I've got some work to do , need to build all this up and get the basics working first.
What method do you use to measure the correct deviation of the C4FM signal?
--- In op25-dev(a)yahoogroups.com, ikjtel <ikj1234i@...> wrote:
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Thinking about this some more, why couldn't
you just split the audio from the RX and TX into two
Yeah, asterisk app_rpt transmitting analog FM voice over standard FM radio hardware is
already a fully solved problem having no need for a USRP.
Our software stack can receive analog NBFM in addition to P25, for one thing because the
software support to do so was not difficult to add.
I thought the USRP_TX app was only for that
hardware. I want to use a conventional radio, just as you described, for both P25 and
NBFM. So sign me up as as experimenter wanting to try that!
OK -excellent. For
starters could we try first to get python/op25_audio_tx.py working - that app does not
support asterisk / app_rpt but will establish a baseline. The results of that test would
direct the next step of the experiment.Â
op25_audio_tx takes input analog voice either from a sound card or from a disk file
(8,000 samples/sec, short signed int16 format). It generates audio at the soundcard
audio line-out port (which should be connected directly to the modulator stage of the FM
TX). You might be able to use the same sound card for both lines, if the card supports
full duplex and if it allows different rates for record and playback. If not the next
best thing might be to use canned test audio files [use the -i option and place the audio
test file in baseband-0.dat].
The biggest problem so far has been group delay distortion. It's also critical to
get the proper audio signal levels going into the TX such that the FM deviation is
correct.
That should be it - let me know of next step(s)
73
Max