Before this:
Digital modulation for trunking: C4FM, π/4-DQPSK & CQPSK
Key takeaways Digital trunking uses only a handful of modulations. C4FM is four-level FSK — P25 Phase 1 runs it at 4800 symbols/s × 2 bits = 9600 bps. CQPSK (also called LSM) is a linear cousin that produces a compatible constellation and is used by simulcast transmitters. π/4-DQPSK is the phase-shift modulation of TETRA, and plain 4FSK carries DMR and NXDN. Each has a recognisable constellation and eye diagram, which GopherTrunk plots live so you can judge a lock at a glance. Understanding these shapes is how you tell systems apart and tune a clean signal.
You met the three families of digital modulation already — FSK, PSK, QAM — and the relationship between symbols and baud. This lesson narrows that to the specific modulations digital trunked systems actually use, because once you can name the shape on the scope, you’re halfway to identifying the system.
C4FM — the P25 Phase 1 workhorse
C4FM (Continuous 4-level FM) is a four-level FSK: the carrier sits at one of four small frequency deviations per symbol. Four levels means each symbol carries two bits (a dibit). P25 Phase 1 runs C4FM at 4800 symbols per second, so:
4800 symbols/s × 2 bits/symbol = 9600 bits per second
The four deviations map to fixed dibits, the same scheme DMR’s 4FSK uses:
| Deviation | Symbol level | Dibit |
|---|---|---|
| +1800 Hz | +3 | 01 |
| +600 Hz | +1 | 00 |
| −600 Hz | −1 | 10 |
| −1800 Hz | −3 | 11 |
C4FM’s appeal is that it is constant-envelope — the amplitude never changes, only the frequency — so it works with the cheap, efficient nonlinear amplifiers in handheld radios without distorting. On a symbol scope you see four stacked levels; on a constellation you see four points.
CQPSK / LSM — the linear twin for simulcast
Here is the clever part of P25: it also defines CQPSK (Compatible QPSK), often called LSM (Linear Simulcast Modulation). CQPSK is a linear modulation — it shifts phase, not frequency — yet it is engineered to produce a signal that a C4FM receiver can decode and that lands on the same constellation. A radio doesn’t care which one transmitted it.
Why bother with two ways to make the same signal? Because of simulcast: large systems broadcast the same channel from many transmitters at once over an overlapping area. For those overlapping signals to combine cleanly rather than smear, the transmitters need the tighter control that a linear modulation gives. C4FM’s nonlinear path is fine for a single handheld but not for phase-precise simulcast, so infrastructure transmitters often use CQPSK/LSM while the constellation a receiver sees stays the same. (This is exactly why simulcast distortion is its own decoding challenge, covered later in the path.)
π/4-DQPSK and 4FSK — the rest of the field
Two more shapes round out digital trunking:
- π/4-DQPSK — used by TETRA. This is a differential phase-shift keying that rotates the carrier phase in steps of a quarter of pi, encoding bits in the change of phase between symbols rather than its absolute value. On a constellation it traces an eight-point ring; differential coding makes it forgiving of phase ambiguity.
- 4FSK — the same four-level FSK family as C4FM, used by DMR and NXDN. DMR and P25 Phase 1 share the underlying 4FSK idea, which is why their constellations and eye diagrams look so alike even though the systems differ in every other respect.
Reading them on the scopes
Every one of these modulations is just a pattern of symbols, and GopherTrunk draws those symbols live. On the Constellation panel you’ll see the four points of C4FM/4FSK or the ring of π/4-DQPSK; tight, well-separated clusters mean a clean signal, while smearing toward the centre signals low SNR, mistuning, or a clock problem. The Eye diagram shows the same symbols against time — for the four-level modes you’ll see three stacked eyes, and the wider they open, the more margin the decoder has.
These views are how you both identify a system and tune it: a P25 simulcast signal that won’t lock often shows a recognisable distortion on the constellation, and the only way to spot that is to look. Later lessons on tuning with scopes lean directly on what you’ve just learned to read.
Quick check: P25 Phase 1 C4FM runs at 4800 symbols/s. What is its bit rate?
Recap
- C4FM is four-level FSK; P25 Phase 1 runs it at 4800 symbols/s = 9600 bps.
- CQPSK / LSM is a linear cousin that lands on the same constellation and is used by simulcast transmitters that need phase precision.
- π/4-DQPSK carries TETRA; plain 4FSK carries DMR and NXDN.
- Each has a recognisable constellation and eye diagram that GopherTrunk plots live.
- Reading those shapes is how you identify a system and tune a clean lock.
Next, we look inside the bitstream itself: framing, error correction and interleaving — the structure that lets digital survive fading.
Frequently asked questions
What modulation does P25 Phase 1 use?
P25 Phase 1 uses C4FM, a four-level frequency-shift keying. It runs at 4800 symbols per second, and because four levels carry two bits each, that yields 9600 bits per second. The same signal can also be produced with CQPSK, a linear-modulation cousin that fits the same constellation.
Why does P25 specify both C4FM and CQPSK?
They produce a compatible on-air signal but suit different transmitters. C4FM uses a constant-envelope FM-style path that works with cheap, efficient nonlinear amplifiers in handhelds. CQPSK (also called LSM) is a linear modulation that simulcast transmitters need so multiple sites can overlap cleanly, so infrastructure often uses it.
What is the difference between 4FSK and π/4-DQPSK?
4FSK shifts the carrier among four frequencies, one per symbol, and is used by DMR and NXDN as well as P25 Phase 1’s C4FM. π/4-DQPSK is a phase-shift modulation used by TETRA that rotates the carrier phase in quarter-pi steps, encoding bits in the change of phase rather than its absolute value.
How can I see these modulations in GopherTrunk?
The Constellation panel plots each received symbol in the IQ plane, so you can see the four points of C4FM or the ring of π/4-DQPSK, and the Eye diagram shows the symbol levels against time. Tight clusters and a wide-open eye mean a clean lock; smearing means errors are near.