[aprssig] BPSK decoding

[scott at opentrac.org]

> That's nowhere near narrow enough!  PSK'ers take for granted the 
> "brickwall" selectivity of soundcard DSP processing that can 
> be easily 
> under 75 Hz wide, and routinely work within 50 or 100 Hz of 
> each other.  

I meant that the center would be at 500 hz, but I can see where it'd be
difficult to implement a narrow enough filter in hardware.

> Further,  since the soundcard DSP schemes are essentially audio 
> "transceivers" with an effective bandwidth of 60 Hz or so without the 
> equivalent of an RIT control, unless you transmit back to the other 
> party on EXACTLY  the same frequency (with 30-50 Hz) they 
> won't copy you.

That could be done, I think.  The audio carrier for the transmitted signal
would have to be the same as the center for the RX filter, but again, that
part doesn't look practical.

Maybe I'll tackle it someday when I've got some more DSP experience.  A QRP
rig with a GPS frequency reference would be nice to have, too - I know
that's been done.  You could build a store-and-forward message system that'd
work with Internet-linked high-power base stations, assigning transmit
channels and timeslots, and prioritizing traffic.  I think there are some
military HF systems like that... would be useful for emergency
communications.  Put it in a go pack with a small PC that can serve a web
interface, and WiFi laptops at the site could all access and send messages
at the same time.  PSK31 is slow, but you can do a lot of health and welfare
and command-and-control type stuff over slow text link.

Scott
N1VG