[op25-dev] OP25 -- can't even resolve the NAC
louiswilen at gmail.com
Tue Apr 14 17:19:17 UTC 2020
I am in Montgomery County, MD trying to use OP25 to monitor the police
and fire system.
The platform is a Raspberry Pi 3b with the OS and all known drivers
GQRX hears and shows a strong signal on the primary control channel
frequency, 853.8625. GQRX also shows a strong signal on alternate
control channel 853.6875. (I got the control channels from
Also, I can turn in local FM, VHF ham radio, etc. stations reliably with
GQRX. The audio is excellent. So my dongle appears to be fine.
Using a standalone scanner, I’ve verified the primary control channel
frequency is what I have specified as the current control channel, 853.8625.
I’m trying to find the NAC by running:
./rx.py –args “rtl” -N ‘LNA:47’ -S 2400000 -f 853.8625e6 2>stderr.2
And that gives nothing on the OP25 console screen except “Frequency
853.862500(0)” in the bottom left corner. This should give me the NAC
and other info in the upper left of the screen, but that area is blank.
Even though I know the control channel is on 853.8625 (based on my
standalone scanner), I figured that I'd try the above command but
specify the alternate, 853.6875e6, as the control channel frequency.
That gives me the following in the upper left corner:
NAC 0x8e2 WACN 0xbee00 SYSID 0x8e0 853.687500/808.687500 tsbks 114
That looks sort of good. But if I then use the secondary control channel
and NAC 0x8E2 in the trunk.tsv file, the OP25 console screen still does
not show any voice activity.
The OP25 spectrum plot (option 1 plot) does show the peak for the
control channel that I’ve selected.
And I still don’t know why I can’t get the NAC or any meaningful OP25
console data for the primary control channel to appear since I know it’s
the control channel that is in use, based on watching my standalone scanner.
The stderr file does not show anything that seems abnormal. Here it is
when specifying the primary control channel in the rx.py command line:
gr-osmosdr 0.1.4 (0.1.4) gnuradio 22.214.171.124
built-in source types: file osmosdr fcd rtl rtl_tcp uhd miri hackrf
bladerf rfspace airspy airspyhf soapy redpitaya freesrp
Using device #0 Realtek RTL2838UHIDIR SN: 00000001
Found Rafael Micro R820T tuner
[R82XX] PLL not locked!
[R82XX] PLL not locked!
Using two-stage decimator for speed=2400000, decim=25/4 if1=96000 if2=24000
Project 25 IMBE Encoder/Decoder Fixed-Point implementation
Developed by Pavel Yazev E-mail:pyazev at 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.
p25_frame_assembler_impl: do_imbe, do_output, do_audio_output,
metadata update not enabled
Allocating 15 zero-copy buffers
Flowgraph completed. Exiting
My dongle is very stable and appears to be 'on frequency'. Still, I've
tried using offsets and changing the frequency a bit to no avail.
So what I’m mainly looking for is the NAC for the system at this point.
Once I get that to appear on the screen when specifying the primary
control channel on the command line, everything else should hopefully
fall into place.
Any suggestions would be appreciated.
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