[aprssig] Looking for 9600 baud equipment

Scott Miller scott at opentrac.org
Fri Oct 3 02:21:44 EDT 2008

> It is possible to make a pair of 56k phone line modems work over a radio
> link.  I've seen it done at UHF (but not done it myself!)  The link used
> was real time analogue both ways, and had at least a 3k audio bandwidth.

Not at 56k, I don't think.  I'm pretty sure 56k requires a direct 
T-carrier connection on one end.  They should only be able to do 33.6k 
peer-to-peer.  That's still pretty good, but that's only going to happen 
when you've got a good solid link and no mobile flutter.  It's also not 
useful for a half-duplex link.

On a semi-related note, I've started synthesizing some 1200 baud AFSK 
test data to augment the WA8LMF TNC test CD.  The first file is a bunch 
of packets back to back with a steadily increasing amount of AWGN.  Each 
packet indicates the noise level in the payload, so you can run it 
through your TNC to get a quick measure of noise tolerance or set it to 
loop and gather more accurate statistics on how many packets were 
decoded at each level.

It still needs some tweaking, but it's working well so far.  I've been 
generating it with a 48,000 hz sample rate to keep each bit time an 
integer number of samples, but I'll probably need to change that for CD 
compatibility.  My MP3 player doesn't like it, either.

I figure I'll also do some files for increasing amounts of clock jitter 
(easy) and twist (might take some more creative filtering).  If anyone 
knows of a good mathematical model for mobile flutter, let me know and 
I'll see what I can do.  Likewise for any other specific defects you can 
think of.


More information about the aprssig mailing list