Transponder (satellite communications)
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A communications satellite’s channels are called transponders, because each is a separate transceiver or repeater. Most comsats are microwave radio relay stations in orbit, and carry dozens of transponders, each with a bandwidth of tens of megahertz. Most transponders operate on a "bent pipe" principle, referring to the sending back of what goes into the conduit with only amplification and a shift from uplink to downlink frequency, as opposed to a 'regenerative' system whereby the signal is used to remake and remodulate the signal.
With data compression and multiplexing, several video (including digital video) and audio channels may travel through a single transponder on a single wideband carrier.
Original analog video only has one channel per transponder, with subcarriers for audio and automatic transmission identification service ATIS. Non-multiplexed radio stations can also travel in single channel per carrier (SCPC) mode, with multiple carriers (analog or digital) per transponder. This allows each station to transmit directly to the satellite, rather than paying for a whole transponder, or using landlines to send it to an earth station for multiplexing with other stations.
