Identifying what you’re hearing

Key takeaways You can often name a system before decoding it by reading four clues. Channel bandwidth on the waterfall — 6.25, 12.5, or 25 kHz — is the fastest first cut. Symbol rate and the constellation shape narrow the modulation family: a four-level pattern means C4FM or 4FSK (P25, DMR), a phase ring means π/4-DQPSK (TETRA). The audio cadence — bursty TDMA versus a continuous stream — confirms it. GopherTrunk’s constellation, eye diagram and histogram make every one of these visible, so identification becomes a glance rather than a guess.

Before GopherTrunk can follow a system, it needs to know which system. Sometimes a database tells you; often you’re staring at an unknown carrier on the waterfall and have to work it out. The good news is that digital trunking uses only a handful of modulations, and each leaves fingerprints you can read with the scopes.

Clue 1 — channel bandwidth on the waterfall

The width of a carrier on the FFT and waterfall is fixed by the standard, so it’s the quickest first cut. Zoom in until a single carrier fills the display and estimate how many kilohertz of spectrum it occupies:

6.25 kHz — a narrow carrier. Think NXDN or P25 Phase 2 (which packs two voice channels into this width using TDMA).
12.5 kHz — the workhorse width. Think P25 Phase 1 or DMR (DMR fits two TDMA slots here).
25 kHz — wide. Older analog/wideband channels, or TETRA (one 25 kHz carrier shared by four time slots).

Bandwidth alone rarely pins the exact system, but it eliminates most candidates in a second. A 6.25 kHz carrier is never P25 Phase 1; a 25 kHz carrier is never DMR.

Clue 2 — symbol rate and constellation shape

Now look at the recovered symbols on the constellation and eye diagram. The symbol rate sets how fast the eye opens and closes, and the shape reveals the modulation family:

Four horizontal levels (or a four-point cluster) — 4-level FSK: C4FM (P25 Phase 1) or 4FSK (DMR, NXDN). The histogram shows four distinct peaks.
A ring of phase points — π/4-DQPSK (TETRA): the constellation rotates in quarter-pi steps and traces a rosette rather than fixed levels.
A tight four-corner QPSK box — CQPSK/LSM, the linear cousin P25 simulcast transmitters use.

A clean lock draws tight clusters and a wide-open eye; smearing means you’re close to the noise floor — useful information, but don’t mistake a weak-signal smear for the wrong modulation. The histogram is the tie-breaker: count the peaks, and a two-versus-four-level question answers itself.

The constellation gives away the modulation family at a glance: four levels for FSK systems, a rosette ring for TETRA's π/4-DQPSK, a four-corner box for CQPSK simulcast.

Clue 3 — audio and burst cadence

Even without decoding, the rhythm of a signal helps. FDMA systems (P25 Phase 1, NXDN, conventional DMR carriers) put one continuous transmission on the channel for the duration of a call. TDMA systems (P25 Phase 2, DMR, TETRA) chop the carrier into time slots, so on the waterfall and in the symbol stream you see a pulsing pattern — bursts with gaps — even during a single conversation. A steady solid trace versus a stuttering one is a strong TDMA-versus-FDMA tell.

Putting the clues together

No single clue is decisive, but two or three together usually name the system. Use this recognition table as a field guide:

System	Channel bandwidth	Modulation	Access	Telltale
P25 Phase 1	12.5 kHz	C4FM (or CQPSK simulcast)	FDMA	Four levels, continuous trace, 4800 sym/s
P25 Phase 2	6.25 kHz equiv.	H-CPM / TDMA	TDMA (2-slot)	Narrow channel, pulsing bursts
DMR	12.5 kHz	4FSK	TDMA (2-slot)	Four levels and slotting on one 12.5 kHz carrier
NXDN	6.25 / 12.5 kHz	4FSK	FDMA	Very narrow carrier, four levels, continuous
TETRA	25 kHz	π/4-DQPSK	TDMA (4-slot)	Phase-ring constellation, wide channel, strong slotting
Analog / wideband	25 kHz	FM	—	No symbol structure; fuzzy noise-like spectrum

Read it top to bottom: bandwidth eliminates rows, the constellation shape confirms the modulation, and the burst cadence settles TDMA versus FDMA. For the protocol details behind each row, the protocol landscape lesson is the deeper reference.

Quick check: you see a 12.5 kHz carrier with a four-level constellation that pulses in bursts during a single call. Best guess?

Recap

Channel bandwidth (6.25 / 12.5 / 25 kHz) on the waterfall is the fastest first cut.
The constellation shape names the modulation: four levels for FSK, a ring for π/4-DQPSK, a box for CQPSK.
The histogram confirms 2-level versus 4-level by counting peaks.
Burst cadence separates pulsing TDMA systems from continuous FDMA ones.
Two or three clues together usually name the system before you decode a single bit.

Next, we’ll find the one frequency worth monitoring — locating and confirming the control channel.

Frequently asked questions

Can I tell which system I'm hearing before decoding it?

Often yes. Channel bandwidth on the waterfall, the symbol rate, the shape of the constellation, and the cadence of the audio all give the system away. A 12.5 kHz carrier with a four-point constellation is likely P25 Phase 1 or DMR; a 6.25 kHz carrier points to NXDN or P25 Phase 2; a wider 25 kHz channel with a phase ring suggests TETRA. These clues narrow the field before you commit to a decoder.

What does channel bandwidth tell me about a system?

Bandwidth reflects how much spectrum each carrier occupies, which is set by the standard. P25 Phase 1 and DMR use 12.5 kHz channels; NXDN and P25 Phase 2 squeeze into 6.25 kHz; older analog and some wideband systems use 25 kHz; TETRA uses 25 kHz shared by four time slots. Measuring the carrier width on the waterfall is the fastest first cut.

How does the constellation help me identify a system?

Different modulations draw different shapes. C4FM and 4FSK show four horizontal levels or a four-point pattern; π/4-DQPSK draws a ring of eight phase points; CQPSK shows a tighter four-corner QPSK pattern. The constellation, eye diagram and histogram together confirm the modulation family, which maps onto a short list of candidate systems.

Why bother identifying before decoding?

Picking the wrong decoder wastes time and can make a perfectly good signal look broken. A quick visual identification — bandwidth, symbol rate, constellation, cadence — tells you which decoder to try first, so you lock faster and avoid chasing a non-existent fault. It is the difference between a confident guess and trial-and-error.