Also known as: 4FSK, four-level FSK, 4-level FSK
4FSK is four-level frequency-shift keying: the carrier is switched among four discrete frequency deviations, so each symbol encodes a two-bit dibit.1 It is the workhorse modulation of digital land-mobile radio — the physical layer of DMR, NXDN, and the C4FM form of P25 — because its constant envelope and modest bandwidth suit efficient handheld transmitters.
How it works
The modulator maps each incoming dibit to one of four evenly spaced frequency offsets above and below the carrier — conventionally +3, +1, −1, −3 deviation units. In P25 C4FM these correspond to ±1.8 kHz and ±0.6 kHz around the channel centre, transmitted at 4800 symbols per second, which yields 9600 bits per second gross. The bit-to-symbol mapping is chosen so adjacent levels differ by a single bit, a Gray code, so the most likely error — slipping to a neighbouring level — corrupts only one bit.
Crucially, land-mobile 4FSK is continuous-phase: the frequency changes are shaped so the phase never jumps, giving a constant envelope that non-linear amplifiers handle efficiently. C4FM in particular is filtered so tightly that it is mathematically equivalent to a CQPSK phase modulation on the same symbol grid — a receiver can demodulate the identical signal either as 4-level FSK or as a phase constellation, which is why C4FM and CQPSK interoperate.
In practice
A 4FSK demodulator recovers the instantaneous frequency, slices it into four levels, and reads off dibits. The four levels appear as four rails on a symbol scope and as four clusters on a constellation or eye diagram; a clean signal shows tight, well-separated groups, while noise, timing error, or deviation mismatch smears them together. Symbol-clock recovery and deviation normalisation are the two adjustments that most affect decode reliability.
Relevance to SDR
4FSK is central to the trunked and conventional digital voice systems that scanner users care about. DMR and NXDN are 4FSK, P25 Phase 1 is C4FM 4FSK, and P25 Phase 2 uses an H-CPM/H-DQPSK 4-level scheme on a TDMA grid. Paging and telemetry protocols also use 4FSK for its bit-per-symbol efficiency over plain 2FSK.
This is the core of GopherTrunk’s decode chain: it demodulates C4FM/4FSK to recover the dibit stream for P25, DMR, and NXDN before framing, error correction, and vocoder decoding. GopherTrunk decodes clear and scrambled traffic; keyed encryption remains opaque regardless of modulation.
Sources
-
Frequency-shift keying — Wikipedia, for M-ary FSK and the four-level (2 bits/symbol) case used in land-mobile radio. ↩