Also known as: AFSK

AFSK (audio frequency-shift keying) represents bits as audio tones that then modulate a radio (usually FM).¹ The classic case is the Bell 202 standard — 1200 Hz and 2200 Hz tones — carrying 1200 bps APRS packet over AX.25.

AFSK sends data as two audio tones over an FM channel — the scheme behind APRS (Bell 202).

How it works

Because the keying is at audio frequencies, AFSK can pass through an ordinary FM voice channel. A receiver demodulates the FM to audio, then detects which tone is present per bit.

Relevance to SDR

GopherTrunk decodes AFSK as part of its APRS pipeline, detecting the mark/space tones after FM demodulation.

Sources

1. Frequency-shift keying — Wikipedia, for audio FSK and the Bell 202 mark/space tone scheme. ↩

See also

• Frequency-shift keying (FSK)
• APRS
• AX.25
• FFSK

Learn it step by step

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