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From: ken@turtlevax.UUCP (Ken Turkowski)
Newsgroups: net.arch
Subject: The problem with high-order truncation of multiplication
Message-ID: <846@turtlevax.UUCP>
Date: Thu, 1-Aug-85 13:48:51 EDT
Article-I.D.: turtleva.846
Posted: Thu Aug  1 13:48:51 1985
Date-Received: Sat, 3-Aug-85 08:42:10 EDT
Reply-To: ken@turtlevax.UUCP (Ken Turkowski)
Organization: CADLINC, Inc. @ Menlo Park, CA
Lines: 17

Signal processing generally uses fixed-point numbers that are normalized:
i.e. have a magnitude less than 1.  When two such numbers are multiplied
together, the product is still normalized.  Rather than throwing away the
most significant bits as in multiplication of integers, you want to keep
these bits, since that's where most of the significance is, and instead
throw away the least significant bits, after shifting out the duplicated
sign bit.

Machines like the WE32100 which automatically truncate products at the top
cannot be used for signal processing.  I'm surprised that AT&T would make
such a machine, since they do so much signal processing for echo cancelling,
phase extraction, etc. in their communication products.
-- 

Ken Turkowski @ CADLINC, Menlo Park, CA
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