A transponder in satellite broadcasting is a transmitter on which one or more services can be broadcasted.
In the old days, a typical satellite transponder had about enough bandwidth (roughly 40 Mbps) for a single analog PAL colour TV channel. If your satellite had 22 transponders, you could transmit 22 TV channels.
Today digital video compression standards like MPEG allow a typical TV channel to be transmitted using much less bandwidth. Typically between 3 and 5 Mbps (depending on picture quality) is required for each channel. Channel data can also be mixed or "multiplexed" - for transmission on a single transponder. This allows a standard 40 Mbps transponder to carry roughly 10 TV channels (or the equivalent in radio and other services).
DVB-S2 is a new transmission standard that has become increasingly common since 2009. It enables increased capacity on each transponder, making it possible to provide high definition (HD) services which require more bandwidth than regular SD channels.
See this article for more details: transponder (Wikipedia).
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