Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!seismo!think!rst
From: rst@think.COM (Robert Thau)
Newsgroups: sci.math,sci.physics,sci.electronics
Subject: Re: Analog/Digital Distinction
Message-ID: <6654@think.COM>
Date: Sat, 1-Nov-86 19:21:47 EST
Article-I.D.: think.6654
Posted: Sat Nov  1 19:21:47 1986
Date-Received: Mon, 3-Nov-86 21:39:02 EST
References: <105@mind.UUCP>
Reply-To: rst@godot.think.com.UUCP (Robert Thau)
Distribution: net
Organization: Thinking Machines, Cambridge, MA
Lines: 39
Summary: Analog representations are not exact
Xref: mnetor sci.math:104 sci.physics:97 sci.electronics:46

In article <105@mind.UUCP> harnad@mind.UUCP (Stevan Harnad) writes:
>"Preserving information under transformations" also sounds like a good
>candidate. But it seems to me that preservation-under-transformation
>is (or ought to be) a two-way street. Digital representations may be
>robust within their respective discrete boundaries, but it hardly
>sounds information-preserving to lose all the information between .2
>volts and 4 volts. I would think that the invertibility of analog
>transformations might be a better instance of information preservation than
>the irretrievable losses of A/D.

I'm not convinced.  Common ways of transmitting analog signals all
*do* lose at least some of the signal, irretrievably.  Stray
capacitance in an ordinary wire can distort a rapidly changing signal.
Even fiber optic cables lose signal amplitude enough to require
repeaters.  Losses of information in processing analog signals tend to be
worse, and for an analog transformation to be exactly invertible, it
*must* preserve all the information in its input.  

Of course, it is possible to keep the loss of information to
acceptable levels.  Any stereo buff owns at least two fairly good
analog systems (speakers).  If s/he is a committed stereo buff, s/he
probably can quote you exactly how much information is lost by the
speakers (at what volume levels, in what pitch ranges, ad nauseam),
and how much it would cost to lose less.  The point is that the amount
of information in the speakers' input which they lose, irretrievably,
is a consequence of the design decisions of the people who made them.
Such design decisions are as explicit as the number of bits used in a
digital representation of the signal in the CD player farther up the
pike.  Either digital or analog systems can be made as "true" as you
like, given enough time, materials, and money, but in neither case is
perfection an option.

>And this still seems to side-step the
>question of WHAT information is preserved, and in what way, by analog
>and digital representations, respectively. 

Agreed.
--
This article was generated entirely by line noise.  Don't believe a word of it.
rst