[filmscanners] RE: scanner dmax discussion

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

> From: Clark Guy
>
> I agree that 9 decades (180 dB) is a WWWWWIIIIIIIDDDDDDDDEEEEEEEE
> range, but maybe possible.
>
> 24bit (144dB) A/Ds are commercially available for digital Audio
> for a while,
> so maybe 30bits A/Ds are being worked on in the lab?  One could sacrifice
> speed for quality more easily in scanning than in audio!
>
> I would expect something on the order of 14 bits  (84dB) or
> better from any
> good scanner.
>
> On further reflection, 30 bits (per channel) would be likely to
> be overkill
> for any but the most demanding instrumentation applications.  A good solid
> 16 bits per channel is probably more than enough for my needs!

Believe me, 24bit audio ADCs don't have 144db of S/N. Not even close. The
bottom few bits are all noise. The only way to measure anything with 180db
range is with an extremely narrow (sub Hz) bandwidth, since thermal noise
power is proportional to bandwidth, or by dropping to near zero Kelvin,
since it is proportional to absolute temperature. But even then, there are
other noise mechanisms that get in the way.

You can certainly get way more than 9 octaves (54db) of range from even a
cheap scanner, but the original post was claiming 0.1% _linearity_ over the
range in question, which from my experience sounds more like what you'd get
over 9 octaves of range, not 9 decades. If I was told you could get 1%
linearity over 15 octaves (4.5 decades), or 10% linearity over 20 octaves (6
decades), that would sound plausible to me, because that's the sort of
accuracy you get for the current/voltage curve of typical silicon junctions.

But these are just my intuitions from years of working with semiconductors.
I have no particular expertise in CCDs, beyond having read a couple data
sheets.

--

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

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