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Re: Digital Radios



>Bandwidth is another problem compared to analog, according to Bell >Labs, 
>it
>takes 64 KB per second of data input to get the human voice through -- >at 
>least that is the rate at which it is first digitized in some >equipment.

Actually that number is correct, but through psycho-acoustic models (i.e. 
what the brain doesn't perceive is lost) the data rate can go down quite a 
bit.  DSP's filter out "unnecessary" information to squeeze down the data 
rate.  There are "robotic" sounding codecs that can go as low as 1200 bps 
(like MELP, CELP and that AMBE encoder) and some decent sounding voice 
quality (64 Kb uncompressed-like) codecs that run in the 9600 bps range.  
MP3 audio is an example of a type of codec, but in this case meant for 
higher data rates/quality.

"Digital" cellphones like GSM use codecs in the 12Kbps range, but there is 
newer technology available than that now for the same quality that is more 
efficient.

Again, especially with a satellite, bandwidth isn't as important as it would 
be at HF.  In fact, a wider bandwidth signal has the capability of some 
processing gain that would make it work in lower signal conditions than the 
current narrowband SSB.  Shannon's law can definately prevail in this 
application.  Mr. Karn knows what he's talking about on this one-- take a 
look at PCS technology if you don't believe it.

Fred W0FMS




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