Simulcast
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Simulcast is a contraction of "simultaneous broadcast", and refers to programs or events broadcast across more than one medium at the same time. For example, Virgin Radio is simulcast on both AM and on satellite radio; the BBC's Prom concerts are often simulcast on both BBC Radio 3 and BBC Television. Another application is the transmission of the original-language soundtrack of movies or TV series over radio, with the television broadcast having been dubbed into a local language.
Simulcasts via satellite can be a challenge, as there is a significant delay because of the distance (nearly 50,000 miles or 80,000 km round-trip) involved. Anything involving video compression (and to some extent audio data compression) also has an additional significant delay, which is noticeable when watching local TV stations on direct broadcast satellites. Even though the process is not instantaneous, this is still considered a simulcast because it is not intentionally stored anywhere.
Multiplexing (also sometimes called "multicasting") is something of a reversal of this situation, where multiple program streams are combined into a single broadcast. The two terms are sometimes confused.
In horse racing, a simulcast is a broadcast of a horse race which allows wagering at two or more sites; the simulcast often involves the transmission of wagering information to a central site, so that all bettors may bet in the same betting pool, as well as the broadcast of the race.