From: gtoal@an-teallach.com (Graham Toal)
Date: Thu, 12 Aug 93 03:18:08 PDT
To: cypherpunks@toad.com
Subject: Secure voice software issues
Message-ID: <4731@an-teallach.com>
MIME-Version: 1.0
Content-Type: text/plain


In article <9308110421.AA29116@jobe.shell.portal.com> hfinney@shell.portal.com writes:
 > A couple of comments on the cryptophone idea.
 > 
 > First, there has seemed to be general agreement in our earlier discussions
 > of this concept that the hard part is compressing the voice to the point
 > where it can go over commonly-available modems.  The government-standard
 > CELP algorithm is too slow for general-purpose home computers.  You need
 > an algorithm that can operate in real time and compress intelligibly down
 > to about 13K bits per second.  It has to be either able to compress and
 > decompress simultaneously or else you need some switching logic to decide
 > which person is talking and which is listening at each moment, with both
 > sides reversing roles in synchrony.

Tony Robinson (who some of you might know already) - the author of the
lossless 'shorten' sound compression program - is working on an ADPCM
version.  Currently compresses at 3 bits per (8 bit) sample.  This plus
a sampling rate of 4000hz instead of 8000hz (keeping it easy, on a sun)
just squeezes in under the bandwidth limitation of v32bis, with a little
left over for protocol overhead.

Graham
PS Has the 'netphone' list died or did I just drop off when I moved house
like I lost my cypherpunks subscription?  I remember there was talk of
letting it drop because the situation seemed to be in control, but I haven't
seen much progress since on vat et al, and the underground effort to do
something on soundblasters that everyone said 'shut up about' has had more
than enough time to come up with the goods...
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