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From: raoul@eplunix.UUCP (Nico Garcia)
Newsgroups: comp.dsp
Subject: Re: US Gov. sends real-time audio at 2400 baud!
Message-ID: <966@eplunix.UUCP>
Date: 13 Nov 90 19:03:28 GMT
References: <1990Nov8.210640.2893@ux1.cso.uiuc.edu>
Organization: Eaton-Peabody Lab, Boston, MA
Lines: 26

In article <1990Nov8.210640.2893@ux1.cso.uiuc.edu>, sl35746@uxa.cso.uiuc.edu (By-Tor) writes:
> Hi.  I heard that the US government uses 2400 baud modems to send real-time
> audio, to get a secure line anywhere in the world.  Does anyone know what
> kind of algorithm they are using for compression?  I think it would be great
> if we could make a real-time audio irc for people with sampling/playback
> capability.  Anyways, consider that 2400 baud=2400 bits per second, meaning
> without compression, you could get 2400 hz sampling at 1 bit resoultion.
> This is totally unintelligable, so how the heck could they compress it
> so much?  Obviously some kind of destructive method must be used.  I thought
> maybe they look for duplicate waves or something.  Any ideas?

One respondent pointed out that 2400 baud may actually equal 9600 bits/second.
But a more fundamental question: who says 2400 bps is unintelligible? I had
to look at some work on speech comprehension with clipped waveforms: a guy
named Lickliter documented that people could understand something over 80%
of speech, even with absolute clipping (equivalent to one bit data). I don't
know if he ever tried reducing the bandwidth, but humans have a marvelous
ability to make speech out of the most corrupted signals. And partial
clipping has been used to turn a 60-dB range input to a 20-dB range
transmission for equipment like transmitters and hearing aids for years.


-- 
			Nico Garcia
			Designs by Geniuses for use by Idiots
			eplunix!cirl!raoul@eddie.mit.edu