What Is a Trunked Radio System? P25, Talkgroups, and Digital Scanning

Updated July 26, 2026 · 7 min read

Reading a feed description and hitting "P25 Phase II TDMA trunked system, talkgroup 8042" is where a lot of people give up. The concepts are genuinely simple once separated from the jargon.

Conventional radio: one channel, one frequency

On a conventional system, each channel is a fixed frequency. Patrol uses 155.4750 MHz, fire uses 154.2050 MHz, and so on. To listen, you tune to that number.

The problem is capacity. A department needing twelve separate channels needs twelve frequencies licensed and permanently allocated — and most sit idle most of the time while the busiest one is congested.

Trunking: a pool of frequencies, assigned on demand

A trunked system takes a smaller pool of frequencies and shares them dynamically. It works like a phone switchboard:

1. A control channel runs continuously, carrying data rather than voice.
2. An officer keys up on the "North Patrol" talkgroup.
3. The controller finds a free frequency and instructs every radio in that talkgroup to switch to it.
4. The conversation happens.
5. When it ends, the frequency returns to the pool for whoever needs it next.

The same conversation may use a different frequency every time. This is why tuning to a fixed number no longer works — the system moved the traffic.

Talkgroups replaced channels

A talkgroup is a logical group of radios: "North Patrol", "Fire Dispatch", "Detectives". It is identified by a number rather than a frequency, and that number is what feed listings reference.

This is why feed descriptions read like TG 8042 — PD Dispatch rather than a frequency. The talkgroup is the stable identifier; the frequency is a temporary assignment.

Practically, this is an improvement for listeners. A scanner following a trunked system follows the conversation automatically. And an app streaming a feed does all of this server-side — you tune to a feed and the volunteer running the receiver has already handled the tracking.

P25 in plain terms

Project 25 (P25) is the digital radio standard used by most US public-safety agencies. It was created so that radios from different manufacturers and different agencies could work together during multi-agency incidents — a direct response to interoperability failures in large disasters.

P25 Phase I uses FDMA: one conversation per 12.5 kHz channel. It is the older and more widely deployed variant.

P25 Phase II uses TDMA: two conversations share one 12.5 kHz channel by alternating in time slots. It doubles capacity on the same spectrum, which is why agencies under FCC narrowbanding pressure moved to it.

For a listener, the difference is mostly invisible — but a hardware scanner must explicitly support Phase II, which is why older scanners suddenly stopped working when a system upgraded.

Digital does not mean encrypted

This confusion is worth clearing up directly. Digital describes how audio is encoded for transmission. Encrypted describes whether it is scrambled so only authorised radios can decode it.

A P25 system broadcasting in the clear is fully receivable by any P25-capable scanner. Most US public-safety radio is digital; only some of it is encrypted. When people say "they went digital and now I can't hear them", the cause is usually either encryption or a scanner that does not support the system's variant — not digital transmission itself.

For more on the encryption side, see why police channels are encrypted.

Other systems you will encounter

• DMR — a digital standard common with businesses and some smaller agencies. Cheaper than P25 and increasingly used by rural departments.
• NXDN — another digital standard, similar niche.
• EDACS and MPT-1327 — older trunking standards, now rare but still running in a few places.
• Simulcast — several transmitters broadcasting the same signal to cover a wide area. Notorious for causing distortion on hardware scanners when signals from two sites arrive slightly out of phase. Online feeds are usually cleaner, because the operator has already positioned the receiver to favour one site.

Why online feeds are usually easier

Following a modern trunked system with your own hardware means buying a scanner that supports the exact variant, programming the system, identifying the talkgroups, and getting a good enough signal to decode the control channel. It is a genuine hobby.

A feed skips all of it. A volunteer has already solved the hardware and programming problem and is streaming the result. You choose a location and listen — which is the approach the Police Scanner app takes.

The trade-off is that you depend on that volunteer's receiver staying online, and you hear whichever talkgroups they chose to stream. For most people that is a good trade.