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Re: AO-40 data mode




I hope I'm speaking for more than myself when I state that any <3 KHz BW 
data signal would certainly be welcome on AO-40 especially on an 
experimental basis.  (There is _plenty_ of bandwidth available for this)

3500 bps could easily be done with QPSK or 8PSK on a satellite link (QPSK 
and 8PSK are established ways for digital TV/data broadcasting, albeit on a 
much wider bandwidth on comercial satellites).

QPSK is 2 bits/symbol/Hz BW (roughly) 3500/2 = 1750 Hz BW approximately + 
error correction.. say a 2/3 punctured code would do it in a 2625 Hz BW more 
or less signal... theoretically possible. And fairly robust.

8PSK gives you 3 bits/symbol, but at a 6 dB (correct me if I'm wrong guys) 
disadvantage.  I don't know if FEC would overcome this enough to make it a 
robust mode, but with the same parameters as above, it would be 1750 Hz BW, 
etc.

G3GUO has done a HF voice modem that uses multitones (that helps with HF 
multipath fading not really applicable on satellites) that is in a 2.7 KHz 
bandwidth if I remember.. it uses an AMBE commercial CODEC.  This month's 
QST has AOR advertising a commercial version of the design for around $550.

For strictly satellite use or non-multipath uses, I think QPSK with FEC is 
the way to go.  I think that speex codec being open source and no patent 
restrictions is also the way of going.  But with the AOR box being 
commercially available, I'd also expect people to use it on AO-40 which is, 
of course, fine.

Speex hooked to a QPSK mod/demod could be done on a soundcard.  Tres' cool, 
IMHO..

It'd also be usable on terrestrial links if the multipath wasn't that bad.  
It'd work on HF as well, but the legalities are cloudy on this in the US as 
the symbol rate is too wide for HF data specs, unless you interpret the 
rules for voice bandwidth as applying to digital voice (which is the way it 
SHOULD be, IMHO).  For that matter G3GUO's modem is in a grey area on this.  
Anyone know the legalities of such modems on HF?  Useable only in the voice 
segments, or not at all?  Yup it's a little off topic, but I'm curious.

Fred W0FMS


>From: DC <dc@dcarr.org>
>To: amsat-bb@AMSAT.Org
>Subject: [amsat-bb] AO-40 data mode
>Date: Thu, 23 Oct 2003 09:38:56 -0500
>
>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
>KD5QGR
>
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