What is Audio Frequency Shift Keying (AFSK)?

Audio frequency shift keying (AFSK) is a modulation technique (a form of frequency shift keying (FSK)) where digital data is represented by changes in the frequency, pitch, of an audio tone, providing an encoded signal appropriate for transmission by way of telephone or radio. Generally, the audio transmitted alternates between two tones: the ‘space', represents a binary zero and the other one, the ‘mark', represents a binary one.

The Audio Frequency Shift Keying modulated signal, in radio applications, is normally being used to modulate a radio frequency (RF) carrier using a conventional technique like amplitude modulation (AM) or frequency modulation (FM) for transmission. Audio Frequency Shift Keying differs from regular FSK by performing the modulation at base-band frequencies.

Audio Frequency Shift Keying is not always used for high speed data communications, seeing as it is far less efficient in both bandwidth and power than most other modulation modes. However, Audio Frequency Shift Keying has the advantage that encoded signals will pass through alternating current (AC) coupled links, which includes the majority of equipment originally designed to carry speech or music.