P25 digital voice

The main North American standard for public safety digital radio.

ModeP25 digital voice

FamilyDigital voice

Audio exampleComing

P25 sounds like a steady, gravelly digital buzz, like a coarse electric razor, with no speech you can make out; control channels buzz nonstop around the clock.

What does it sound like?

Plate 45Digital voice

A steady, gravelly digital buzz.

Field marks4-level FSK, 4,800 symbols per second (Phase 1)Public-safety radio in North America

Who uses it?

Police, fire, EMS and government agencies, mainly in North America.

Where and when can I hear it?

VHF, UHF, 700 and 800 MHz public safety bands.

The science

Classic P25 (Phase 1) gives each conversation its own 12.5 kHz channel and sends 4,800 symbols per second, each symbol carrying 2 bits, for 9,600 bits per second. Phase 1 radios transmit with C4FM, continuous four-level FM, in which the carrier shifts among four frequencies. The IMBE vocoder turns speech into 88 bits every 20 milliseconds (4,400 bits per second of voice description), and the rest of the 9,600 bits per second is forward error correction (extra redundant bits that let the receiver repair damage) and signaling. P25 Phase 2 trunked systems instead split a 12.5 kHz channel into two alternating time slots so two conversations can share it, using a half-rate vocoder.

Why use it?

P25 was built so radios from different manufacturers, and different agencies, can talk to each other through a common air interface, with vendor competition and spectrum efficiency as stated goals. Its strong error correction helps receivers cope with poor signal conditions and extends usable range, and P25 radios must also work in analog FM, letting agencies move to digital gradually.

Lore

Project 25 grew out of a September 1989 meeting; under an agreement among APCO (the Association of Public-Safety Communications Officials), NASTD (the state technology directors' association) and federal agencies, its steering committee formed in 1990. The published standards are administered by the Telecommunications Industry Association (TIA) committee TR-8. The standards also define optional encryption that some agencies enable.

What do the decoded parts mean?

Talkgroup and unit ID: Which group is talking and which radio is transmitting.

Easily confused with

DMR digital voice: DMR pulses as two conversations take turns
NXDN digital voice: NXDN is often narrower

How to recognize it

The most common version, P25 Phase 1, sends 9,600 bits per second on a 12.5 kHz channel using four-level frequency-shift keying. Through an ordinary FM scanner that becomes a harsh, even buzz for as long as someone is transmitting. Phase 2 fits two talk paths into one channel by taking turns in time slots, which gives it a slightly pulsed texture more like DMR.

On trunked systems, one channel is the control channel. It carries the system's traffic directions (which radio is talking on which frequency) all the time, so it sounds like a data buzz that never stops. Voice conversations hop between the other channels.

Easily confused with:

DMR: similar raspy tone, with a faster flutter from its time slots.
NXDN: thinner, and often narrower.
Trunking control channels of other systems: also continuous data, with a different rhythm and sound.

Where and when to hear it

P25 is used mainly by police, fire, emergency medical services and government agencies, mostly in North America, and increasingly in some other countries. It is found in the VHF (136-174 MHz), UHF (around 380-512 MHz), 700 MHz and 800 MHz public safety bands. Many states run statewide P25 networks shared by many agencies.

Public databases such as RadioReference list which systems are P25, their frequencies and whether they are encrypted.

A trunked system is easy to recognize on a wideband display: one frequency carries a constant data signal day and night, while several nearby frequencies light up briefly with voice bursts that come and go as people talk.

A little history

Project 25 began in 1989 as a joint effort of the Association of Public-Safety Communications Officials (APCO), state telecommunications directors and U.S. federal agencies. The goal was interoperability: radios from different manufacturers and agencies should be able to talk to each other, a lesson learned from emergencies where responders could not communicate. The resulting standards are published by the Telecommunications Industry Association as the TIA-102 series.

Phase 1 radios appeared in the 1990s. Phase 2, standardized at the end of the 2000s and first put into service in 2012, doubled capacity per channel through time-division, as regulators pushed public safety radio into narrower channels.

Common questions

Is it legal to listen to P25?

In the United States, listening to unencrypted public safety radio is generally lawful under federal law, and scanners that receive P25 are sold openly. Some states limit scanner use in vehicles or using what you hear to commit a crime, and some countries allow listening only to transmissions meant for general reception, such as broadcasts and amateur radio. Check the rules where you are.

Why can I hear P25 but not understand it?

Digital voice is compressed data. A standard analog scanner plays the raw data as a buzz; you need a receiver or software that supports P25 and its voice codec to hear speech.

Why do some P25 channels stay silent even on a digital scanner?

Many agencies encrypt some or all of their traffic. Encrypted transmissions can't be heard by the public, and decrypting them without authorization is illegal.

How do people follow unencrypted P25?

Digital scanners from major scanner makers support it, and free software such as SDRTrunk and OP25 can follow unencrypted P25 systems with a software-defined radio.