Also known as: FFSK
FFSK (fast frequency-shift keying) is a coherent form of audio FSK in which the mark and space tones are exact integer multiples of the bit rate, so each bit contains a whole number of cycles.1 This makes detection clean, bit timing self-evident, and the transmitted spectrum compact — the properties that made FFSK the standard for short control bursts on analog land-mobile channels.
How it works
FFSK’s defining trick is choosing the two tones so that each represents a whole number of cycles per bit period. A common form keys 1200 Hz for a mark and 1800 Hz for a space at 1200 baud: at 1200 bps a bit lasts 1/1200 s, in which 1200 Hz completes exactly one cycle and 1800 Hz exactly 1.5 cycles. Because both tones cross zero at the bit boundary, the transmitter can switch between them with no phase discontinuity, giving a continuous- phase, constant-envelope waveform — closely related to minimum-shift keying — that a saturated FM transmitter passes cleanly. The integer-cycle property also means a receiver can count zero crossings or correlate against the two tones with the bit clock aligned automatically, so bit and symbol timing fall out of the signal itself rather than needing a separate recovery loop.
Those traits — clean detection, easy timing, narrow spectrum, robustness to the non-linearities of an FM channel — suit short data bursts far more than sustained throughput, which is why FFSK shows up in signalling systems that piggyback brief control messages onto otherwise-analog voice channels.
Variants
FFSK is closely tied to the same continuous-phase, integer-index ideas behind MSK; the difference from generic AFSK is precisely the coherent, integer-cycle tone choice, which AFSK (e.g. Bell 202’s 1200/2200 Hz at 1200 baud, where 2200 Hz is not an integer number of cycles per bit) does not enforce. Different systems pick different tone pairs and bit rates, but the coherent principle is shared across MDC1200, maritime DSC, and MPT 1327.
In practice
FFSK carries MDC1200 unit-ID and emergency signalling on analog two-way channels, DSC digital selective calling on marine VHF/HF (the distress-alert layer of the GMDSS), and MPT 1327 trunking control signalling — typically at 1200 bps. In each the FFSK burst is short: an MDC PTT-ID is a fraction of a second at the start or end of a transmission, and an MPT 1327 control channel interleaves signalling frames with call setup.
Relevance to SDR
GopherTrunk detects FFSK bursts on analog channels to decode signalling such as PTT IDs and trunking control data: it FM-demodulates the channel, then correlates the audio against the mark/space tones with bit timing derived from the integer-cycle structure. Because the bursts are short and self-clocking, reliable capture depends mostly on getting clean FM audio in the first place.
Sources
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Frequency-shift keying — Wikipedia, for coherent/fast FSK and continuous-phase tone signalling. ↩