SDR and TRUNKING for a newbee

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Jan 19, 2003
Howard County
Hello, I just received the hardware. Is there... or can someone supply a brief paper on exactly what steps to follow to be able to monitor/track Digital/ Analog and APCO-25 Common Air Interface and Project 25 Phase II. Using one dongle?

I have been reading through the forums and the more I read about bad plug-ins and versions, the more confused I get.
Please help make it simple.

MANY thanks!..



Feb 24, 2001

There's no one magic answer. Different systems imply different choices. What you want is generally done by two different programs - one for control channel decoding and another for digital voice decoding.

Are these the systems you're trying to follow?

For the first one, you can use Trunk88 or Unitrunker.

Howard County Trunking System, Ellicott City, Maryland - Scanner Frequencies

For the second one, you can use Trunk88 or Unitrunker with VBCable and DSD or DSD+.

Maryland Transit Administration Trunking System, Baltimore, Maryland - Scanner Frequencies

For the third system, you can use Unitrunker (or possibly op25) with VBCable and DSD or DSD+.

Central Maryland Area Regional Communications Committee (CMARC) Trunking System, Various, Maryland - Scanner Frequencies

For the last one, you can use Unitrunker or possibly op25. However, piping voice discriminator audio through DSD / DSD+ is problematic for TDMA since there's no easy way to tell it which TDMA slot to decode.

First Responders Interoperable Radio System Team (FIRST) Trunking System, Various, Maryland - Scanner Frequencies

Specific questions belong in the trunked radio decoder forum:

Trunking Control Channel Decoding - The Forums

Links to the control channel decoding software:

Each program has different requirements and capabilities.

Questions about DSD and DSD+ go here:

Digital Voice Decoding Software - The Forums

Good luck.


Apr 8, 2005
Springfield MO
Everything you're asking to do is relatively easy but having just one dongle is where you'll encounter difficulties: for proper and useful trunked system monitoring you're going to have to have at least 2 dongles (one for the control channel receiving + decoding and one for receiving the voice channels). The other potential is having a discriminator tapped scanner/receiver for the control channel info and then use the dongle for the voice channels, either solution is workable.

Aside from that the one snag in your future plans is P25 Phase II decoding which isn't possible at this time using anything on Windows, period, and at least as of this post I'm making the only potential I'm aware of on any platform is under Linux using OP25, one of the devs made a post recently saying they had just added Phase II support to that software suite but didn't comment on the specific usefulness or actually working nature of the support, just that it's been finally added to the dev tree.

I'm sure that as time passes we will see an update to DSD (the original version) to allow for P25 Phase II decoding, and I have seriously high hopes that whoever created and hopefully continues to maintain DSD+ (the other version) will be able to add P25 Phase II support in a future version. I've never had a lot of P25 content to monitor in my area, and soon I'll have 2 or 3 very large/major systems converting over to P25 Phase II so without some kind of software decoder support over time I'm going to be left out totally. I simply can't afford $500+ scanners for handling P25 Phase II decoding.

As for a list of steps, there are several dozen guides out there for getting things set up, mostly copies of each other with a variation here and there but you'll figure it out like the rest of us. Might take a few go-throughs but sooner or later you'll get that "Cool, it actually works..." feeling. ;)

Just for the record, a clarification so it's more... well, clear I suppose: DSD and DSD+ currently support P25 Phase I (the original 10+ year old protocol) as well as Motorola's Phase II variant which is known as X2-TDMA that was created as an interim product while Phase II was being ratified as an actual standard. Make no mistake, however: X2-TDMA is not Phase II as Phase II exists now and is being rolled out as. In fact, X2-TDMA never really had any traction in the real world that I know of. We have a system here in Las Vegas that claims to have a few talkgroups on a large system that are X2-TDMA in nature (just those specific talkgroups) but I can't say that I've ever noted any activity on them or noticed DSD or DSD+ kicking in X2-TDMA mode decoding.

X2-TDMA does exist, of course, but now that Phase II is a proper standard and being put to use, I suspect that X2-TDMA will just up and vanish as time passes and only P25 Phase I and Phase II will remain, with Phase I being "phased out" also as time passes.
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Jul 19, 2007
NW Ohio
I wonder how SDR's and their software perform receiving P25 simulcast systems? The commonly available scanners are lousy and I would like to get something a little better without spending a young fortune.
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