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From: aweinste@Diamond.BBN.COM (Anders Weinstein)
Newsgroups: net.ai,net.cog-eng,sci.electronics
Subject: Re: The Analog/Digital Distinction
Message-ID: <1670@Diamond.BBN.COM>
Date: Wed, 29-Oct-86 11:37:55 EST
Article-I.D.: Diamond.1670
Posted: Wed Oct 29 11:37:55 1986
Date-Received: Wed, 29-Oct-86 22:20:16 EST
References: <15@mind.UUCP>
Reply-To: aweinste@Diamond.BBN.COM (Anders Weinstein)
Organization: BBN Labs, Cambridge, MA
Lines: 41
Keywords: analog, digital, continuous, discrete, quantization, sampling rate
Xref: mnetor net.ai:1257 net.cog-eng:314 sci.electronics:23


> From Stevan Harnad:
>
>>	Analog signal -- one that is continuous both in time and amplitude.
>>     ...
>>	Digital signal -- one that is discrete both in time and amplitude...
>>	This is obtained by quantizing a sampled signal.
>
>                                          Question: What if the
>original "object" is discrete in the first place, both in space and
>time? Does that make a digital transformation of it "analog"? I

Engineers are of course free to use the words "analog" and "digital" in their
own way.  However, I think that from a philosophical standpoint, no signal
should be regarded as INTRINSICALLY analog or digital; the distinction
depends crucially on how the signal in question functions in a
representational system.  If a continuous signal is used to encode digital
data, the system ought to be regarded as digital.

I believe this is the case in MOST real digital systems, where quantum
mechanics is not relevant and the physical signals in question are best
understood as continuous ones. The actual signals are only approximated by
discontinous mathematical functions (e.g. a square wave).

>                                              The image of an object
>(or of the analog image of an object) under a digital transformation
>is "approximate" rather than "exact." What is the difference between
>"approximate" and "exact"? Here I would like to interject a tentative
>candidate criterion of my own: I think it may have something to do with
>invertibility. A transformation from object to image is analog if (or
>>to the degree that) it is invertible. In a digital approximation, some
>information or structure is irretrievably lost (the transformation
>is not 1:1).
> ...

It's a mistake to assume that transformation from "continuous" to "discrete"
representations necessarily involves a loss of information. Lots of
continuous functions can be represented EXACTLY in digital form, by, for
example, encoded polynomials, differential equations, etc.

Anders Weinstein