Tutorial dPMR PMR446

Tutorial: dPMR and the PMR446 channels

What it is

dPMR is a narrow-band digital radio standard (6.25 kHz) used in licence-free PMR446 handhelds in Europe and by some businesses. OnAir has a scan of all 16 PMR446 digital channels and logs calls with the own and called IDs, colour codes, status messages and packet data. Voice needs an mbelib you install yourself.

What you need

• Any SDR that tunes the band: an RTL-SDR covers roughly 24 MHz to 1.7 GHz at up to about 2.4 Msps (see Radio-RTL-SDR), and so do HackRF, Airspy, SDRplay and others (Supported-radios). The narrow signals here need only a small sample rate; the mode asks for 2 Msps.
• Antenna: a UHF vertical or the handheld's whip antenna on the table is enough near the transmitters.
• Frequencies (database): the 16 PMR446 digital dPMR channels, 6.25 kHz apart, from 446.103125 MHz (channel 1) to 446.196875 MHz (channel 16). Licensed dPMR channels: type the frequency.

Before you start

Set the gain so that the ADC level in the status line stays clear of full scale; use Auto-tune or Optimise gain (Your-first-reception). Too much gain makes strong neighbours (paging, FM broadcast, repeaters) leave spurs in the channel. Make sure the radio is on frequency: a cheap RTL-SDR can be several kHz off at 430 MHz, so enter its correction (ppm) in Radio settings or watch the carrier offset the panel shows. Narrow digital voice channels need an offset under about 1 kHz.

Step by step

1. Connect the radio and antenna and start OnAir. Open the group Two-way radio and choose dPMR (the entry reads Digital voice, 6.25 kHz).
2. Press scan all 16 to step through the PMR446 channels; the panel says scanning the PMR446 channels, no call heard yet. Once a call is heard it stops on the channel (PMR446 n). Or type a frequency.
3. Press Start; Now shows ON AIR with the kind of call, the colour code and the IDs.
4. Open Calls (Carrier, Time UTC, Kind, From, Type, Details, Duration), Messages (status and short messages, packet data) and Radio / Heard (the radios heard).
5. Install mbelib yourself if you want to hear voice: Voice-codecs lists the file names and folders. Without it the panel says no mbelib: frames counted only.

What you should see

Calls with IDs and the colour code; the bit errors corrected by the Hamming code, the frame sync counters (FS1 to FS4), the CCH and data block counters, and the end frames. emergency call is shown when set.

Reading the screen

Now: current call and channel. Calls: the log. Messages: headers and short data (no headers yet if none). Radio: IDs heard.

Options and settings

• scan all 16: step through the PMR446 channels.
• Copy a call row.

Troubleshooting

• Nothing on PMR446. Most PMR446 radios are still analogue FM, and dPMR is not common. Check the radios nearby.
• Scan too slow. Stay on a channel you know is in use.
• Calls with no sound. mbelib missing.
• Decoding with bit errors. Offset or weak signal; the panel shows the offset and inverted if needed.
• Wrong channel. The digital channels are 6.25 kHz apart; the analogue ones 12.5 kHz.

Tips

• Record the channel (Tutorial-Record-IQ-then-the-Signal-workbench) while it is busy and replay it to try settings.
• A scan (Main-window, tab Scan) helps to find an active channel.

Legal and privacy notes

PMR446 is licence-free; listening to digital voice is regulated differently in each country: follow the rules where you live. Calls can be private. OnAir decodes only what is in the clear and does not decrypt. The AMBE+2 vocoder is patented, so the sound needs your own mbelib (Voice-codecs). Do not publish what you hear. See Legal.

Under the hood

dPMR is a 4FSK system at 2400 symbols/s (4.8 kbit/s) in 6.25 kHz FDMA, described in ETSI TS 102 490 (dPMR for PMR446); the channel has a colour code, frame sync words FS1 to FS4, a Hamming-protected header, slow data and AMBE+2 voice.

Related

Mode-dPMR-PMR446, Tutorial-DMR, Tutorial-NXDN, Tutorial-Digital-voice-DMR-D-STAR-P25, Voice-codecs, Radio-RTL-SDR, FAQ