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From: jbuck@galileo.berkeley.edu (Joe Buck)
Newsgroups: comp.dsp
Subject: Re: A-law and u-law companding
Message-ID: <11858@pasteur.Berkeley.EDU>
Date: 10 Mar 91 20:18:43 GMT
References: <1991Mar8.052136.11121@trl.oz.au>
Sender: news@pasteur.Berkeley.EDU
Reply-To: jbuck@galileo.berkeley.edu.UUCP (Joe Buck)
Organization: U.C. Berkeley -- ERL
Lines: 24

In article <1991Mar8.052136.11121@trl.oz.au> m.summerfield@trl.oz.au writes:
>I am currently investigating implementation of A-law and u-law (that's
>mu-law :-) companding techniques. I am aware of the formulae which are
>used to generate these, however I am more interested in practical fast
>software and digital hardware implementations, in which the calculation
>of logarithms is impractical.

No one computes logarithms to do u-law or a-law companding.  Decompression
is easy to accomplish with a lookup table: only 256 entries are required.
You can go the other way in only a small number of instructions; I suggest
checking out the handbook for any DSP chip; all that I've seen include
fast algorithms for u-law and a-law compresion and expansion.

>on the subject, I was wondering if anyone out there can point me in the 
>direction of any examples of previous work in this area. I would be
>particularly interested in any examples of custom (VLSI) implementations.

There's really a very small amount of computation required; it would be
easy to do in hardware.

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