P25 Decoding

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Premium Subscriber
Jul 23, 2017
Go easy on the new guy. I've been decoding with SDR for going on 4 days now.

I've actually figured out much of what is going on however I keep building on it and creating problems for myself. I'm using New Bern NC frequencies for trunking.

I started with SDRtrunk on Linux and was able to decode local P25 Phase 1 stuff with no problem. Got my city Public Services trunking frequency and all is well. Even hooked an AirSpy up and got the whole 6MHz of channels as well. At this point I'm happy.

Then I decided to add another trunking channel. VIPER. The old laptop I had seemed to choke a bit on two control channels. Then I decided to put Unitrunker on a Windows 10 laptop. Let me first preface this by saying that later when I put SDRtrunk on the same laptop I get both control channels fine. (Public Services, VIPER). I think one frequency is outside of my bandwidth and that's not an issue for me.

One control channel (Public Services) decodes with LSM in SDRtrunk. The other channel (VIPER) seems to decode in C4FM. Again both are working EXACTLY as I want.

However in Unitrunker the VIPER channel gets 100% health and decodes perfect whereas the Public Services channel gets health in the yellow and red. 67% max (and yes I read about the 40 packets thing and this being a good signal). The problem is it drifts into the red and audio is awful and choppy.

I've tried 5 different antennas and even drove downtown with my laptop. No matter where I am I can never get a solid signal that isn't all choppy and inaudible.

I suspect this is a P25 decoding issue with Unitrunker which I think is supposed to autodetect between LSM and C4FM. Using latest preview version of Unitrunker.

The fact that I can get a clean trunking signal with SDRtrunk at home makes me know it isn't a signal problem but rather a software issue.

I guess it doesn't matter since I can do what I want with SDRtrunk pretty much but it's driving me nuts that I can't get Unitrunker working. Is there any way to specify the P25 protocol?

Any and all help appreciated.
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