Secure phones - STU3

[Phil Karn]

As far as I can tell, basic compatibility among STU-IIIs calls for the
2400 bps LPC-10 coder as a minimum. So it's universal. It works, but it
sounds pretty bad, so there's been a lot of work on newer and better
vocoders. Correct me if I'm wrong, but I suspect the Motorola coder you
mention is probably one of several manufacturer-specific algorithms,
sort of like the manufacturer-specific high speed modes you find on some
fax machines.

FED-STD-1016 CELP (Codebook Excited Linear Prediction) at 4800 bps seems
to be the up-and-coming standard for newer phones, and it indeed does
provide *much* better voice quality than LPC-10. Unfortunately, it also
requires many more DSP cycles.

Phil