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FLDIGI on DMR

Discussion in 'General Technical Questions and Answers' started by KN4EHX, Nov 12, 2018.

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  1. KN4EHX

    KN4EHX Ham Member QRZ Page

    Greetings,

    I have attempted to use FLDIGI MT63-2KL to pass a message over DMR. When we tried this experiment for the simulated emergency training in my county it was discovered we could not receive the messages over DMR, but we could over traditional analog 2 meter wide band. (Acoustically)

    My first assumption was maybe the speaker was too loud and was over driving the mic; we adjusted to volume on the speakers and tried again with the same results. Next guess would be bandwidth on DMR.

    Does anyone know why this happened and if I can not use MT63-2KL is there another mode that I can use with DMR?

    I have searched Google and have not found anything involving FLDIGI over DMR.

    FYI we used simplex for the test and didn't torture anyone on the repeaters.

    Perhaps another mode?

    Thanks,
    Nate
    KN4EHX
     
  2. KK5JY

    KK5JY Premium Subscriber QRZ Page

    Most DV modes use voice modeling to extract human speech patterns, and then encode them efficiently for transmission over the digital channel.

    Most such codecs aren't designed for communicating arbitrary waveforms, such as those generated by digital modems. It's almost certain that the waveform is being fatally disorted by the codec during transmission.

    If you want to encode MT63 onto a VHF+ frequency, the best way to do so is with a SSB transmitter. That's the way it was designed to be transmitted. Any analog voice mode should work, but the most efficient way is with SSB.
     
    KA9JLM likes this.
  3. KC9YGN

    KC9YGN Platinum Subscriber Platinum Subscriber QRZ Page

    Um, well, why? FT8 is already a digital mode ideally suited for poor propagation conditions. It was never intended to be encoded a second time into DMR and transmitted that way. You're defeating the whole purpose of using FT8 in the first place. Basically what you're doing is taking a highly efficient method of digital communications and trying to convert it into a form that is much less efficient.

    In any case, the DMR encoding is going to rather drastically alter the tones used in the FT8 signal being fed to it, there is going to be even more alternation of the received signal on the receive end and, well, I don't see how it could work at all.
     
  4. KN4EHX

    KN4EHX Ham Member QRZ Page

    Perhaps you know something I don't know, but I'm under the impression FT8 and MT63-2KL are two very different modes.

    Basically I need to be able to send a message with actual text rather than just my call. Ex. Multiple trees have fallen across Main St. taking down power lines 30 homes without electricity. Requesting chainsaws and a plow truck.

    Is MT63-2KL a form of FT8?
     
  5. KC9YGN

    KC9YGN Platinum Subscriber Platinum Subscriber QRZ Page

    Apologies for that error. Apparently I have ft8 on the brain of late. But we're still talking about the same situation. Won't work to the best of my knowledge. MT63 is designed for use in difficult communications situations, is tolerant of mis-tuning issues and is sent with the transmitter in normal SSB mode. It is already a type of digital communications mode. So you're trying to take one digital signal, re-encode it into a second mode that is less tolerant of difficult conditions than MT63 is, then decoding it at the receiving end back into MT63 and then trying to decode that... And I don't think DMR can even handle that kind of thing anyway. As KK5JY said just send it standard SSB if you need to use MT63
     
  6. KG2C

    KG2C XML Subscriber QRZ Page

    I would try something less sophisticated that won't be damaged as much by the audio compression inherent in DMR and similar modes. I would give PSK31 and RTTY a try.
     
  7. WA9ZZZ

    WA9ZZZ Premium Subscriber QRZ Page

    KK5JY gave the basic reason this did not work. There is a lot of digital compression going on to get the bit rate low enough for the available channel. This is why even voice gets distorted. A non-voice signal will be even worse.

    If what you need is to send text messages, you should take a careful look at the radio reference manual. DMR standards do support text messaging capabilities. You would have to check your radio documentation to see if your radios have the capability and how to use it. To send text, the radios would just bypass the voice coder and send the messages directly as packets.
     
    KC9YGN likes this.
  8. AA5CT

    AA5CT Ham Member QRZ Page

    re: "There is a lot of digital compression going on"

    It's NOT even "compression". IT IS a whole different animal as KK5JY posted. This has been the case with "digital" cellular starting back in the 1990's. The 'dispatch' radio services (using P25, DMR, etc.) are just 'catching up' so to speak to where 'cellular' has been for twenty years.
     

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