🚀 go-pugleaf

In article <BxHn0t.MCL@watserv1.uwaterloo.ca> lcmyu@crypto2.uwaterloo.ca (Luis Ola Toshio Chi-Ming) writes:
>Can someone tell me how these digitally encrypted phones provide
>reasonable security in light of the following problem.  Phones lines
>can tolerate raw data rates of 9.6 kbps whereas voice requires 
>64 kbps.  Let's say the CELP compression reduces the voice
>requirement down to 9.6 kbps (voice becomes noticibly choppy thereafter).

Actually, a common data rate for CELP is 8 kbps. This is the rate used
in the TDMA digital cellular system, and it is also the peak rate of
the vocoder in our CDMA system. Our vocoder also supports three lower
speeds (1000, 2000 and 4000 bps) with the rate changing dynamically
with voice activity. This would let you multiplex other things (e.g.,
data) with vocoder data, using pauses in speech to full effect.  In
overload situations the vocoder can be told to run at a lower average
rate with a slight degradation in quality.

Error correction is not really an issue. We have a few bits in each
frame devoted to a CRC on the most important bits, but voice is so
redundant that you can withstand a fairly high frame error rate
(several percent) before the quality degrades unacceptably. Voice is
very redundant.

>this would imply blocks of duration 6 seconds.  An uncorrected channal bit
>error would result in white noise for 6 seconds.

I don't know where you get this figure. Our vocoder operates on 20
millisecond frames. Lose a frame and you lose audio for 20
milliseconds. All processing is done on a frame-by-frame basis, and if
you wanted to add cryptography it too could be done separately on each
frame.

Phil