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From: jbuck@galileo.berkeley.edu (Joe Buck)
Newsgroups: comp.dsp
Subject: Re: US Gov. sends real-time audio at 2400 baud!
Message-ID: <39469@ucbvax.BERKELEY.EDU>
Date: 8 Nov 90 23:33:28 GMT
References: <1990Nov8.210640.2893@ux1.cso.uiuc.edu> <2449@cod.NOSC.MIL>
Sender: usenet@ucbvax.BERKELEY.EDU
Reply-To: jbuck@galileo.berkeley.edu (Joe Buck)
Lines: 29

In article <2449@cod.NOSC.MIL>, hammond@cod.NOSC.MIL (John A. Hammond) writes:
|> I suspect that there is some misinformation here.  The article
|> is probably refering to STUIII's which have a data port which
|> currently runs at 2400 baud.  The instrument samples and sends
|> voice at a higher bit rate that 2400.  Some of the original
|> glossies on the beast promised 9600 baud data but that is
|> still a dream as far as I know.

There's misinformation, but only in article <2449@cod.NOSC.MIL>.  Yes,
the STU-III boxes are 2400 bps speech codecs; they use linear predictive
coding (LPC) to achieve this rate.  It doesn't sound all that great,
but it's 2400 bps, it's understandable, and you can recognize who is
at the other end (at least if there isn't background noise).

Commercial and military 2400 bps speech codecs have been around for
quite a while.  Conceptually they are very simple: break up the speech
into frames of about 20 msec, average; estimate the pitch, do LPC
spectrum analysis, and send parameters corresponding to pitch, LPC
parameters (reflection coefficients or line spectrum frequencies),
and gain.  Then you do lots of tricks to improve the speech quality.

There are now speech coders running at 9.6 Kbps that sound nearly as
good as a telephone line for voice; they typically use CELP (code
excited linear prediction) or a related algorithm.


--
Joe Buck
jbuck@galileo.berkeley.edu	 {uunet,ucbvax}!galileo.berkeley.edu!jbuck