From: wcs@anchor.ho.att.com (Bill_Stewart(HOY002)1305)
Date: Wed, 11 Aug 93 19:27:28 PDT
To: hfinney@shell.portal.com
Subject: Re: Secure voice software issues
Message-ID: <9308112126.AA25524@anchor.ho.att.com>
MIME-Version: 1.0
Content-Type: text


> 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. 

I downloaded a copy of the GSM 06.10 software (gsm-1.0) from some machine
at Technische Universitat Berlin, tub.cs.tu-berlin.de, which does
a 13.3 kb/s voice coding, and has conversion for Sun, linear, mu-law, and A-law.
(Jutta Degener and Carsten Bormann, Copyright 1992.)
It runs in two different modes - a strictly-follow-the-standards mode
and a cheat-a-little-using-floating-point mode.
In standard mode, piping the compressor into the decompressor ran epsilon faster
than real time on a Sparcstation 2, and epsilon slower on a Sun ELC.
Cheating mode took about 2/3 as long as standard mode.

Looked like it was fairly portable.  I compiled it with GCC.
One of the readmes says that compression and decompression run faster than
realtime on Sparcs, but I assume that means doing just one at a time,
not both simultaneously.  As is typical, the decompression is about
twice as fast as the compression.  I assume it won't be quite fast enough
on a 486 box, but a "Version 1.0" of anything can often be made faster;
I haven't looked at the algorithm to see how much optimizing can be done,
but the code's cleanly written and has a bunch of medium-sized tables and
unrolled loops suggesting they've at least done some work on speed.


Both of these were on samples that were a little fancier than /dev/tty voice;
one was /usr/demo/SOUND/sounds/sample.au, and one was the 106.au
"It's 106 miles to Chicago, we've got a full tank of gas...." 
Blues Brothers sound-byte from the net.  Haven't tried it on other samples
yet, and I need to try running it across the net.
I also don't have two 14.4kbs external modems to play with on Sparcs,
so I haven't been able to verify which V.42/V.42bis/MNP options will let
you squeeze out the start/stop bits to let you fit 13.3 kbps of data over it,
but people tell me it should work ok.


			Bill Stewart