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Author Topic: RTLSDR based scanner for P25  (Read 7591 times)

Offline IQ_imbalance

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RTLSDR based scanner for P25
« on: January 15, 2019, 0239 UTC »
Ran across this tutorial, and it worked very well for getting a somewhat operational (still working on audio issues!) scanner going based on a rtl-sdr stick and a Raspberry Pi...

https://www.hagensieker.com/wordpress/2018/07/17/op25-for-dummies/
LOG/NE-SW unterminated BOG/DJ-130/800Mhz Yagi
AFEDRI SDR-Net ICF-SC1 SDS-200 various RTL-SDR
Central MD

Offline Josh

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Re: RTLSDR based scanner for P25
« Reply #1 on: January 15, 2019, 0442 UTC »
Very nice. Still waiting for SDRuno to scan and/or do ap25.
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Offline IQ_imbalance

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Re: RTLSDR based scanner for P25
« Reply #2 on: January 15, 2019, 2136 UTC »
ARGH.  It's decoding the traffic (or at least recognizing broadcasts from the various talkgroups) but I can't get it to push the decoded audio to my headphones.  The headphones and software audio device works (at least for speaker-check!), but I don't know if it's a decode problem or a communications problem between rx.py and the audio system :(

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Offline Josh

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Re: RTLSDR based scanner for P25
« Reply #3 on: January 15, 2019, 2234 UTC »
This is a classic example of why I want everything radio related in my shack to be a windows box. A laptop or tablet may take a lot more current than a hacked phone, but the versatility is worth it.
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Offline JimIO

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Re: RTLSDR based scanner for P25
« Reply #4 on: January 16, 2019, 0601 UTC »
I have an RTL-SDR v3 so I looked at that link. I'm not interested in listening to the cops so  I'm not going to do anything. Like all things Linux the answer is waiting for you on the web. I did a quick search and found peoples posts from over a year ago. I found this too:

https://www.microsoft.com/en-us/p/ubuntu-1604-lts/9pjn388hp8c9?activetab=pivot%3Aoverviewtab

Made me LOL when I read you have to reboot before it will work!

Offline k5fms

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Re: RTLSDR based scanner for P25
« Reply #5 on: January 19, 2019, 0023 UTC »
I followed instructions from this page:
https://www.rtl-sdr.com/rtl-sdr-radio-scanner-tutorial-decoding-digital-voice-p25-with-dsd/
and it worked fine. The problem is when trying to decode Project 25 Phase II. RTL-SDR doesn't do this as of yet.

Edit: Windows doesn't do this. I read that it works with Linux.
Tecsun PL660, RTL-SDR v3, Radio Shack discone, homebrew antenna tuner & longwire South of Ft. Worth, TX

Online MDK2

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Re: RTLSDR based scanner for P25
« Reply #6 on: January 19, 2019, 1442 UTC »
Very nice. Still waiting for SDRuno to scan and/or do ap25.

I think I read that SDRuno 1.3 will have scanning. Scanning isn't important to me so I kind of glanced past it, but they were talking about it on Fakebook in the SDRplay group the other day.
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Offline IQ_imbalance

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Re: RTLSDR based scanner for P25
« Reply #7 on: January 19, 2019, 1516 UTC »
I'm not a hardware (or software!) guy so probably not the most informed opinion....but i wonder if dedicated hardware still has the edge for scanning purposes.  I think the fast scanners do about 100 channels/s (of course, you have to identify the channels first!), but with the wide bandwidth of the SDRs it would make sense that they could identify and track transmissions a lot faster _as long as the transmissions are in that reception bandwidth_!  I think it would be an interesting exercise to build something on top of a framework like rtl_power that grabs a power spectrum over a chunk of bandwidth and then shifts to the next chunk, etc. while looking for repeated transmissions on a frequency over a user-defined time frame.  Unfortunately by the time i have those skills everything will be digital and encrypted, probably  :-\ >:(

Anyone care to comment on the bottleneck is for SDR based scanners....data transfer over the network, demodulation, ??? 

There's also the issue of with things going digital, it's not so much the # of different frequencies you can scan but whether or not you can parse the data coming in.
LOG/NE-SW unterminated BOG/DJ-130/800Mhz Yagi
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Central MD

Offline Josh

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Re: RTLSDR based scanner for P25
« Reply #8 on: January 19, 2019, 1901 UTC »
I don't know much about the new gen of scanners that are sdr internally and have handheld or desktop enclosures and displays, but the current sdr apps that scan are mediocre at best as far as scanning rate and so on. I'm sure things will get better with time.
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Offline IQ_imbalance

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Re: RTLSDR based scanner for P25
« Reply #9 on: January 20, 2019, 0256 UTC »
OK update on my 'audio issues'.  Turns out they didn't have anything to do w/ the audio routines in the OP25 software...I just wasn't hitting the control frequency.  I'm still trying to fine tune things, and understand why tuning the center of the control freq. doesn't apparently work (don't think there's a DC spike....).  The control freq. for a system near me is 774.043750 MHz, but my RTLSDR shows it as 774.044563 (more or less).  However, i don't get good decoding if I tune into the latter...in fact, i have to bump it up to 774.044900 MHz or so to get good decoding and actual audio out.

