> > 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...
Well spotted! Yes it would.
> 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.).
I think you're overstating this, but the principle is correct.
> > 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?
Of CCD filmscanners, AFAIK only the Olympus used adjustable gain amps on
each channel. I believe older PMT drums all did it this way, with an ADC
tacked on the end.
Regards
Tony Sleep
http://www.halftone.co.uk - Online portfolio & exhibit; + film scanner
info & comparisons
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