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Before infra red hand controls became common, ultra-sonic was used as the transmission medium. It used sound to carry messages rather than light, a little like Morse Code messages. The frequencies normally chosen were between 38 and 40 KHz, well above human hearing ability but low enough to be able to drive low cost transducers (similar to very small loudpseakers).
As manufacturers changed to infra-red, the carrier frequency and the signal data often remained identical to the ultra-sonic transmitters but simply used a light emitting diode instead of a transducer. Infra-red quickly became the standard medium because of its greater immunity to interference. The carrier frequency has tended to stick with many control protocols even though the requirement for 40KHz carrier frequencies has faded to almost nothing.
Many of the more recent remote control protocols have now moved away from these carrier frequencies to allow more data to be sent in a given space of time.
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