Project - DMR MotoTRBO Connect Plus Decoding Without Expensive Scanner

As a radio hobbyist, I've enjoyed listening to scanners and hearing local law enforcement, fire, rescue, dispatch, etc.  But these days, it seems that everything has gone digital.  I have spent plenty of money on ham radio gear, but couldn't justify the cost of a fancy digital scanner capable of listening to the DMR MotoTRBO Connect Plus trunk system in use in my county.  Talking with a good friend and mentor Ron (KA3JIJ) led me to a project.  Using a PC, SDR dongle(s), and software to listen to the system.

I started by gathering the information about the system from RadioReference.  They had the system ID number, frequencies, talk groups etc.  Ron gave me an SDR dongle he had lying around.  I downloaded SDR#.  Though this software was not needed for this project, it was fun to play with and gave me a key piece of information...the PPM correction for the SDR.  I watched a few tutorials and then downloaded DSDPlus.

I got knee deep in the configuration before I realized I would need either another SDR or a discriminator tapped scanner.  The reason for this is due to it being a trunked system.  One SDR and instance of DSD apps is needed to track the "control channel" and another SDR and instance of DSD apps is need to track the "voice channels".

I modified the batch file for the control channel FMP app to use my SDR, have the PPM correction value, control channel frequency, and to output to port 20001.  I set the control channel DSD app to bring audio in from port 20001and for now output audio to my speakers.  I modified the frequencies file to have the frequencies, Channel (TS) Number, etc.  I modified the groups file to have nicknames for each of the TG's.

Once I started the 2 control channel batch files, the decoding began. I could see calls coming through and which channels they were on! The system auto populated some TGs, RIDs, and neighbor sites.  I could even hear traffic if it was on the control channel frequency!  The next step when I get another SDR will be to set it up for the voice channels and I should be able to listen to all the traffic as it goes through the trunked system.

73 de W4LSG sk

UPDATE: 12/12/2019:
Another friend of mine, Clayton (W4CMB), had an SDR that he didn't use. He mailed it to me and I plug it into my PC.  I configured the voice channel FMP to use the new SDR and the PPM correction.  The other values were all left at default.  I had to modify the control channel DSD app so it didn't output sound.  It's purpose is to create traffic files that the VC DSD uses to follow the conversation.  There is no need for audio out of the CC DSD.

Now everything is working. I'm able to see the traffic on the control channel and hear the audio as it routes through the trunked system.  I've assigned priorities to the TGs so it will follow only 1 conversation at a time.  The only issue I'm having at this point is when there are multiple TGs active at the same time.  It's as if I'm losing the signal and the audio will be choppy or not there at all.  Once there is only one TG active, the audio comes through fine. This may be a gain or PPM correction setting...

All in all it was fun project. It's an alternative way to hear digital systems without buying an expensive digital scanner.  However it is not portable at this point nor feasible to leave this laptop PC running all the time.  I will investigate getting this to run on a Raspberry PI or on a small form factor PC.  The goal is to have a smaller self-contained system that I can plug up and it will auto start and function much like a scanner does.