From: Bill Stewart <stewarts@ix.netcom.com>
Date: Mon, 5 Aug 1996 06:34:49 +0800
To: cypherpunks@toad.com
Subject: Re: Digital Telephony costs $2
Message-ID: <199608042050.NAA12518@toad.com>
MIME-Version: 1.0
Content-Type: text/plain


At 05:09 PM 8/3/96 -0800, Jim Bell talked about mixing telephony,
voice compression, and modem functions on future modems,
and how doing a 28.8 modem uses up most of a DSP chip,
while 2400bps voice coding and decoding also each use up about half,
making full duplex tough.

One advantage of higher-speed modems is that you can get away with
16kbps ADPCM coding, which is dirt-simple computationally;
your 386 probably has enough horsepower to do it, though a PC's
interrupt structure may make it tough to shove all the data in and
out in real time.  You still need a sound card that'll do the
A/D and D/A conversion simultaneously if you want full-duplex;
that wasn't part of the original market vision of Soundblaster,
so vanilla sound cards don't all do it.  It also has the advantage
that the data is being moved through your CPU, so encryption is
an easy add-on, rather than having one combined modem/voiceblaster
card which doesn't have any hooks for crypto or other processing.

>Sure,  it may not be necessary to compress voice audio all the way down to 
>2400 bps, since the current modem standards allow 28.8kbps and beyond, but I 
>suggest that decreasing net traffic by a factor of 12 (28.8k to 2.4k) is a 
>desirable goal.  

One problem is that tighter compression methods are far more sensitive
to network latency than crude ones, and need to process more milliseconds
of speech before putting out a packet on the net (e.g. a 64-byte tinygram
is 200ms of speech at 2400bps, vs. 32ms at 16kbps.)  For modem-to-modem
communications, this is no problem; for Internet random delays it is.

Also, another big difficulty with full-duplex transmission is that you
need echo-cancelling, especially with high-latency circuits.
Half-duplex is annoying, but it doesn't echo, and it's more tolerant
of delay because you're not expecting it to have natural timing...

>The reason I think a system I've described has a future is that modem 
>manufacturers have a PROBLEM.  Their problem is that they've pretty much run 
>out of room to improve the bit-pushing through a 3 KHz bandwidth.  

Given that the "3KHz" is almost universally transmitted over 64kbps
digital channels, there's really no point in pushing past 33.6 with
analog-based coding; better to just do ISDN.  (You can still do analog-only
calls if you're on an analog central office talking to someone else
at the same exchange, but it'd be a flat-rate local call anyway.
If there's anybody still using analog trunks between offices, 
it's some mom&pop rural telco, and you can't get 28.8 on barbed-wire...)

#			Thanks;  Bill
# Bill Stewart, +1-415-442-2215 stewarts@ix.netcom.com
# <A HREF="http://idiom.com/~wcs"> 	Defuse Authority!