[filmscanners] RE: Density vs Dynamic range>AUSTIN (2a)

["Paul D. DeRocco" <pderocco@ix.netcom.com>]

> From: Anthony Atkielski
>
> No.  In fact, you contradict yourself in this statement.  A steady DC
> voltage does not change, and yet you speak of "the number of different DC
> voltages that can be distinguished."  How will these different voltages be
> distinguished, given that DC does not change?

Why is this so difficult for you to grasp? The information carrying capacity
of something depends upon the number of different states it _could_ have,
not the number of states it actually attains. A page of text may have a few
thousand characters on it, representing maybe twenty kilobits of
information, even though there aren't 2 to the 20000 power different pages
that actually exist in the universe. And it would have the same information
carrying capacity even if one particular page happened to be utterly blank.

> The information carried by
> the medium resides in the transitions embodied in that medium:
> changes from
> one voltage to another, or from one place to another, and so on.  The only
> way to use different DC voltages is by providing for a transition between
> voltages; but a transition is a variation in the voltage, and is thus
> effectively AC.  If there are no transitions, the voltage never
> changes, and
> it really is DC; but in that case, it carries no information,
> either, and so
> it is not a signal.

Not true. It carries information about what the voltage is, as distinguished
from what it might be.

> No.  It conveys information only if it changes.  The whole purpose of a
> signal is to communicate information, and the only way to communicate
> information is through change.  The only utility in a signal that does not
> change for a million years arises when the signal _does_ change;
> if it never
> changes, it may as well not exist, since it communicates no information.

> A DC voltmeter has a dial, allowing for changes in voltage.  If
> the incoming
> voltage never changed, it wouldn't need a dial; it wouldn't even need an
> on/off light, since the voltage would always be there.

But just because it doesn't change doesn't mean that it couldn't change.
You'd still need a voltmeter to know that it hadn't changed. The distance
from Los Angeles to San Francisco is a fixed number, but it's still a number
that has meaning; it conveys information.

> These are practical applications of information theory, and they are
> unavoidable.  Understanding them is important to understanding
> many aspects of digital imaging.

Look, this is getting stupid. This began with a discussion of whether a
scanner captures AC or DC information. If DC doesn't represent information,
then removing the DC component should have no effect on the functioning of
the scanner. But of course it does. To make a scanner, you need a frequency
response down to DC. You don't capacitively couple the output of your CCD
into the A/D converter. It won't work. Period. Now _that's_ practical, and
it happens to coincide exactly with the theoretical, too.

--

Ciao,               Paul D. DeRocco
Paul                mailto:pderocco@ix.netcom.com

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