Re: Patents and AX.25

[Phil Karn <karn@unix.ka9q.ampr.org>]

>Would SSB just be for convenience (existing equipment), or
>could/should other modulations be used (ok, so it might not be legal,

Yes, I picked the 3 Khz bandwidth for convenience in quick implementations
using existing HF transceivers. The wider the better, but wider bandwidths
will require new or modified RF hardware (as opposed to just DSP code).

>details details!) Modern DSPs should be able to spit out HF directly
>not just for piping into an audio input; only a linear amp and filters
>would be needed (right?).

Well, sort of. Modern DSPs are still limited in their data rates so
baseband processing is still much easier than at IF or RF. But that's
coming.

>How do military HF radios work? 

Don't know much about them, but I've found one MIL-STD that seems to
cover a few HF modulation techniques. A quick browse of
MIL-STD-188-110A seems to suggest a basic rate 1/2 convolutional code
with interleaving over multiple independent QPSK carriers. Other code
rates are achieved with puncturing and symbol repetition.

BTW, there's active discussion of HF modem topics on the hfsig@tapr.org
mailing list.

Phil