Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!nrl-cmf!ukma!rutgers!att!skep2!wcs
From: wcs@skep2.ATT.COM (Bill.Stewart.[ho95c])
Newsgroups: comp.sys.next
Subject: Re: Voice Mail Format
Message-ID: <360@skep2.ATT.COM>
Date: 13 Dec 88 17:03:30 GMT
References: <19355@ames.arc.nasa.gov> <21949@apple.Apple.COM> <2062@super.ORG>
Reply-To: wcs@skep2.UUCP (46323-Bill.Stewart.[ho95c],2G218,x0705,)
Distribution: na
Organization: AT&T Bell Labs Center 4632, Holmdel, NJ
Lines: 22

In article <2062@super.ORG> rminnich@duper.UUCP (Ronald G Minnich) writes:
:In article <21949@apple.Apple.COM> desnoyer@Apple.COM (Peter Desnoyers) writes:
:>just a hair different. The binary data is standard mu-law PCM voice at
:                                                    ^^^^^^^^^^
:Is it possible to explain this in, say, less than 500 words. 

Well, to start with, PCM is Pulse Code Modulation - you sample the analog
waveform at N samples per second (telephone business uses 8000 samples/sec)
and send a digital code representing the amplitude of the signal.
There are two standard encodings around - USA & Japan use mu-law and Europe
uses A-law.  Both have basically the same approach - 8 bits of data,
with a non-linear representation.  Because human hearing is non-linear,
you preserve the most sound fidelity if you represent low amplitudes more
precisely, and higher ones less precisely.  So the change in sound level
between byte values 2 and 3 is much smaller than the change between  126
and 127.  I don't have my formulas handy, but the basic difference between
mu-law and A-law is whether there's a code to represent 0, or whether
the encoding is symmetric, with +/- epsilon represented by codes 0 and -1.
-- 
#				Thanks;
# Bill Stewart, AT&T Bell Labs 2G218 Holmdel NJ 201-949-0705 ho95c.att.com!wcs
#
#	News.  Don't ask me about News.