| 摘要 |
<p>A wide band frequency modulated DBS signal of the type having time sequential compressed chrominance and luminance (video) components is received by an aerial (1), frequency converted in a down converter (2) and applied to the input (4) of a conversion unit. The conversion unit has a tuner (5), i.f. stage (9, 10) and a frequency demoduiator (11) to produce baseband video which is applied after de-emphasis at (12) to a modulator (27) where the video is amplitude modulated onto a vision carrier from an oscillator (28). The modulated carrier is applied through a vestigial side-band filter (29) and an adder (30) to an output (31). Sound/data bursts present in the DBS signal are recovered in a stage (13) and processed in a processor (14) where the sound/data bursts are expanded to occupy approximately a line period at a bit rate which is an integral sub-multiple of the bit rate for the sound/data bursts in the received DBS signal. The expanded sound/data and clock frequency are digitally modulated in a modulator (32) onto a sound carrier from an oscillator (33), the modulated sound carrier being applied through a bandpass filter (34) and the adder (30) to the output (31). …<??>The output (31) is connected to a cable distribution system for which the narrow band signals produced by the conversion unit are suited as such systems cannot cope with high bit-rates. The processing of the sound/data bursts such that their bit rate is substantially reduced also simplifies the handling and demodulation processes for these bursts.</p> |
