Digital BW, The Print

Hi Truman,

> So what you are saying is that film scanners have no amplifaction on the
> sensor prior to the A/D?

It's really a voltage matching circuit.  The typical A/D has an input
voltage range of say +3V to -3V (which is 6V peak to peak), and the CCD
output, let's say, has a voltage swing of 2.8V to saturation.  You have to
adjust the output voltage of the CCD to the input voltage of the A/D in
order that you take advantage of the full swing of the A/D.

Let's use the number we used above of the CCD having a 2.8V swing, and a 6V
swing for out A/D.  That would be a gain of 2.143 provided by the analog
section between the CCD and the A/D.  If the CCD were putting out 1V, the
A/D would see 2.143V...  There can be some curve adjusting in this analog
section, some scanners do that.

> This means that tonal range for higher bit
> scans is accomplished by interpolation (call in what ever you want,
> filtering, smoothing, etc.)?

No, not at all.  How did you arrive at that conclusion?

The CCD will output a voltage, and that voltage will pretty linearly
correspond to the number of electrons the CCD sensor "sees".  The lower the
number of electrons it sees, the lower the output voltage, and the higher
the density that it saw...  The converse is true as well, the higher the
number of electrons it sees, the higher the output voltage, and the lower
the density that it saw.

Austin