Austin wrote:
> May be I'm slow today...but that paragraph is really unclear
> to me, and I know this stuff quite well. What exactly do
> you mean by 'for a given sensitivity from the analog circuitry'?
OK, let me put it another way and try to avoid some of the ambiguous terms.
You have an analogue circuit which starts with a CCD and produces meaningful
variations in voltage for a certain minimum and a certain maximum light
intensity (I'm ignoring colour for simplicity's sake). This voltage is
fed into an analog to digital converter (ADC). The real minimum and maximum
light intensities which the circuit can resolve into a digital number is
determined by the analog CCD circuit NOT by the number of bits used in the
ADC. The number of bits in the ADC *only* determines the number of steps
between the minimum voltage it can resolve into a number and the maximum
voltage it can resolve into a number.
The only reason I can see that a greater number of bits would help is that
when you are at the extremities of the CCD's range, more bits should help
resolve meaningful data from noise, or by reducing the size of the steps,
reduce the loss of image information which lies between the steps at a lower
bit depth.
Ultimately, it's the CCD circuitry which determines the minimum and maximum
light intensities that the scanner can (in theory) resolve, not the number
of bits used to convert it to a digital image which just determines the
smoothness of the conversion.
What Ed's demonstration of a relationship between bits and dynamic range
demonstrated to me was that the numbers for some scanners simply *seem*
to be the theoretical maximum determined by a mathematical relationship
from a given bit depth *not* a real measurement of the DR as determined
by the kind of tests Dave Hemingway described.
In fact, referring back to my argument above, there's no reason why an 8
bit per channel scanner couldn't have a dynamic range of (say) 4 if the
analog circuitry is capable of measuring that range of light intensities.
Rob
Rob Geraghty [EMAIL PROTECTED]
http://wordweb.com
