Identify Digital Voice Modes Using Analog FM Receivers

In the SF Bay Area there has been a recent flurry of activity about digital transmitters on various VHF frequencies which has led some to falsely presume that the signals were D*Star. As it turns out, the signals were from an amateur MotoTRBO repeater. Due to the inability of the local hams to identify the signal type, the trustee of the D*Star system was falsely accused of generating QRM on frequencies 25 kHz away from his repeater.

Identifying digital voice modes without digital equipment, by listening with 5 kHz analog FM receivers, isn’t easy but there are some things you can listen for. D*Star has a fairly unique sound in that every transmission begins with a short 2400 Hz tone burst; if you hear a very short “beep” at the beginning you’re hearing D*Star. MotoTRBO (which is the Motorola branded variant of ETSI DMR Tier 2) is a TDMA mode and as such it has a “sputtering” or “machine gun” sound on 5 kHz analog FM gear. Then there’s P25 Phase 1, P25 Phase 2, NDXN, etc etc. (Note: I don’t know of any amateur NXDN or P25 Phase 2 systems on the air – yet.)

To help clarify some of the current confusion, I’ve dedicated some time this weekend to generating audio recordings of various digital audio modes as received by a 5 kHz analog FM receiver. I’ve also generated spectrum plots for these modes.

Please download and play “How to Identify Digital Phone Modes on VHF/UHF” (PowerPoint 2003 format) from: