Re[2]: [Elecraft] K3 Questions

[Corboy-Poteet]

David. let me pose a question: if you converted the digitized voice
signal to mp3 or ogg format, is this similar to (or the same as) a
freq domain data stream?  The data compression from CD wav file to mp3
is around 10 to 1; you are certainly not storing a digital copy of the
sound wave.  Voice compression may not be so good but still should
result in the need to transfer considerably less data in order to
assure a useful voice recovery on the receiving end.

Mike  W5FTD





 Stuart Rohre wrote:
 In the basics of digitizing the signal you have to clock the conversion at 
 least twice the highest frequency you want to reproduce in the voice (or 
 audio) signal.

 That's only at the input to the encoding chain (and technically it is 
 twice the bandwidth, not twice the highest frequency).

 However, I would imagine all digital modes that would be used for 
 communications, rather than broadcasting (and for that matter, also 
 those used in modern broadcasting systems) don't send time domain data.
   One way or another they send frequency domain data, often in the form
 of just the formant frequencies used in a model of vocal tract resonances.

 The critical rate for these systems is the syllable rate, not the 
 frequency of the highest component.  I seem to remember that the 
 military were using 2400 bits per second codecs maybe a couple of 
 decades ago.I suspect that was partly do do with how fast they could
 encrypt.

 Mobile phone codecs tend to be vocal tract model based, although they 
 use more than 2400 bps so that the voice sounds reasonably natural (but
 try them on modem tones or even music!).

 To get much better than 2400 bps, I think you would probably have to 
 recognize phonemes, which is basically the continuous speech voice 
 recognition problem.  I think that might get you down to about 300
 bits per second.


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Re[2]: [Elecraft] K3 Questions. ..DVoice

[Corboy-Poteet]

Stuart, what bandwidth do the Public Service people use on their
digital systems?  And let me ask you my earlier question: what happens
on digital voice when you decrease the bandwidth on reception? Can you
significantly improve signal-to-noise by narrowing the bandwidth (with
only a loss of fidelity)?  Or does digital voice have no tolerance for
bandwidth reduction; that is, you need basically all of the bits in
order to provide intelligible audio out.

Mike   W5FTD



 Absolutely good points about the fail soft nature of analogue signals.
 While the new digital systems usually have a threshold, and fail hard, ie,
 the message does not get thru even partially, after a certain loss of signal
 point!

 This is why analogue ham communications get thru in disasters, and many
 Public Safety systems do not, as they depend more and more on go/ no go
 digital systems.

 -Stuart
 K5KVH
 retired fireman 



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