Re: Compressed/Encrypted Voice using Modems

[jim@tadpole.com (Jim Thompson)]

> And why are you limiting this to V.32 (9600bps)?  V.32bis (14.4k bps) modem
> chips cost maybe 20% more than v.32 chips in quantity.
> 
> Even higher speeds are available if you're willing to go that far.  Zyxel
> v.32bis modems have proprietary 16.8 kbps and 19.2 kbps full duplex raw
> modulation rates, but they use DSPs instead of modem chips like the
> ones from Rockwell, AT&T, and Intel.  I believe there are some v.FAST
> (not CCITT compliant) modems like the one's from Motorola (Codex) that
> can do 21.6 kbps and 24.0 kbps.  I believe the final speed of v.FAST once
> standardized by the CCITT will be 28.8 kbps.

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.

The Rockwell (RC96AC/RC96ACL/RC144AC/RC144ACL) modem chip (set) has an
on-board codec that does ADPCM in hardware.  It makes for a very nice
programable answering machine.  Interestingly, the designers guide has
someting to say about bit rates.

At a sampling rate of 7.2 KHz, (the only sample rate this chipset
supports) 8 bit samples are presented at a bit rate of 57600 bps.  Add
in a start/stop bit, and you now need a DTE rate of 72,000 bps.  Most
UARTS don't support this rate, and thus you will need to find one that
will run at 115.2 Kbps.

Drop to 4 bit samples, and you get a bit rate of 28,800 bps, for a
required async DTE speed of 36,000.  (or 38400 bps on most uarts.)

I'm also fairly sure that recent Zyxel modems (e.g. the U-1496) use
this Rockwell chip(set), and not a dedicated DSP.

Jim