RTL-SDR Newbie

[Micdisc16]

To introduce myself, I'm an Electrical Engineer who has recently become very interested in learning about digital radio. Particularly since my Police department went to the P25 Simulcast system. The department went to this system in 2013 and I have dealt with not having a scanner since that time. Now I am trying to listen to this system, but I am not having much luck.

I have followed this guide:
RTL-SDR Tutorial: Decoding Digital Voice (P25, DMR, NXDN, D-STAR) with DSD - rtl-sdr.com

And have been partially successful.

So to get to my question:
My understanding of the P25 system was that you have control frequencies and voice frequencies. However, when I go to my County's RadioReference page, I only see control channels.

https://www.radioreference.com/apps/db/?ctid=439

So the question is, what do I tune to in order to hear voice? If I go to these control channels listed on the website, but I see no information coming through with SDR#. No spikes. I have DSD+ 1.51 running as well by the way. If I scan around the spectrum of 851 I can find that frequencies are definitely being used all over the place, however, when I tune to these channels.....
I do indeed get information coming through DSD+, but never voice. I get NAC addresses (different ones), and then some NXDN96 and 48 coming through.

P.S. - Also, using Unitrunker on the control channels doesn't yield anything.

Any help or advice is appreciated.

 

[br0adband]

The actual RR entry for the P25 Phase I system in your area is at this link:

https://www.radioreference.com/apps/db/?sid=7430

Since every single frequency there is showing in red which is meant to convey control channels (red for primary, blue for secondary) I'm going to assume that there's a mistake someplace since obviously a modern trunking system can't really work well with every assigned frequency being capable of use as a control channel. I would guess that it's possible, sure, just not particularly efficient in the long run.

Having said that and since I/we don't know precisely which of those 10 listed frequencies are actually in use as a control channel (should be only one of them at any given point in time), you're going to have to use SDR# to tune them one at a time until you hear the familiar P25 Phase I control channel signal. You can hear samples of what to listen for here:

Project 25 (P25) - Signal Identification Wiki

When you find which of those frequencies is a control channel, that's what you use with Unitrunker for the Signal receiver which is the RTL stick assigned for the duty of receiving the control channel signal for decoding. But before you go that far you'll need to ensure that your setup is functional meaning you need basically three components to decode P25 Phase I traffic with a computer (not counting the computer itself + the RTL stick(s); it can be done with just one stick but two is the preferred configuration with one assigned as the Signal (control channel) receiver and the second for Voice channel/frequency reception. The three components are:

1) The software for controlling the SDR device to be able to tune in the actual frequency or frequencies necessary and be able to decode the P25 Phase I control signal (this would be Unitrunker but you can use SDR# or something else for non-trunked P25 Phase I traffic)
2) A virtual audio cable which will pass the baseband audio signal from the SDR software to DSD+ for decoding the digital formats (mentioned in the other thread I'm about to provide a link to)
3) DSD+ for decoding the Voice channel traffic which carries the P25 Phase I content

Instead of typing this out all over again I made a few posts in another thread recently in the attempt of helping another person do what you're attempting to do so I'll point you to that thread for more reading:

http://forums.radioreference.com/vo...coding-software/326941-airspy-unitrunker.html

Now the person that started that thread is using an Airspy SDR device but since it is controlled by the RTL2838U chip (for the USB connectivity) it works pretty much exactly the same with respect to being used by SDR# or Unitrunker so the information is still relevant.

Feel free to ask questions if necessary but if you do please at least mention what specific hardware you have (and whether it's one or two or more of them because with the RTL sticks you'll really need at least 2 to do proper trunk tracking reception, if it's an Airspy or something with a much wider bandwidth window then it's possible to use just that one device).

Based on what you said, if you see activity with DSD+ and NACs and other info being listed then yes, you've got a somewhat working configuration (you wouldn't see anything if it wasn't working properly) - now it's probably just a matter of bringing Unitrunker into the mix (where SDR# would then be removed from the config entirely so Unitrunker can do its thing).

 

[ka3jjz]

Just as a point - P25 system by Harris can use any channel - not just 1 or 2 - as a control channel. I suspect that is what we have here, although a Georgia resident would have to confirm that

Mike

 

[br0adband]

Didn't even know that about Harris systems, learn something new every day, go figure.

