Audio frequency-shift keying (AFSK) is used to send digital data as sound, over radio or wire. Normally, one tone is used for "mark" or binary 1, another for "space" or binary 0.

AFSK is used in the U.S Emergency Alert System (EAS), first designed by the National Weather Service for weatheradio, then adopted by the FCC as an upgrade to the Emergency Broadcast System. AFSK-encoded data (7-bit ASCII) is sent before the warning, indicating who, what, where, and when, so that digital receivers can decode the information and be activated only when necessary. It is also used at higher bitrates for Weathercopy used on Weatheradio, by NOAA in the U.S., and more extensively by Environment Canada.

DTMF is a related technology.

See also generic frequency-shift keying for RF applications.