Path: utzoo!attcan!uunet!seismo!sundc!pitstop!sun!amdahl!apple!desnoyer
From: desnoyer@Apple.COM (Peter Desnoyers)
Newsgroups: comp.sys.next
Subject: Re: Voice Mail Format
Keywords: voice mail
Message-ID: <22109@apple.Apple.COM>
Date: 13 Dec 88 17:24:23 GMT
References: <19355@ames.arc.nasa.gov> <21949@apple.Apple.COM> <2062@super.ORG>
Distribution: na
Organization: Apple Computer Inc, Cupertino, CA
Lines: 35

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. 
>An equation maybe? This is outside of my domain ...
>Thanks,
>ron

Sure. Mu-law encoding is what everyone uses (e.g. Sprint) unless
they're really cheap. (Note that almost all telephone transmission
except the local loop to your phone is now done digitally, usually in
this format.)

The way it works can be viewed thus: Sample voice at 13 bits per
sample, 8000 times per second. Apply a sort-of-logarithmic function to
squeeze these 13 bits into 8: +1 -> +1, while +4096 -> +128. Unsqueeze
it at the destination. Thus your sampling noise is very small for
quiet signals at the cost of increased sampling noise for loud
signals. The trade-off must be optimal - Bell Labs spent years
researching it. (1/2 :-)

The function is actually a piece-wise linear approximation to
something sort of logarhythmic, most likely because it was easier to
do back in the 60's when they first started using digital
transmission. (Yup, even then it was cheaper to spend _lots_ of money
on digital hardware than it was to find space to string more wires
underground in Manhattan. I think it was the first application of
transistors in the public network.)

[I apologize for any technical inaccuracies or vagueness. I couldn't
find any of my references on mu-law. I have a mu-law to linear table
on line, but the machine is down. oh well...]

				Peter Desnoyers