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:

http://www.bay-net.org/articles.html

Best,

…dtw

 

Comments

  1. FYI: As of 10pm PDT 28-Mar I have uploaded a new version (v1.1) of the presentation to the Bay-Net website. Changes:

    1. Modified pages related to DMR/MotoTRBO so that they now indicate the recordings and spectrograms as being of the uplink “Mobile Station” type.

    2. Added recordings and spectrum plots of the DMR/MotoTRBO downlink (aka “Base Station”) signaling. Downlink and Uplink signaling are as it turns out very different.

    Please download the updated version from:

    http://www.bay-net.org/articles.html

    Best,

    …dtw

  2. Chris KC2SYK says:

    Thanks for this. You should make it a web page so it gets indexed by google and people can play the audio in their browsers. MS powerpoint isn’t exactly a portable format.

    Thanks and 73.
    -Chris KC2SYK

  3. Thanks for the resource but I also would appreciate a website version. The powerpoint is easy to save as a single offline file, but I don’t have Microsoft Office, and open office apparently can’t play the audio files. I can see the slides just fine though. You could use soundcloud or one of these: http://www.makeuseof.com/dir/chirbit-record-audio-clips/ to put the clips somewhere you can embed into your page.

    73s
    James
    N9XLC

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