Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!optilink!elliott
From: elliott@optilink.UUCP (Paul Elliott x225)
Newsgroups: comp.dsp
Subject: Re: US Gov. sends real-time audio at 2400 baud!
Summary: Vocoder?
Message-ID: <4825@optilink.UUCP>
Date: 9 Nov 90 16:45:35 GMT
References: <1990Nov8.210640.2893@ux1.cso.uiuc.edu>
Organization: Optilink Corporation, Petaluma, CA
Lines: 39

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?  [...]

The following might be obsolete information...

The Gov't has used voder / vocoder technology (are those terms correct?) to
send very low bit-rate voice communications.  This technique breaks up the
speech input into multiple frequency bands, measures the energy level in 
each band, and sends the level info at a fairly slow rate ( approx 10 ms?).
At the receiving end, the data is used to control the amplitude of multiple
fixed-frequency sources.

This was originally done with an all analog approach, but obviously FFT
and digital synthesis could be used.  The trade-offs are the number (and
placement) of the frequency bands, and the sampling rate.  The signal
quality is poor, but understandable.  Individual voice characteristics
are pretty much lost; the voice usually sounds robot-like.

I've been told that on the Pink Floyd Album "Wish You Were Here", on the
song "Sheep", they use a vocoder for the "Through the vally of death"
spoken portion.  To my ear, it sounds more like a DSB (ring) modulator,
but on the otherhand I've never heard a vocoder in person, so it could be.

The Heinlein novel _The Moon Is A Harsh Mistress_ has a nice section where
a vocoder is connected to "Mike", the self-aware computer.

And since my technical education on the subject of vodocer technology
is derived largely from Heinlein novels and Pink Floyd records, you are free
to take this with a grain of salt ;-)




-- 
      Paul M. Elliott      Optilink Corporation     (707) 795-9444
            {uunet, pyramid, pixar, tekbspa}!optilink!elliott
                          "Think Blue, Count Two."