DSD+ and Unitrunker Help

[scantopeka]

I'm new to SDR and I've been trying to monitor my local P25 system. I've got two SDR dongles and Unitrunker is tracking the system and following the voice channels with the other dongle.

Unfortunately that's it, I can't get any voice traffic at all. I'm using the free virtual A/V cable but DSD is not playing any audio and does not appear to be decoding the P25 voice. It's possible I don't have the audio channels routed correctly, or it could be the voice signal is too weak to decode. Any suggestions?

 

[SCPD]

If DSD won't work for you, try DSD+.

First thing is to make sure discriminator audio is actually making it to DSD(+).

DSD+ has an integrated oscilloscope. If you're using DSD, run UniScope and point it at the same virtual audio path as DSD (they can share the same device) to see the wave form.

 

[scantopeka]

I'm using DSD+ and the built in scope is showing the voice signal for the voice channel.

I haven't tried DSD because I read it doesn't do as well with weak signals.

 

[SCPD]

Good. Make sure your correction value for the voice following receiver is as dead-center as possible.

An easy way to tell if you are off-center (without the benefit of a waterfall display), is to select the scope tab and click the start button while watching the waveform. If the waveform starts off high before settling on the middle of the scope window, the real signal is above (higher in frequency) which means your correction value is too low. If the waveform starts off low before settling in the middle, the real signal is below (lower in frequency) which means your correction value is too high. Stop the receiver, adjust the warp value, and repeat the process until the waveform stays centered vertically in the 'scope tab.

 

[scantopeka]

It sounds like that is most likely the issue. The waveform in the scope is very large and takes up the whole box every time. If a good signal is centered in the scope that's probably what's causing the issue. I'll try and post a photo or video of the scope so you can see it.

Thanks for the help!

 

[Amraks]

What is DSD+ command prompt doing?
You should see everything getting decoded.

I had a my virtual cables settings wrong the other day, it would detect the sound but you couldn't hear the voice.
So tweaked it back to the original settings.

 

[scantopeka]

Yeah it's not showing any kind of decoding at all. I think it's possible I have the virtual cables setup wrong. On the free virtual cable program I'm using I can't figure out how to change them. I might try the paid version and see if they work any better.

My signal health on Unitrunker control channel varies widely from 0 to 90 and its constantly jumping up and down. I'm going to work on a better antenna first and see if it helps. I'm using the little antenna that came with it and a home brew that might not be any better. Once I have good antenna installed I'll repost the results.

 

[SCPD]

You can also try VB cable instead of VAC.

If you're able to decode the control channel at 100%, you should see good results on the voice channels.

 

[scantopeka]

So I think I'm getting closer, I have one of the receivers locked in on the control channel and tracking the system, the signal is 90%+ almost the entire time.

I changed to the alternative virtual audio cable and now if I tune to a P25 voice channel using SDRSharp, DSD+ begins decoding and I can hear the audio.

I still can't get voice traffic to work in Unitrunker though. The receiver is hopping to the different voice channels but DSD+ is not decoding and I can't hear any voice traffic. I have the virtual audio cable selected for digital output.

I suspect the issue now might be I don't have the correction set correctly. I have used SDSharp to figure out what the proper correction should be, but when I enter it into the correction field it seems to move the frequency up instead of down. Any tips or suggestions?

 

[Amraks]

How many LCN are there and how far apart are they?

 

[scantopeka]

There are 15 showing in Unitrunker, and the system I'm trying to monitor is Topeka/Shawnee County Kansas P25 System

 

[Amraks]

park the voice receiver on the control signal to make sure its calibrated right.
correct it if it needs to be.

 

[scantopeka]

I think that's where I'm getting tripped up. How do I correct it?

With the current control signal I used SDRSharp to help me zero in on the correct frequency and then I put that into UT with no correction value. With the voice signals I can't do that because they're always changing. I determined what I think that offset value should be but when I enter it, the value pushes the frequency up not down.

For instance say the correct frequency is 851.000, I determine the receiver is getting the best signal on 850.7565 so my offset or correction should be .2435. Problem is when I put that value into the correction field in UT it changes the frequency to 851.2435 instead of taking it down to the correct frequency.

Of course I may not be doing this right at all so let me know if I'm missing something.

 

[Amraks]

In unitrunker are you setting your correction in the settings which is ppm in sdrsharp?

Set this ppm value to 0 in sdrsharp to start off with.
Then enter your frequency it won't matter if its not spot on at this stage.
Now set your ppm until you line up with your signal I would use the control signal to calibrate it.
doesn't matter if you are using it for voice we only need to find out the correction value of this receiver as they vary for each individual receiver.

Once you have the right PPM(correction) you can input this into unitrunker on that particular receiver...

http://www.atouk.com/SDRSharpQuickStart.html

 

[scantopeka]

Thanks I think I'm making good progress, the system is being tracked well and I'm seeing the voice in the scope on DSD+ but it's not decoding the voice yet. I'm wondering if the gain is too high on the voice channel, the signal fills the entire scope. Hopefully with a little tweaking I'll have it. I'll try and post a photo of the DSD scope when I get home.

(I was a little surprised how much correction my second receiver needed. The first one was only 5 ppm but the second one was 50 ppm!)

 

[scantopeka]

Here's a capture of the DSD scope.

 

[Amraks]

what are your virtual audio cable settings? for each connection

Can you send me a screen shot of the dsd decoding and send me a screen shot of the virtual audio cable window.

 

[DSDPlus]

Here's a capture of the DSD scope.

Is this flat audio? No filtering / de-emphasis?

 

[scantopeka]

Getting closer!

Heard the first audio traffic last night, I continued to tweak the gain and sampling rate and finally started to get some audio. It only works about 50% of the time and some of it is very garbled but it's progress. I really appreciate the help.

I have posted screen caps of each of the current settings, as you can see on the DSD decoding it's not getting clean data yet because it's saying "encrypted" but the voice traffic on those channels is not encrypted. If you see anything I should try tweak please let me know.

 

[Amraks]

Your system is encrypted by the looks of that.