Audio frequency-shift keying (AFSK) is a modulation technique by which digital data is represented as changes in the frequency (pitch) of an audio tone, yielding an encoded signal suitable for transmission via radio or telephone. Normally, the transmitted audio alternates between two tones: one, the "mark", represents a binary one; the other, the "space", represents a binary zero.

AFSK differs from regular frequency-shift keying in that the modulation is performed at baseband frequencies. In radio applications, the AFSK-modulated signal is normally used to modulate an RF carrier (using a conventional technique, such as AM or FM) for transmission.

AFSK is not generally used for high-speed data communications, as it is less efficient than other modulation modes. In addition to its simplicity, however, AFSK has the advantage that encoded signals will pass through AC-coupled links, including most equipment originally designed to carry music or speech.

Applications

Most early telephone-line modems used audio frequency-shift keying to send and receive data, up to rates of about 300 bits per second. The common Bell 103 modem used this technique, for example. Some early microcomputers used a specific form of AFSK modulation, the Kansas City standard, to store data on audio cassettes. AFSK is still widely used in amateur radio, as it allows data transmission through unmodified voiceband equipment.

AFSK is also used in the United States' Emergency Alert System to transmit warning information. It is used at higher bitrates for Weathercopy used on Weatheradio by NOAA in the U.S., and more extensively by Environment Canada.

The CHU shortwave radio station in Ottawa, Canada broadcasts a digital time signal encoded using AFSK modulation.

Related technology

Dual-tone multi-frequency (DTMF) is another encoding technique which represents data using pairs of audio frequencies.