Hi,
I am not interested in the question too much, but when I have read the last
message by Austin...
ALL USEFUL SIGNALS (= INFORMATION) ARE AC SIGNALS, "CONSTANT SIGNAL" IS ONLY
AN OFFSET.
This is true in "picture" domain too. Sound strange ?
Look at this example:
You are reading (maybe) this my e-mail. Well, you see pixels (apparently DC
signal) on the screen, but only the sequence of pixels with different
luminosity has sense for you. If you go away from the monitor, I could see
that the (spatial) frequency of this AC signal becames too high for your
sensor (your eye) and you cannot read anymore the words. What this example
means ? It means that I must ever model what I observe and what I am
interested to. For example, if I want listen to the music, I am not
interested to supersonic frequencies (another spectrum region, as Paul
wrote) then I can ignore the (S+N)/N ratio in this region. Incidentally,
this statement is used in audio to "model the noise".
However, yes, every single pixel of the CCD outputs a AC signal, a signal
that varies in function of the "luminosity", sampled by an sample&hold
mechanism and converted in digital by a ADC converter (the instrument that
misure it in given instant). If this AC signal is, for example, too low
compared with the termal noise in the CCD, it becames no more discernible
from the noise (the electrical SIGNAL in part generated by the atomic
agitation, that is function of the temperature) in the region of interest.
Note that if I want misure the temperature with a CCD, could happen that the
noise is the signal and the signal is the noise. Obviously, the CCD isn't
the best temperature sensor... :-)
About the audio: the audio signal is ever nearly pure AC. On your hifi
speaker try a "DC signal", as a car battery, on the woofer, and look to what
happens... ;-)
Ciao,
Isidoro
>From: "Austin Franklin" <darkroom@ix.netcom.com>
>Reply-To: filmscanners@halftone.co.uk
>To: isiora02@hotmail.com
>Subject: [filmscanners] RE: Density vs Dynamic range>AUSTIN (2a)
>Date: Fri, 14 Jun 2002 13:27:28 -0400
>
>Hi Julian,
>
> > You are talking about *DC signals* and are therefore concerned about
> > offsets!!
>
>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).
>
>I'll look over the rest of what you said when I get the time, but to answer
>a question you raised, NO, I did NOT make up these terms, they are straight
>from my reference material, though my references don't all use consistent
>terms. I have done nothing but try to CLARIFY them, since there seems to
>be
>some confusion, and as I said, not everyone uses the same terms...so unless
>we can agree on a set of terms, we really can't talk about this.
>
>The start of this sounds positive...so you have raised my hopes ;-)
>
>Regards,
>
>Austin
>
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_________________________________________________________________
Scarica GRATUITAMENTE MSN Explorer dall'indirizzo
.
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