So, it's just a lot of semi-informed trial and error at the moment.  At least now I know the setup works, and what a properly tuned QPSK signal should look like on the gnuplot datascope...
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AFEDRI SDR-Net ICF-SC1 SDS-200 various RTL-SDR
Central MD

Offline ChrisSmolinski

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Re: RTLSDR based scanner for P25
« Reply #10 on: January 21, 2019, 0138 UTC »
I finally got OP25 installed on my RPi, but am having some issues.

Here's the trunking info for my county: https://www.radioreference.com/apps/db/?sid=8201

852.9375 seems to be the currently active control channel.

I use this command to run OP25:

./rx.py --args 'rtl' -N 'LNA:47' -S 2400000 -f 852.9375 e6 -o 25000 -q -2

It runs. but nothing happens. So I hit 1 to bring up the spectrum display with gnuplot. I can see the signals, although they seem to be offset, by the 25 khz offset in the cmd line arguments.
Question: should I offset the control frequency by this amount? Or is the spectrum display offset a bug?

I then get the 6 or so zeros displayed, which I interpret as I have the wrong ppm value (-q -2). I have played around with the ppm setting, but it never seems to help.

I understand I can get a constellation plot with 2, which shows up, but how to interpret it, to know when I am correctly tuned?

Any other pages I can check out for details on using OP25 on a RPi?
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 900 ft Horz skyloop / 500 ft NE beverage / 250 ft V Beam / 58 ft T2FD / 120 ft T2FD / 400 ft south beverage / 43m, 20m, 10m  dipoles / Crossed Parallel Loop / Discone in a tree

Offline IQ_imbalance

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Re: RTLSDR based scanner for P25
« Reply #11 on: January 21, 2019, 0235 UTC »
Check out the SDR forum on radioreference.com...I was going through the same teething pains.  Try tuning to whatever freq your rtlsdr sees as the control channel (and without a -d or -q offset) and see if you start pulling in talkgroups (audio or not).  What I did from there was a lot of trial and error with the control freq in trunk.tsv....Id try fine tuning in the op25 program and then punch that corrected freq into trunk.tsv as the control freq.

For the constellation plot (2) apparently youre trying to minimize the spread of the data points within each of the 4 sections.  I THINK what you want in the datascope (4) is 6 very distinct and clear openings in the data stream....this isnt perfect but gives you some idea of what Im talking about...
https://goo.gl/images/q453W6

In short...tuning is a lot of trial and error!!!!

UPDATE: apparently you should put the actual control freq. from the RR database into the trunk.tsv file, and then correct any RTLSDR tuning errors within OP25 or with the -q and -d options.  Not exactly sure why....

Also, here's a screencap from gnuplot on a successful tuning effort...
« Last Edit: January 21, 2019, 1638 UTC by IQ_imbalance »
LOG/NE-SW unterminated BOG/DJ-130/800Mhz Yagi
AFEDRI SDR-Net ICF-SC1 SDS-200 various RTL-SDR
Central MD

Offline ChrisSmolinski

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Re: RTLSDR based scanner for P25
« Reply #12 on: January 21, 2019, 1812 UTC »
Yes, tuning is indeed a lot of trial and error.  I finally ditched the RPi, and created an Ububtu VM on the Mac, and got things running fairly quickly. Here's what I did, should be the same steps on Windows: http://www.radiohobbyist.org/blog/?p=2601
Chris Smolinski
Westminster, MD
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netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 900 ft Horz skyloop / 500 ft NE beverage / 250 ft V Beam / 58 ft T2FD / 120 ft T2FD / 400 ft south beverage / 43m, 20m, 10m  dipoles / Crossed Parallel Loop / Discone in a tree

Offline Josh

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Re: RTLSDR based scanner for P25
« Reply #13 on: January 21, 2019, 2019 UTC »
Emulations saves the day once again.

Also just saw this;
"Re: Update SDRuno issue

    Quote

Postby Tech_Support Sun Jan 20, 2019 6:51 am
RSP1 owners have not been forgotten or ignored. Release 1.23/4 were developed to provide support for the RSPduo, but they also give a lower CPU load than release 1.22. However, there is a bug that affects the gain control on the RSP1. This is being corrected in the next release (1.3), which also includes a raft of new features including Frequency Scanning. The intent is that all RSPs will be fully supported in release 1.3 and all future releases."
« Last Edit: January 21, 2019, 2029 UTC by Josh »
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Offline Pigmeat

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Re: RTLSDR based scanner for P25
« Reply #14 on: January 27, 2019, 1310 UTC »
Don't you nimrods know if you're going to run Uno, you're going to need to get the cards?

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