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



Hi Isidoro,

> Hi Austin,
>
> your message that I refer to is at the bottom of my e-mail, this is:
>
> >Sorry to sound "snide", but er, DUH!  Do you know of a CCD that outputs
> >AC???  I don't!!!  It's ALL D/C measurements as it relates to
> scanners (and
> >actually, even audio).
>
> The statement in capitals is instead mine.

Thanks.  Yes, I did say that.  The signals for a scanner (and audio) are
sampled DC signals at an instant in time.  No A/C is involved in the
sampling at all.  Of course, for audio playback, to hear anything, the
signal OUTPUT needs to vary...since you are replicating not only amplitude,
but frequency over a fixed time.  But, the signal is still treated as DC and
the sampling is still done as DC.  The digitized signal (all the samples)
does not know anything about the A/C nature of the signal.

> I claim that all useful signals are AC, or that can be modeled as AC
> signals.

For audio, yes, but not for imaging.

> The DC measurement isn't the signal itself, but only a measure,
> just so, of part of its nature in a given instant.

Agreed.

> Then I claim
> that the CCD
> outputs a analog AC signal,

The CCD does not output an analog AC signal.  It outputs a "static" DC
signal for each sensor element in the linear array, and that row of static
DC signals are presented to the A/D to be digitized one sensor at a time.
Remember, the light shining through the film, is reasonably constant, and
the amount of light presented to each CCD sensor is pretty much constant
(for our purposes)...and that the outputs from the CCD are then sequentially
"sampled"...then the CCD moves (or the film moves) to the next "position".

Though A/C (frequency) can certainly be ascribed to imaging, that has
nothing to do with the sampling, digitization, and reproduction of the
image.  Also true for audio, as I said above.  I do agree audio is an A/C
signal, no doubt.

> At last, I want underline that no one can talk about signal and
> noise if he
> don't state what the signal is and what the "noise" that MAY obscure it.

Agreed.

> Only after you "modeled" your system, you can apply the formulas
> (and yours
> are correct).

Oh, why thank you ;-)

Regards,

Austin

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