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Re: Digital modes and antenna questions



Hi guys,

Rod asks:
 > Also decoding telemetry from the birds, is there a
 > "one size fits all" software or does each satellite
 > have specialized software for doing this?

There is no one single program that does everything
for every amateur radio satellite.

However, you can do much of the work completely in
software. For example, when AO-40 was still active it
was common to receive the 400 bps telemetry signal
with your radio, then demodulate the signal with
a soundcard and AO40Rcv (software) and then display
the telemetry with P3T, another piece of software.
These days you can accomplish a similar feat for
currently active satellites like AO-51, albeit using
totally different software packages.

If you're really interested in digital satellite modes
you might want to check out the AMSAT Digital CDROM -
it comes with several digital program and test
signals for everything from 1200 baud to 38k4 baud.
(Disclaimer: I had a hand in the making of a couple
of the items on the CDROM.)

broberts@mta.ca wrote:
> Apparently nobody has yet written software that can
> demodulate 38600 bps (or 38k6). 

Actually, it's 38,400 bps (38k4) and, yes, there is
software that will demodulate the signal using only a
soundcard. I know because I wrote it. :-) For my
version, see the paper "A First Look at a New
Soundcard DSP Modem for Satellite Telemetry" in the
2005 AMSAT Symposium Proceedings, or just email me
with your questions.

The 38,400 bps signal will just fit into the bandwidth
of a soundcard running at 48,000 samples per second.
(No, this doesn't violate Nyquist.)

And I'm not the only one to write software for 38k4.
There's also a 38k4 DSP modem for the DSP56002EVM,
if you have one of those. (I don't.) See

http://bips.bi.ehu.es/prj/modem/tossa/index.html

for their 38k4 software modem that runs on a
DSP56002EVM. There's probably others too.

The tricky part, in my opinion, is receiving the
38k4 signal and getting it (undistorted) into the
soundcard. At RF you need a very wide filter to
pass the whole FM signal, and as an audio signal,
its spectrum goes from almost zero Hertz up to
19,200 Hertz. Some FM receiver chips, which would
work fine at 9600 baud, have a frequency response
that cuts off before 19,200 Hertz. With the effects
from the RF filter, and the FM receiver chip
combined, in some receivers the 38k4 signal will
never make it to the soundcard. But I digress...

Anybody know a good drop in replacement for
something like an MC13135P that would work up
past 20,000 Hertz?

Douglas KA2UPW/5
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