[op25-dev] OP25 working with UHD and GRC [2 Attachments]

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Balint balint256 at hotmail.com
Wed Sep 26 23:47:20 UTC 2012

Hi folks,

I had the pleasure of sitting with some major OP25 contributors (Max and
Mike) at the GNU Radio conference today, and also managed to get OP25
working with a USRP N210 using UHD. You can see in the attached image that
I'm using the OP25 block from gr-baz in GRC (the flowgraph sends the decoded
audio to the speakers via the Audio Sink). The voice frames' data is being
printed in the console, and four-level dibits are shown on the scope. The
other tabs contain FFTs for the entire baseband and the selected channel,
and make use of the new click-handler feature that triggers a callback
containing the frequency you clicked on. You can therefore tune (really
change the offset in the Freq Xlating Filter) to a signal you see appear on
the FFT just by clicking on it (the same way you can with the Python apps,
e.g. usrp_p25_rx/audio_p25_rx, however this is all from GRC).

The IT++ 4.2 BCH problem still exists, so I've used the alternate
implementation that Max also uses in the 'repeater' code to handle
identification of the frame header.

The GRC flowgraph obviously uses the UHD Source block to capture the
baseband data, but the Python scripts (e.g. those mentioned above) also now
work with UHD. The legacy USRP code is actually untouched as I've written a
thin wrapper for UHD that makes any UHD-compatible device accessible through
the legacy interface (i.e. brought 'from gnuradio import usrp' &
usrp.source_c back). Not the best approach to force projects to move to UHD
properly, but a temporary measure in the meantime ;)

I'll post some code soon!

Kind regard,


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