Is it possible to decode DMR using a SignaLink and a plain old analog radio (in my case a Kenwood TM-V71A)? I had a hunch that people must have been doing it otherwise what was the point of software such as DSDPlus and MultiPSK? Both of these pieces of software claim to allow decoding of DMR (and P25, D-STAR, NXDN and other modes) using a sound card interface which takes the audio out from a radio into what looks like a "line in" or "microphone" on a computer.
TL;DR - The results are disappointing so far, but it doesn't look impossible to achieve. Next steps for me are to improve the signal quality into the radio. The steps below I think have maximised the quality coming out of it.
There are a few key lessons I learned in this process. This will save you a lot of time as this info isn't neatly assembled anywhere else that I could find:
- The Kenwood TM-V71A has 4 audio outputs (5 if you include the built-in speaker) and they are not all the same:
- There are the two External Speaker jacks labeled SP1 and SP2
- There is Pin 5 on the Data Terminal 6-Pin DIN connector which is labeled "PR1 Output 1200 (bps) repeat signal"
- There is Pin 4 on the Data Terminal 6-Pin DIN connector which is labeled "PR9 Output 9600 (bps) repeat signal"
- For the purposes of using a SignaLink you should use the outputs on the Data Terminal DIN. Those outputs are taken before the squelch and volume controls on the front. This means they are always "on"
- The SignaLink vendor sells a ready-made cable to interface with the radio (Part #SLCAB6PM) which makes attaching the radio to the SignaLink a simple plug-n-play process.
- The SignaLink vendor also sells a ready-made "jumper block" (Part #SLMOD6PM) which fast-tracks the process of configuring the jumper wires as they show in their instructions. THIS WILL NOT WORK FOR DMR. You will need to manually configure the jumpers as follows: G->2, PTT->3, MIC->1, and importantly SPKR->4 (if you use the jumper block it will connect SPRK->5).
- Why is this a problem? Well, the pin 5 takes you to the "1200bps" repeat signal which is too filtered. You need to use the "9600bps repeat signal" which is the most unfiltered audio output you can get.
- I contacted Tigertronics (the SignaLink vendor) in Feb 2022 and they advised that they are aware of this issue and will be creating a replacement product that lets you select between "1200" and "9600"
- There is only a tiny nugget of info about the difference between these ports on pp 83-84 of the Kenwood user manual.
- Via either the menus or the MCP-2A (see below) software, you should configure the following radio parameters:
- Menu 517 "Data Band" (DAT.BND) - this selects which band is used for TX/RX. Since the Air Bands are only available on Band A and since you can use a SignaLink to decode ACARS on the Air Band, I set mine to Band A.
- Menu 518 "Data Speed" (DAT.SPD) - This selects whether you are using Pin 5 or Pin 4. You should set this to "9600" which is Pin 4 on the connector. In practice I noticed that even when set to "9600" there is still audio coming out of Pin 5. I haven't investigated further.
- There is also a setting on the Kenwood TM-V71A that is ONLY accessible via the MCP-2A software. This is Kenwood's official "Memory Control Program" which allows you to change the various settings.
- Load the MCP-2A software and make sure it's connected to the radio via the correct COM port
- As a "best practice" download the data from the radio so you don't unintentionally over-write anything
- Select Edit - Data Terminal and in the window you can see a setting "PR1 Pin Output Level" with an option "For Packet Mode"
- By default this is at 9. In my case I've had to play around with this and found that 6 is a good level.
- There is no other way to set this parameter except via the MCP-2A software (let me know if this is not correct)
- Write all the parameters back to the radio (the settings don't take effect until you do this).
- You now have the cleanest audio available from the radio coming into the computer. From here it's all about the software. If you just want to decode ACARS, APRS and other "simple" protocols you are all set. To do DMR I tried MultiPSK with zero luck. With the DSDPlus "Fastlane" release I was able to get a smattering of decoded audio. If you want to give it a go, there is some raw audio I collected: https://vk2.net/resources/[email protected]. That's about as good as I got (note that you need to decode that raw .WAV file to get anything useful). There are a bunch of tuning steps with the DSDPlus software including ways of automating the tuning process (see here: https://forums.radioreference.com/threads/dsdtune-for-dsdplus.305050/).
Kenwood TM-V71A connected to a SignaLink