Study of a Time-Compression Technique for TV Transmission Using a Chirp Filter and Envelope Detection

We study a time-compression technique motivated by the longstanding interest in transmitting multiple broadcast-quality color TV signals through a single satellite transponder, i.e., a usable RF bandwidth of 36 MHz in a communications satellite such as COMSTAR. This can be done by the use of frequency division multiplexing (FDM). However, the nonlinearity of the transponder can cause serious intelligible crosstalk and intermodulation interference between the FM carriers unless the satellite power amplifier is backed off substantially. Such a backoff, in turn, leads to a reduction in the downlink carrier-tonoise ratio. As a result, there exists an optimum trade-off between the crosstalk and s/n's, which limits the overall system performance, and achieving broadcast-quality TV transmission becomes difficult. It is possible to time compress each scan line of a color TV signal by 2373 the use of a linear chirp, a linear dispersive filter (LDF) and an envelope detector. Two or more time-compressed scan lines from different, but synchronized, TV signals can then be time multiplexed together in the time duration of an ordinary TV scan line. This concept of timecompression multiplexing (TCM) is not new,1,2 but recent advances in fast analog-to-digital converters, digital-to-analog converters and charge-coupled devices have greatly facilitated the implementation of time compression or expansion. However, because of their present limitations on bandwidth and speed, time-compression factors for achievable TV signals are only 2 or 3.