 

[Voyager]

Just as a point - P25 system by Harris can use any channel - not just 1 or 2 - as a control channel. I suspect that is what we have here, although a Georgia resident would have to confirm that

Mike

And Motorola can use 1 to 4 channels as control channels. (one primary and 0 to 3 alternates)

 

[Micdisc16]

My problem is that when tuning to each of these frequencies I cannot see anything. I live relatively close to the center of the antenna coverage in the City so I don't believe that reception should be a problem. I did Occasionally see a spike on the first frequency listed (851.275), however, it wasn't a constant stream like a control channel would be.
My last resort is to just buy an actual scanner, however, the enthusiast in me would like to get this working. I'm only using one tuner right now, but I might get a second tuner for Unitrunker just because they are cheap. If it doesn't work, then I'm not out that much.
Something tells me I'm missing something simple though, just can't figure out what that is. As far as the br0adband's first reply, I do have all the software installed and setup using the Virtual Audio Cable with the correct default recording device selected, and correct output on SDR# selected. In any case.......

Thank you for your help.

 

[dave3825]

The first thing you need to do is figure out is why your not getting anything on the systems freqs. I would start by checking and asking in the Georgia forums about this system to confirm that those freqs listed are indeed correct..


Are you close to the jail? With sdsharp, try tuning to this freq

853.3500 Jail DC Jail (Detention Center) P25

Does anything come in?

 

[br0adband]

P25 isn't hard to recognize so if necessary just go stepping through the entire 850 MHz band to locate P25 traffic. You said in your original post that you had activity from DSD+ showing NAC identifiers - if you land on a P25 Phase I control channel that's all you'll get: NAC info which just constantly scrolls by, and never any voice traffic to decode. I personally have no experience monitoring a Harris manufactured P25 system as noted above so I can't help there if it's using some funky style or way of doing things but even so there should be a constant P25 Phase I control channel somewhere in that 850 MHz range for that system.

With 10 towers in the system it should have pretty damned good coverage considering but there's never a perfect setup no matter how much hardware is present or how much money is spent I guess.

As noted, perhaps checking out the Georgia subforum here at RR might yield more info and if not then posting a question specific to that system might get some more relevant info. That simplex P25 frequency for the County Jail might yield something of use as well, seems a bit odd to have just one channel/frequency for that use but it could be a situation where they intend to switch everything over to P25 at some point and are just using that as a test case, who knows.

If you're getting DSD+ activity showing NACs then you've got the hardware set up properly at least to some degrees, now it's just a question of acquiring some voice traffic. With just one RTL stick it is possible to monitor a trunked system but it's not very efficient overall - a second stick really makes things work as they should so consider grabbing another one at your earliest convenience.

The current FCC licenses for that system (two licenses) show the correct frequency information matched up with what's posted on the RR database entry as well.

 

[Voyager]

Make sure the gains are sufficient on your dongle. One of the channels should have a control channel stream if it's trunked.

 

[Micdisc16]

Figured it out....Somewhat

Okay. I have figured out why every frequency in the database is listed as a control channel. It looks like at any given time the control channel can change between these frequencies. The other frequencies at that point become voice frequencies. I figured this out by finally finding one of the control channels and then Unitrunker started decoding the site properly. The next night I went back to the same frequency and it was no longer broadcasting the control. It had become a voice frequency. After searching some more, I found that another frequency in this group had become the control.
This explains why the database listed all of them as control freqs.
Has anybody ever encountered this type of setup?

So with all of that said, my next problem is decoding the voice streams. I followed the tutorials perfectly and even ordered a second dongle with a strong antenna from RTL-SDR blog for setting up Unitrunker. Unitrunker tunes to the correct freqs each time a person is queued up. And DSDPlus starts decoding the P25 stream, but it is mostly garbled. I've tried messing with every gain I can find. Even tried a different Virtual Audio Cable.
I found the DSDTune and tried that route of tuning the recording and then starting back DSDPlus with that setup information. That helps, but it is still mostly garbled mess.

Any ideas on what I'm doing wrong? Do you think that DSDPlus won't do the P25 that Harris has set up?

 

[BM82557]

Have you adjusted the dongles PPM settings? Being off freq can cause what you describe.

 

[buddrousa]

If control channels hop (change) it is done every 24 hours most of the time. These are settings in the prom in the controller.

 

[Micdisc16]

PPM

Yes, I have adjusted the PPM for each dongle. The first was a cheap Ebay dongle and set at 41 PPM, the second is temperature compensated and is right around 0 to 1.

I'm not sure why the sound is coming in the way it is. Seems to be a decent signal strengh. Every once in a while ( like 1 in 50 transmissions or less I can understand a clear statement with words)

Kind of baffled by this. I wonder if Harris P25 isn't a standard P25 Phase I decode. Looking at the Contract on our City's engineering webpage, it looks like they are using Harris radios which use the IMBE vocoder. I wonder what DSDPlus is using.