Re: Compressed/Encrypted Voice using Modems

[Phil Karn <karn@qualcomm.com> (Phil Karn)]

At  7 Feb 04:56 CST, Jim Thompson wrote:

>I don't see how a 28.8kbps (raw) data rate is possible, as the Shannon
>limit for a POTS line is 22kbps.  Certainly parts of the phone system
>no longer impose the narrow bandwidth that are part of the 'spec', but
>one can not always depend on getting a line that exceedes the published
>parameters of the phone system.

Where do you get this figure of 22 kbps? I would tend to dispute it since
I use a Codex FAST modem on my SLIP link and it really does run at 24.0 kb/s
on the wire (not counting compression). The throughput display often reads
30 kb/s even when I'm shipping a pre-compressed or encrypted binary file,
although that's a phony figure because it includes the asynch
start/stop bits that aren't actually sent over the wire.

The usual Shannon limit of a phone line is more like 30 kb/s, although
it can vary enormously. Generalizations are dangerous. At the very least,
you can certainly say that it's no greater than 64kb/s, since it's almost
certain that your call passes through a mu-law codec somewhere.

Back to vocoders, their quality does tend to be a strong function of data
rate. 8kb/s CELP is really not that bad - a little warbly when there's
background noise, but not objectionably so in my opinion. In a mobile
telephone
environment (where I'm familiar with it), it's *much* less objectionable
than the usual impairments you get from ordinary FM analog transmission.

4kb/s is noticeably worse. Things get rapidly better as you go above 10-12
kb/s with present algorithms. Also, vocoders need not be constant rate.
Ours selects
one of four rates on the fly depending on voice activity, which doubles
capacity in a CDMA radio environment. It'd also be useful in a packet
network, although
the small frame sizes (2/5/10/22 bytes) can make header overhead rather
significant.