Also known as: C4FM

C4FM (compatible four-level FM) is the four-level FSK modulation used by P25 Phase 1 and System Fusion. The carrier sits at one of four frequency deviations per symbol, carrying 2 bits each.¹

C4FM is four-level FSK: each symbol sits at one of four frequency deviations, carrying two bits.

How it works

C4FM runs at 4800 baud (9600 bps) and is paired with CQPSK: the two are designed so a single receiver detects both transmit paths. Its four levels appear as four clusters on a constellation and three stacked eyes on an eye diagram.

Relevance to SDR

Recognising healthy C4FM symbols is central to decoding P25 Phase 1 and Fusion; the scopes reveal SNR, tuning, and timing problems at a glance.

Sources

1. Project 25 — Wikipedia, for the C4FM modulation, symbol rate, and its use in P25 Phase 1. ↩

See also

• Frequency-shift keying (FSK)
• CQPSK
• Project 25 (P25)
• P25 Phase 1
• System Fusion (YSF)
• Symbol rate (baud)

Learn it step by step

• Digital modulation & constellations

From the blog

• SDR Internals, Part 6: Demodulation