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  #1 (permalink)  
Old 11-10-2018, 7:02 PM
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Default Finding Control Channel with OP25

Good Evening,
I hope this message finds you all well. I'm in the process of setting up my Ubuntu Linux box with OP25. I've got it all set up and ready to go, just need to find my frequencies of choice. Now here is the link to the ones I want. I'm still new and learning, and yes i've Googled and searched the forums, but I'm not sure I'm looking for the "Right Stuff". I can't seem to find the control channels. Do they not have any?

P25 Frequency List:
https://www.radioreference.com/apps/db/?ctid=1761

Thoughts?

Thanks in advance!
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Old 11-10-2018, 8:26 PM
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Those are not trunking systems, so no, they do not have control channels.
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Old 11-10-2018, 8:30 PM
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The conventional (non trunked) P25 can probably be monitored via op25 by using the -f <freq> command line option without the inclusion of -T
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Old 11-10-2018, 8:48 PM
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Thanks guys. I didn't think they were trunked, but I wasn't 100% sure. I'll give it a try and see how it goes. I'll document it for everyone if it works. Thanks!
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Old 11-10-2018, 9:29 PM
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Please forgive my Noobness....here's what I've got so far, doesn't seem to be doing it.

./rx.py --args 'hackrf' -N 'LNA:47' -S 1600000 -f 155.7980e6 -V

Now to be fair...I'm not sure whom is Tx'ing on 155.798 Mhz. The County Dispatch Freq seems to be silent on the one that's posted on the website.

I guess the other one I could try is Laconia PD...they aren't dispatched through the county.
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Old 11-11-2018, 8:47 AM
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You may need -D fsk4 although that is not by any means certain (try it with and without).
Also, I recommend you switch over to plot #2 (constellation), plot #3 (symbol) and/or plot #4 (datascope) to see if you're actually demodulating the signal correctly.

Finally, you can turn the log level up to -v 10 to see if any p25 frames are being decoded. (tail -f stderr.2)
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Old 11-11-2018, 9:03 AM
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Thanks! I actually was able to figure out the two frequencies. I didnít have gqrx dialed in correctly so it seemed like it was just static. Iíll give your suggestions a try and see what we come up
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Old 11-11-2018, 8:31 PM
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Played around with some setting and have got op25 dialed into the P25 Freq (154.0900).

I adjusted the gain options with some info found here: https://osmocom.org/projects/op25/wiki/SignalScopePage

I can definitely tell when someone is transmitting. See the constellation plots.

I'm not sure if I'm suppose to see anything in the OP25 window, but I'm still not seeing or hearing anything.

Here's the last command I ran

./rx.py --args 'hackrf' -N 'RF:0,IF:16,BB:20' -S 1600000 -f 154.0900e6 -v 10 -V -2 -U 2> stderr.2

Here is the srderr.2 file

gr-osmosdr 0.1.4 (0.1.4) gnuradio 3.7.13.4
built-in source types: file osmosdr fcd rtl rtl_tcp uhd miri hackrf bladerf rfspace airspy airspyhf soapy redpitaya freesrp
Using HackRF One with firmware 2014.08.1
Using two-stage decimator for speed=1600000, decim=16/4 if1=100000 if2=25000
Project 25 IMBE Encoder/Decoder Fixed-Point implementation
Developed by Pavel Yazev E-mail: pyazev@gmail.com
Version 1.0 (c) Copyright 2009
This program comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to redistribute it
under certain conditions; see the file ``LICENSE'' for details.
op25_audio:pen_socket(): enabled udp host(127.0.0.1), wireshark(23456), audio(23456)
p25_frame_assembler_impl: do_imbe[1], do_output[0], do_audio_output[1], do_phase2_tdma[1], do_nocrypt[0]
audio device: default


Thanks!
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Old Yesterday, 6:28 PM
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Any thoughts?
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Old Yesterday, 6:40 PM
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Quote:
Originally Posted by braymond View Post
Any thoughts?
Not a qpsk signal. If it were, you'd have 4 corners of a box.
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Old Yesterday, 7:00 PM
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Ahhh hence the Quadrature part. Ok so I should add -D fsk4 to the command ?


Sent from my iPhone using Tapatalk
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Old Yesterday, 7:54 PM
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Ok I think its still working with the -D fsk4 options, but still not getting any audio output or display in the OP25 window. Here are the Symbol charts with and without transmissions.
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Old Yesterday, 8:46 PM
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Its not a control channel signal. Looks more like an unmodulated carrier wave, possibly some sort of spurious signal. Note that you are not seeing any symbols being decoded, hence neither fsk4 nor qpsk.
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Old Yesterday, 8:51 PM
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Default Finding Control Channel with OP25

Thanks Boatbod, but I thought we already determined that the county wasnít utilizing a control channel but just transmitting a conventional P25 signal. I know thatís the correct frequency because there is someone sharing their scanner feed on broadcastify and I can correlate the transmissions. Please forgive my noob-ness but I definitely plan to document all this for other new users.

https://www.broadcastify.com/listen/feed/2792

Receiver in Laconia NH. Channels are VHF analog and APCO P25 digital conventional. Updated 5/26/12 receiver is now a Motorola XTS3000 Model I.

Please note: Some additional channels have been added. If the feed gets too congested then some channels may be removed. Recently added Belmont PD, Gilford PD.
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Old Yesterday, 9:27 PM
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Quote:
Originally Posted by braymond View Post
Thanks Boatbod, but I thought we already determined that the county wasnít utilizing a control channel but just transmitting a conventional P25 signal. I know thatís the correct frequency because there is someone sharing their scanner feed on broadcastify and I can correlate the transmissions. Please forgive my noob-ness but I definitely plan to document all this for other new users.

https://www.broadcastify.com/listen/feed/2792

Receiver in Laconia NH. Channels are VHF analog and APCO P25 digital conventional. Updated 5/26/12 receiver is now a Motorola XTS3000 Model I.

Please note: Some additional channels have been added. If the feed gets too congested then some channels may be removed. Recently added Belmont PD, Gilford PD.
At best it's an idle channel. There are no symbols being decoded - just a flat line stuck at level 0 which is inconsistent with expected P25 framing. Does this ever change? Perhaps you could listen to the broadcastify feed and try to correlate any changes in the symbol display with audible transmissions (bearing in mind there can be a significant broadcastify delay).
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Old Yesterday, 9:30 PM
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Default Finding Control Channel with OP25

Yea it goes from the bottom image to the top image when comms are going on...maybe my input settings arenít fully correct but I definitely see the change between the two. I get about a 10-15sec delay between seeing the changes in my graphs to the audio coming through on broadcastify.
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