain amplifier. The sampling frequency of the signals at the outputs of the programmable gain stages is reduced to 128 KHz (i.e. the transmitted baud frequency). A Viterbi decoder corrects errors in these in-phase and quadrature baseband 'soft-decision' signals. The resulting 192 kb/s output data stream is decompressed and converted to stereo audio sound.