Transmission of Two NTSC Color Television Signals Over a Single Satellite Transponder Via Time-Frequency Multiplexing

We describe a time-frequency multiplexing (TFM) method for transmitting two television signals over one satellite transponder, which gives significant improvement over existing methods. The complexity of the hardware implementation is similar to existing time-division multiplexing (TDM) systems, but the picture quality, when received on a 10 m earth station, approaches that which is currently being requested by the major television networks. Unlike existing frequency-division multiplexing (FDM) systems, TFM needs no backoff of the satellite transponder. The general philosophy of TFM is described, including the benefits of using a difference signal to reduce the overall required bandwidth. Also discussed is companding of the difference signal as a means of improving the overall signal-to-noise ratio. Several alternative methods of implementation are considered, along with those which have been explored in some detail. We then describe an experimental TFM system which was implemented using digital field memories and tested with a 10 m earth station operating through the COMSTAR D2 satellite. Received picture quality was excellent in all cases. Weighted SNR was 53-56 dB, and in practically all instances subjects could not tell any difference between single-picture FM transmission and TFM dual-picture transmission.