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[filmscanners] Re: Density vs Dynamic range



<http://www.prosig.com/signal-processing/dynamicrange.html>

This is just one reference to show that RMS measurements *are* used to
derive dynamic range for *some* applications.

In a scanner, we are using a CCD device to make static charge measurements.
The noise can still be characterized by an RMS measurement, but the upper
limit is determined by a saturation effect.  So it is not surprising to
find:

<http://www.kodak.com/US/plugins/acrobat/en/digital/ccd/ccdTerminology.pdf>

On page 16:  "Dynamic Range (DR) is the ratio of the maximum output signal,
or saturation level, of an image sensor to the dark noise level of the
imager.  The dark noise level, or noise floor of an imager is typically
expressed as the root mean square (rms) variation in dark signal voltage."
[The definition continues, with equation showing how to form the dB value]

On 6/12/02 8:25 PM, "Julian Robinson" <jrobinso@pcug.org.au> wrote:

> Austin - of course RMS measurement applies to dynamic range.  I think the
> fact that you say this points to where your view differs from the rest of
> the world, but I'm damned if I can work out how...
>
> Remember the definition from the book *you* posted and *you* agree with:
>
>> the Dynamic Range equation out of "Digital Signal Processing in VLSI":
>> DR (dB) = 10log10(largest signal/smallest discernable signal)
>
> How exactly are you going to measure "largest signal" and "smallest
> discernable signal"?  Most people would use RMS, or at least try to
> approach that with a mean measurement if they didn't have the true-RMS
> gear.  Alternatively you could use peak measurements, but that is a bit
> tricky with the noise and you have to involve some statistical assumptions,
> and as Julian V says, sometimes it can change the results depending on your
> choice of peaks in the HiFi world where short term peaks can be a lot
> higher than sustainable peaks.
>
> Why on earth would you say "RMS doesn't apply to dynamic range."?
>
> Julian R
>
>
>
> At 23:33 12/06/02, Austin wrote:
>>>> SNR also is an RMS based measurements, and RMS doesn't apply
>>>> to dynamic range.

--
Julian Vrieslander <julianv@mindspring.com>

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