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AO-40 data mode

As I was listening to AO-40 the other day I wondered if it would be 
feasable to send voice as highly compressed data in order to achieve 
better S/N than with SSB phone.  Based on my experiments thus far 
3500bps (bits per second) is the lowest bit rate I've been able to 
achieve.  This limit is not a quality limit but its the lowest that I 
force the reference speex codec to go.  At this rate quality is similar 
to that of conventional telephone.  This rate is utilizing a 8khz audio 
bandwidth but the speex library features a 4.8khz bandwidth option that 
should drop the rate even lower.  My ultimate goal is to have better 
quality speech than SSB for a given S/N ratio with 1200bits per second.

My question is what data modes are allowed on AO-40?  Also as this is my 
first real step into radio data transmission what modulation schemes 
should I look into.  My priorities for a modulation scheme are best 
minimum S/N requirements, smallest bandwidth, and not so important 
standardized.  Having the coding be an established standard isn't all 
that important because my audio compression (speex) while open source 
certainly isn't standard either.  What kind of bandwith am I going to 
need for 1200bps or 3500bps?  Thanks for your input,
-David Carr

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