[op25-dev] World's cheapest P25 receiver: $20 RTL2832 dongle + new OP25 Decoder block with DES-OFB for GRC!

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Balint balint256 at hotmail.com
Thu Apr 5 12:25:13 UTC 2012

Hi all,


Firstly, thank you to Max for the Slashdot mention!


I'd like to continue the ultra-cheap SDR theme with this video that
demonstrates the RTL2832 USB DVB-T dongle successfully receiving an
encrypted P25 signal, which is then decrypted and decoded by OP25
(represented by the new 'OP25 Decoder' GRC block in gr-baz
<http://wiki.spench.net/wiki/gr-baz#rtl_source_c> ), and sent to the




The block lets you specify a key to enable DES-OFB decryption of an incoming
stream. As Matt Robert suggested, this could be further developed by having
a 'key map file', that contains the correct key for a specific AlgID/etc.


All the RTL2832 code is in gr-baz. The patch enabling decryption support is
from a rather old version of the source (r219), so if anyone would like to
make it current for submission into the repo, please let me know and I can
send it out!


I also added another new block called 'Message Callback', which is used to
trigger callbacks in the GRC generated code based on messages sent by other
blocks to messages queues. If you watch the video or visit my wiki, you'll
see all the tuning/channel selection/filtering/audio is done by the
appropriate blocks in GRC - separate from the OP25 code (this is different
from it all being in the one Python file, e.g. audio_p25_rx). You may recall
the FSK4 block outputs auto-tuning messages to the Frequency Translating FIR
filter (channel selection block) before the FSK4 block. This feedback loop
is handled internally in the existing stand-alone Python files, but needs to
be replicated in GRC - hence the new block. The 'Message Callback' block
connects to the 'tuning' output of the OP25 Decoder block, and is told about
the 'auto_tune_freq' GRC variable to update when a message arrives with a
new value. This GRC variable is subsequently used to update the Centre
Frequency in the Frequency Translating FIR filter (channel selection block)
to fulfil the FSK4 block's request.


If you're into GNU Radio, I just posted a much longer update with the latest
RTL2832 developments (including a completely re-written 'librtl2832++') to
the GNU Radio Discussion list.


Kind regards,

Balint @spenchdotnet <http://twitter.com/spenchdotnet> 


From: op25-dev at yahoogroups.com [mailto:op25-dev at yahoogroups.com] On Behalf
Of ikj1234i
Sent: Sunday, 1 April 2012 10:20 PM
To: op25-dev at yahoogroups.com
Subject: [op25-dev] congrats to Balint: on Slashdot ! ! !




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