Digitized Audio

Frank Gentges fgentges@mindspring.com
Sat, 24 Jun 2000 20:02:20 -0400


At tacos today I mentioned that I was looking for a good method to
digitize the 300 Hz bandwidth CW output of the RX320 to transmit
remotely at something around 16 kilobits per second.  I have looked at a
few available options in WIN98 and at Xing's MP3 encoder.

I would like to find the best option that will provide a good
spectrogram with Spectran at the remote end.

If I were to set the RX320 BFO for a 250 Hz, then the band should extend
from 100 Hz to 100 + 300 = 400 Hz.  But, the RX320 at 300 Hz bandwidth
has quite a bit of energy beyond 500 Hz and you can hear the beat note
come through zero which means significant artifacts could creep into the
spectrogram.  Simply put, the 300 Hz bandwidth has quite a bit of
transition band beyond the 300 Hz edges before the signal is far enough
down to ignore.

One option would be to set the BFO for a 1 kHz center frequency like we
do now for driving Spectran.  The signal could be digitized and further
filtered digitally in real-time yielding a 16 kilobit per second
stream.  A reverse process could then be used on the remote end.  If
this could have limited processing load it could be done in the PC. 
While we are at it we need to multiplex into the stream the RX320 signal
strength data, but lets not get ahead of ourselves.

Another option would be to use a streaming audio process like MP3 or the
like to encode the audio.  MP3 is an open specification and we should be
able to use it freely.  

RealAudio might be an option but it is proprietary and does not seem to
have a low rate option.  Neither do we know the impact on Spectran of
its artifacts.  It would be nice to know how much we might be missing

In the end, I would like to be able to put a remote RX320 and computer
anywhere in the world and with a modem based internet (or modem direct)
connection, be able to listen to the LF band.

Any thoughts?  Even better, any volunteers to work on this problem so we
can put your solution in our handbook?

Frank Gentges 
Check out our LF web page at <http://amrad.org/projects/lf>