Digital BW, The Print

> When you speak of noise, this is noise that is a by product of
> the scanners
> circuitry, like hum, which I suppose has a certain
> predictability to it, or
> noise like extra data bits that don't hold statistically meaningful data
> bits.

They are kind of both the same thing.  Typically, a system is designed such
that the data you get from the A/D won't contain noise, except in the lowest
bit(s).  The system should be designed above the noise.  All the bits, but
the LSB (least significant bit) should be valid in most cases, at least as
far as system noise goes...noise in the material to be scanned is a
different story.

> I guess I mean values which are too closely spaced to be
> distinguishable as significantly different from one another. Would this be
> non-integer ratio values?

There can be no non-integer ratio values in this type of system.  Since
everything is relative, as in, is a ratio...based on what the CCD/AD
measures as "1"...which is the noise floor, and since you can only measure
in increments of the noise, there are no intermediate values.  If you halve
the noise, you change the baseline ratio value (what was 1 is now 2, and
what would have been 1/2 if you could have measured it, is not 1), so you
still get no intermediate values.

I believe this (the ratio thing) is a very important concept to understand
by the way.

> I'm a bit over my head here, but Dan Margulis
> explained that when the scanner gets into the highest density regions it
> doesn't just hit a wall and call everything that dense and denser
> black (in
> the case of a chrome), it tend to sort of guess a value, and there is a
> randomness to it. I think this is the noise we generally associate with
> "noisy shadows" of a scan.

Yes, the lower bit(s) will be random, but that's what noise is.  Anytime the
low bits are "into the noise level" they will be random...  You can design a
system such that no bits are in the noise level, and the LSB will have a +-
1/2 value error...and will be subject to a small amount of noise...but I
disagree with Dan on this, it is very design dependant, and is not a general
rule.

> But with negative materials this occurs in the
> highlights of the IMAGE, though it's still considered shadow in this
> context, correct?

Yes, correct.  That is why all this "shadow detail" stuff isn't important to
negatives...only chromes.

> In the context of scanners, shadow is the film density
> region, yes?

Yes.

> Okay, to confirm, whatever tone the scanner measures to be darkest (or in
> the case of the random noise I spoke of, assigns to be darkest)
> is assigned
> the tonal value of zero, and each next whole integer value is assigned in
> order up the scale?

It depends on calibration...but suffice to say, 0 means the scanner can't
read anything...and 1 is really the baseline value the scanner starts
"reading" at.  Whatever the scanner determines has a value of 1, the value
of 2 is going to be twice as bright as 1.

> It is the A/D converters function to assign
> tonal values
> to the voltages received by the CCD?

YES!

> So, the lower the noise, the more tones the front end can distinguish,

in the dark region....

> and
> the higher the bit depth (BD) of the A/D required, to keep up, so
> to speak?

YES.

> With more noise you can get away with a lower BD A/D cause
> anything more is
> wasted.

YES.

> Okay, so this is where the manufacturers claims become suspect
> because they speak of the bit depth of their scanners relative to
> their A/Ds
> without specifying the S/N ratio of the front end which is the primary
> limiting factor?

YES ;-)

> In the instance where a manufacturer does put a high BD A/D in with a high
> noise CCD, I get that you will still have a lot of shadow noise
> (which again
> are tones which have been spuriously assigned by the scanner, because it
> can't truly distinguish them as discreet from one another, or
> from the noise
> of the front end?). But when you get out of that threshold area,
> do you then
> have more useable tones further along the scale than if you used the lower
> BD A/D with that CCD?

That's takes a bit of an explanation.  If you have a 20 bit A/D...and a CCD
whose noise really only give you 10 bits of good signal...you will get noise
in all the lower bits, throughout the entire scale...but that noise that is
between values won't really mean anything, but the noise at the end of the
scale will be the shadow noise.  So, no, you don't get more usable tones.

> Nice job Austin. I think I'm getting it so far (roughly anyway). I look
> forward to the next chapter. ;-)

Thanks.  I think I got the above right.  Let me know if I didn't understand
something you asked.