[op25-dev] OP25 and GRC

r_johnson2020@yahoo.com [op25-dev] op25-dev at yahoogroups.com
Fri Apr 7 13:09:28 UTC 2017


As I remember it, I had to first disable the File Source block and the Throttle block, they were causing problems for some reason. Then of course the proper source had to be enabled. Then the first FIR filter in the chain had to be connected to the USRP or RTL2832 source (in this case I'm using a RTL device). I received a warning about that and the flowgraph would not compile/run without it being connected.

One of the issues with the OP25 flowgraph example from gr-baz github, in that it seems to be two circuits (flowgraphs) in one. In the flowgraph, there's one circuit that uses the Combined OP25 Decoder block, and a separate circuit which uses the Simple OP25 Decoder block (it may have been much simpler to have two distinct flowgraphs). 

I decided to use the circuit with the Combined OP25 Decoder block, and carefully and meticulously clear away any blocks that were not necessary for the operation of that circuit. I cross referenced it with another flowgraph using the Combined OP25 Decoder block and everything looks correct.  When I run the flowgraph and observe the symbol decoder graph (DiBits), at first I could not see the symbol stream lines at all. - There are three parameters which can be adjusted. The Fine Offset, Xlate Bandwidth and Gain. After fiddling with these parameters for a time, I started to see two symbol lines appearing, the uppermost and lowermost, although they weren't perfectly well defined. I also noticed that by slowly adjusting the Fine Offset and BW, I was able to get one symbol-stream at a time ("scroll through" the four symbol streams), but never all four at once (you need all four to reconstruct the audio). 

I believe that sample rate has a lot to do with it, since every block in a flowgraph depends on the master samp_rate block. At best, I was able to get the four symbol streams, but never parallel with each other. In other words, the symbol stream was jumping between all four positions, fragmented, but never all four streams flowing concurrently.

I attempted this with different P25 radios, different antennas on my RTL devive, high-power low-power etc, but still am not able to decode it. The flowgraph seem to be very finicky and requires fine adjusment. But it also clearly works, as shown in an older video of Balint's. He makes it look very simple.

 
World's cheapest P25 receiver w/ decryption: GNU Radio + OP25 + $20 RTL2832 DVB-T Dongle
https://www.youtube.com/watch?v=wShOLgW2tmI https://www.youtube.com/watch?v=wShOLgW2tmI


Rob
 
 

---In op25-dev at yahoogroups.com, <superstardodge at ...> wrote :

 I could not get anything out of the flow graph either , I can see a signal on fft, I was going to try using a LNA with single maybe a stronger signal is needed. I have since replaced my HDD and had to reinstall grbaz and cannot get op25.grc to run at all now without crashing 
 What did you use for grbaz install instructions?
 --------------------------------------------
 On Thu, 3/30/17, r_johnson2020 at ... mailto:r_johnson2020 at ... [op25-dev] <op25-dev at yahoogroups.com mailto:op25-dev at yahoogroups.com> wrote:
 
 Subject: [op25-dev] OP25 and GRC
 To: op25-dev at yahoogroups.com mailto:op25-dev at yahoogroups.com
 Received: Thursday, March 30, 2017, 5:23 PM
 
 Hello,
 
 Regarding Balint's newer
 (2016 version) OP25 flowgraph for GRC, has anyone been
 sucessful in getting it to work yet? 
 I see
 that there is a recent thread here regarding OP25 command
 line version from op25.osmocom.org/trac/wiki. I've been
 able to get that version working on the command line. No
 problems there. But when it comes to running the OP25
 flowgraph in GRC, well, it seems to require some black magic
 to get it to work (I've heard rumors of a few people
 getting it to work, but unverified).  The command line
 version is great and works wonderfully, but it doesn't
 really teach us much about the inner workings of DSP and
 P25. 
 
 I'm running a
 dedicated Ubuntu 14.04 machine. I installed OP25, GNU Radio
 etc verbatim from the install instructions on the OP25
 install page. Every thing surprisingly went well. Regarding
 Balint's flowgraph, I can get the flowgraph to compile
 and run. I set the center frequency to the frequency I'm
 transmitting on. All the graphs and scopes seem to
 functioning correctly. They show a signal being received
 (from my transmitter), but it just can't seem to get
 around to decoding the P25 signal. 
 
 I know that gnuradio-companion (GRC) and
 flowgraphs aren't technically a part of the OP25
 project, but I thought someone else here may have fiddled
 with it and got it to work. I've been trying on and off
 for several days without success. 
 
 http://wiki.spench.net/wiki/Gr-baz#op25 http://wiki.spench.net/wiki/Gr-baz#op25
 http://wiki.spench.net/wiki/Gr-baz#op25 http://wiki.spench.net/wiki/Gr-baz#op25
 https://github.com/balint256/gr-baz https://github.com/balint256/gr-baz https://github.com/balint256/gr-baz https://github.com/balint256/gr-baz
 
 Here are the links to
 Balint's wiki and git page if anyone cares to play
 around with it. I believe the wiki page still contains the
 outdated (2013) version of the flowgraph, whereas the
 example at the git page was updated in late 2015 for GNU
 Radio v3.7. 

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