[aprssig] Looking for 9600 baud equipment

[Dave Baxter]

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.
I think the link modulation was FM, but I'm not sure on that, it was a
while ago now, it was not a HAM application.

I also know from first hand experience that some of the early GUNN diode
based 10GHz microwave links worked in full duplex, more as a "feature"
than by design, one say 100MHz from the other, and the resulting IF
processed by a "recycled" broadcast FM radio.  Of course, you got your
own outgoing signal too in your ears (because you directly modulated the
GUNN voltage, resulting in your own "local oscilator" being modulated)
nothing a bit of crafty analogue engineering couldn't make manageable of
course.

I suspect you also need to "actively" drive the modems to force them to
re-train and make the link (or fake telephone dial and ring signalling
etc) Looking through the on disk help doc's for the built in modem in
this Tosh laptop, it says there is a command to go "Off Hook" and force
a re-train sequence, but doesn't tell me what the command is,
strangely..   No doubt a bit of web trawling could reveal that info if
needed.

I don't think though it's a technology you can easily use (AFIK) for
"point and squirt" use like APRS.

73.

Dave G0WBX.


> -----Original Message-----
> From: jimlux [mailto:jimlux at earthlink.net] 
> Sent: 30 September 2008 14:36
> To: aprssig at lists.tapr.org
> Subject: Re: [aprssig] Looking for 9600 baud equipment
> 
> 	I still think we need something MUCH faster than 1200 
> or 9600 -- 56k ought to be a cinch, since we can buy 
> telephone modems to do it, but of course we'd need to go to a 
> much higher frequency band as well.
> 
> 	7 3
> 	Earl
> 
> --------
> 
> Heck, 802.11 goes 50 Mbps..
> 
> It's all a matter of how much power and bandwidth you have, 
> both of which are limited in typical inexpensive ham gear. 
> 1200 and 9600 are both compromises driven mostly by the 
> desire to use an existing 2m radio, and, in the case of 1200 
> bps, the desire to use surplus modems based on the Bell 202 
> standard from the 1950s.  Bell 202, in particular, was really 
> designed to accommodate typical wireline properties. 9600 is 
> a bit better, being specifically designed to work with 
> typical 1970s era 2m radios.
> 
> The 56kbps phone line modems are totally unsuited to RF links 
> for a variety of reasons, but that doesn't mean that there 
> aren't other modulation schemes that can get 10-20 bps/Hz 
> sorts of spectral densities.  The point to point microwave 
> links that do GigE are an example (using 64QAM and lots of 
> coding).  In practice though, I would question whether that 
> much sophistication is worth it. Those modulations tend to be 
> very tough to use in a varying multipath or doppler 
> environment. Why not go higher in frequency where there's 
> more bandwidth, and do something easy, like QPSK or 8PSK.
> 
> Jim, W6RMK
> 
> 
> 
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