[Lf] Re: Digitized Audio

[Frank Gentges]

Dave,

Thank for the good comments.  I tried a little MP3 encoding last night
and by the time I got the data rate down low enough to be interesting
the audio sounded reverberant.  It indeed, encodes in strange ways on
our signals.  I have not had a chance to set up the notebook next to the
desktop so I can try Spectran on it to see what it looks like.

Your comments tend to lead me where I was thinking about going.  We need
our own compression/digitizing algorithm.  Perhaps based on some
existing algorithm like MPEG1 or MPEG2 with different clocking speeds.

Perhaps better yet is a sharp skirted digital bandpass filter centered
on 1 kHz followed by some sort of digitizer.  If the digitizer is too
adaptive it might choose to adapt to enhance the "noise" and suppress
the "signal".  MPEG 1 and 2 are public and available in Linux source
code as I recall.

If you want to try some stuff out I can make some typically poor s/n
.wav files to test it out on.

George brought by a couple of interesting pictures, one of you receiving
the AMRAD award, the other a building where you work.  Congrats David.

Frank

"David W. Borden" wrote:
> 
> Frank,
> 
>   I wanted to do this last year but Shawn pointed out that MP3 encodes in
> strange ways.  It picks the loudest frequency and declares it dominant and
> trashes everything else.  For example if you are recording a room full of
> people and they are all talking quietly and then some one starts shouting,
> MP3 will encode the shouter at the expense of all others.
> 
>   My idea was to have a number of remote receivers all sending their stuff
> by internet streaming audio to a central point where the audio was combined
> and then sent to the DSP for recovery of stuff below the noise.  Jim
> Broyles (KA9GCS) produced me a combiner circuit (a bunch of op amps) that
> combined all the streams to send to the DSP.  It sounds like a neat idea
> (the old space diversity) but MP3 won't do it as it will trash the very
> good stuff I want to recover with the DSP.  So maybe Real Audio.  You have
> to pay money to get the Real Audio encoder.  My professor at UM did it last
> year for his classes, he sent us his lecture in Real Audio.  It sounded
> super, but then there was not any super quiet signal in the background to
> recover.......david
> 
> At 20:02 6/24/2000 -0400, you wrote:
> >Hi,
> >
> >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
> >here.
> >
> >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
> >--
> >Frank Gentges
> >K0BRA, ex AK4R, W3FGL
> >Check out our LF web page at <http://amrad.org/projects/lf>
> >
> >_______________________________________________
> >Tacos mailing list
> >Tacos at amrad.org
> >http://www.amrad.org/mailman/listinfo/tacos
> >

-- 
Frank Gentges 
K0BRA, ex AK4R, W3FGL
Check out our LF web page at <http://amrad.org/projects/lf>