Path: utzoo!utgpu!water!watmath!clyde!att!ttrdc!ttrde!pfales
From: pfales@ttrde.UUCP (Peter Fales)
Newsgroups: sci.electronics
Subject: Re: CODEC - How to get one?
Summary: National makes a nice one
Message-ID: <723@ttrde.UUCP>
Date: 13 Oct 88 14:04:32 GMT
References: <375@limbic.UUCP>
Organization: AT&T, Skokie, IL
Lines: 31

In article <375@limbic.UUCP>, gil@limbic.UUCP (Gil Kloepfer Jr.) writes:
> Can anyone provide input on what the mu and A law compression schemes are,
> and (especially) how I would go about getting a CODEC chip (prices, locations,
> etc).  Would a company like Motorola, AT&T, or TI provide evaluation kits
> with these chips?

All the companies you mention as well as others make CODEC chips.  However,
most of them share a common drawback for experimenter's use in that the
digital information is input and output serially.  I have used a chip
made by National Semiconductor which is much nicer for microprocessor
use because it has a parallel data interface (though the data bus does
use a somewhat non-standard clocking and chip selection scheme).  It 
comes in two versions: the TP3051 uses mu law and the TP3056 uses A law
companding.

Companding is explained in any reference book on digital
telecommunications.  Simply put, it is a method of improving the sigal-to-
noise ratio without increasing the number of sampling bits by using
finer sampling for low amplitude signals.  In other words, the 
relationship between analog input and digital output is not a straight 
line, but a curve, and the shape of that curve is defined by the mu law 
or A law equation.

My source is National's 1984 Telecommunications Databook.  There are
probably more recent devices out there that I am not aware of.

-- 
Peter Fales			AT&T, Room 2F-217
				200 Park Plaza
UUCP:	...att!ttrde!pfales	Naperville, IL 60566
Domain: pfales@ttrde.att.com	work:	(312) 416-5357