[Lf] Re: lf digest, Vol 1 #146 - 1 msg

Sun Jun 25 23:43:41 CDT 2000

Hi Frank and the list,

here are my opinions on your problem:

if you want to digitize a receiver outbut and still be able to detect
very faint signals buried under the noise, forget about audio
compression, like MP3, RA, GSM, and so on! 

The reason is quite simple: compression (as well as denoising!) is done
designing a suitable mathematical model that is fitted to the signal.
Best fit is obtained by minimizing the energy of the difference signal
(input minus modeled signal): computationally efficient methods exist to
compute the signal model that fits at its best the incoming data. The
difference between input signal and the model is simply discarded, and
the transmitted information is the one needed to build up the model.

MP3 adds to the best fitting process a psychoacoustic model: suppose
that you listen to a 1000 Hz signal. Now add a 1500 Hz signal, and
increase its amplitude until you hear it. Turn it off, and do the same
for a tone at 1010 Hz: you will need quite a larger amount of signal to
become aware that the tone at 1010 Hz is played with the steady 1000 Hz
tone. MP3 encoders recognize the presence of tone pairs, and apply
dynamic compression, i.e. they use a coarser signal quantization when a
weak tone is close to a strong one. Imagine this kind of processing
applied to VLF signals....

The best compression to do is Nyquist compression: if you have B Hz of
bandwidth, sample them at a rate equal to 2B (maybe a little more, just
to avoid aliasing), and use a suitable number of bits to get the needed
dynamic range. So for a data stream of 16 kbit/s, you can transmit 500
Hz of bandwidth with 16 bit (i.e. about 96 dB of dynamic range, not
bad...).

Amplitude data can be encoded into the data stream replacing say a
sample every 16k, so getting an amplitude measurement every second. The
"lost" sample will be silently ignored also by the most sophisticated
DSP engine.

However there is a more efficient method: why don't you simply transmit
the frequency spectrum ?!? I suppose you have a two way data channel, so
you can think of sending commands to the "DSP server" in order to
transmit you the desired frequency slice. What do you think about this
?!?

vy 73

Vittorio IK2CZL

> Today's Topics:
> 
>   1. Digitized Audio (Frank Gentges)
> 
>   ------------------------------------------------------------------------
> 
> Subject: [Lf] Digitized Audio
> Date: Sat, 24 Jun 2000 20:02:20 -0400
> From: Frank Gentges <fgentges at mindspring.com>
> Organization: K0BRA
> To: LF at AMRAD.org, tacos at AMRAD.org
> 
> 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>
> 
>   ------------------------------------------------------------------------
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-- 
*************************************************************************
Vittorio De Tomasi         ik2czl at amsat.org
Home page:                 http://space.tin.it/scienza/vdetomas 
My DSP page:               http://www.freeyellow.com/members/padan

"Wir muessen wissen; wir werden wissen" (David Hilbert)