[Xastir] Off-topic: Rx eight freq. at once via RTL SDR: Send to Icecast

[Curt Mills]

This is off-topic for Xastir/APRS but figured some might be interested.

I wanted to set up an RTL SDR dongle to receive our SAR frequency
(high-band VHF, FM narrow) and send to an audio streaming server. I did it
with a pipeline of commands on Linux. I mentioned it on another list:
Another SAR guy two counties away (Hi Tom!) mentioned that he wanted the
same thing but with multiple frequencies so he could choose which frequency
to listen to.

The idea is that SAR members and their family members could attach their
portable devices to the stream and listen to live SAR traffic during the
mission. For SAR members this is most useful on the way -to- a SAR mission:
They can hear the recall code or perhaps more specific directions or
important facts as they are coming in to the search area. All very useful
if they don't own a SAR radio but have a cell-phone (and are in range of
the cell network of course).

So... Ken, N7IPB, and I redid the scheme and can now receive eight AM -or-
FM frequencies at once per RTL SDR receiver, using one piece of software
instead of a pipeline of commands. There are also free audio servers where
you can send these streams so you don't have to run your own Icecast server
either.

I spent one day walking around the house with earbuds in and my cell-phone
listening to a 220 repeater this way. I set up eight 220 frequencies on the
RTL and could choose which audio stream to listen to. Since it was
squelched most of the time and sending samples of zeroes, the cell-phone's
battery didn't get depleted. Note: There is quite a delay with this method,
in my case 20+ seconds from RF to when I hear it in the earbuds, but much
of that is probably audio buffering in the client on my phone. I'm using
VLC on the phone.

One must patch the RTL_Airband code to send constant samples of zero when
each channel is squelched to keep the audio channels streaming, but other
than that it's pretty simple. 8 AM or 8 FM frequencies can be set up this
way, anywhere between 24 MHz and 1.77 GHz, as long as all 8 are within the
2.4 MHz passband where you have the RTL tuned.

If anyone wants to do the same I'm happy to share my configuration, minus
the login/password for my Icecast server.

Total investment: $25 for a TCXO RTL SDR dongle, and a whole bunch of time
playing and testing until we figured out the magic formula.

-- 
Curt, WE7U
http://we7u.wetnet.net
http://www.sarguydigital.com