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Re: [Discuss-gnuradio] Error in OP25 Decoder block

From: ikjtel
Subject: Re: [Discuss-gnuradio] Error in OP25 Decoder block
Date: Sat, 19 Jul 2014 07:59:42 -0700

Hi Doug

It appears that more extensive changes (than just the tiny patch I sent yesterday) would be needed to gz-baz to make it compatible with the latest (3.7) versions of GR and of op25.  Also (AFAIK) there is no support for P25 Phase II in gr-baz.

The version of op25 (the GR-3.7.x compatible one) that you currently have installed *does* have full RX support for P25 Phase II (but not TX).   Unfortunately there is "some assembly required"; the P25 phase II RX support has not been integrated into the GUI app scope.py - so at this stage of the project the phase II support is primarily oriented to other developers and system builders. If you're willing to roll up your sleeves, however, you can decode Phase II audio with it.  Below is a sketch of the commands you'd need to run (once you have captured a complex I/Q sample of the signal, that is).  The sampling rate (2M in this example), relative signal frequency (706.3 KHz), NAC, SYSID, WACN, and TDMA slot ID are all variables that you would need to set properly for your own local environment.

Note that you may need to set the proper git branch in the gr-op25 repo to access the py scripts detailed below.

You also should have the scope.py app installed - you should be able to use that to acquire the trunk control channel and extract the pieces that you'll need (NAC/SYSID/WACN).  The site seems to be having problems, but the URL for that is http://op25.osmocom.org/trac/wiki.png/wiki/SignalScopePage



#! /bin/sh

# resample from 2M rate to 96K rate, translating frequency by +706.3 Kc
python arb-resample.py  -i ~/20130605-774mhz-2mhz -s 2000000 -r 96000 -g 10  -c 706300 -o 2013-96k.dat

# demodulate
python cqpsk-demod-file.py -i 2013-96k.dat -a -6 -r dbit.dat

# process the demodulated file of dibits (dbit.dat) into audio
tdma_audio.py -i dbit.dat -n 0x4a1 -s 0x4a2 -w 0xbee00 -t 1

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