> Remember the CCD talks voltages, and because the device is linear
I guess you could consider it linear for the purpose of this discussion.
But the fact is, there is really no such thing as linear...especially in
A/D converters, but no need to go there in this discussion.
> But now consider 2 bits (binary values)
>
> 00 whitest white
> 01 25% grey
> 10 75% grey
> 11 blackest black
>
Wouldn't 01 be %33.3...and 10 be %66.6...
> The ratio of DMin:DMax is constrained by this, giving a maximum value of
> OD of which the scanner is theoretically capable which is determined by
> the bit depth. Otherwise we'd all be perfectly happy with 8 bit scanners,
> possibly even less.
With 8 bits, you only get about 60-70db SNR (signal to noise ratio), and
using a higher number of bits gives you a better SNR. As I'm sure you
know, A/D converter systems don't give you ALL the bits perfectly, the LSB
is always off, and possibly the two or more LSBs just aren't accurate, so
to get truly 8 usable bits, you would need more than 8 bits...and you drop
n LSBs to get 8 (or use 14 to get 12...etc.).
> Drum scanners and CCD scanners which use analogue shaping of the voltage
> to a non-linear gamma function upstream of the ADC aren't constrained
like
> this, as much wider ratios can be compressed to fit within the bit depth.
This can be done in the digital domain (after the ADC). I can't imagine
why anyone would want to do that in the analog domain. Who does that?
====================================================================
The filmscanners mailing list is hosted by http://www.halftone.co.uk
To resign, <mailto:[EMAIL PROTECTED]> with UNSUBSCRIBE FILMSCANNERS in the
title, or UNSUBSCRIBE FILMSCANNERS_DIGEST if you are reading the Digest.
