Chuck,
> > Actually, it's Nyquist, and it's 2 to 1.
> Check your Engineer's Handbook: it's Shannon for sampling and Nyquist for
> stability. The concepts overlap at times, but not this time.
I don't have to check, I know that you have to only sample at 2x the highest
frequency to reliably detect that frequency, and you simply to NOT need 3x
as you say. It is good to sample at slightly more than 2x. This is XY, and
to sample in a diagonal (worst case), the number is different, of course,
but it's simple geometry, and 2x applies, just the dimension used is 1.414 x
the X/Y dimension.
Perhaps you ought to look at some sampling theory primers:
The scanner IS sampling a frequency, and Nyquist perfectly applies. BTW,
you CAN detect at f, but not reliably. What a sampling system gives you is
in fact a range, 1/2f to f.
> > That is simply part of basis
> > sampling theory, and is inherent in any discussion of sampling, scanner
> > design included. Your comment said that the "steps" (of the A/D) had
> > nothing to do with the intrinsic properties of the CCD and that is
simply
> > NOT true, which was my point, and the A/D IS in fact sized to the noise,
> > and that sizing is 2:1 typically.
> I think you are wrong on this technical point.
What I said above is %100 true, and I'd bet on it. I'm not sure why you
don't believe it, but clearly one of us is missing something.
> The film "noise"...
Film noise has nothing to do with the discussion. The scanner doesn't care
a wit about what is on the film. I think this is the reason we're not
connecting.
> is
> adequately and wholly captured by HP's 12 bit A/D, according to their tech
> notes.
Yeah, but that has nothing to do with sampling theory. The scanner doesn't
care if it's noise on the film (what EXACTLY do you mean by "film noise"
anyway"?) or actual image data.
> I believe that the CCD noise in scanners is substantially smaller
> than film variabilities (ie, "noise"), though it is significant.
That depends on the film.
> CCD
> offsets (eg, fixed cell-to-cell zero light values) are likely to be much
> larger than what is called "noise" like Johnson noise on CCD spec sheets.
Larger or not, and it's deterministic and easy to correct to the noise
resolution of the CCD, analog front end, and A/D.
> That would mean that the models with 14 and 16 bit A/Ds are using a finer
> step that is not directly sized to film noise or intrinsic CCD noise.
Film noise has nothing to do with this, and I believe that's the reason
you're not understanding. As I said, the A/D IS in fact matched to the
noise level of the CCD, period, which is what the question was. It has not
a thing to do with film noise.
> > I'm not sure what you mean by "The A/D is scaled..." because it is not.
> > The analog front end simply "matches" the output voltage range of the
CCD
> > with the input voltage range of the A/D, which is not scaling the A/D at
> > all. It IS "scaling" the voltage TO the A/D.
> Difference of viewpoint only I think:
My statement isn't a viewpoint, it's a statement of how the analog front end
is designed in a typical film scanner. There is no other way/reason to
design it otherwise in this application.
> I'm used to doing basic design where the A/D
> subsystem characteristics can be scaled but the physics of light detection
> or film variabilities cannot.
Who said anything about physics of light detection or film variabilities?
They, as I've said a number of times already, have nothing to do with the
design of the CCD/AFE/AD.
> > The size of the lsb is just not relevant nor is it significant (as
> > for frame-to-frame waveform analysis/synthesis) in these
> scanner systems.
>
> The size of the LSB IS in fact relevant, and IS a property of the noise,
> Maybe we differ in terminology reference
> alone since we're not debating existence of the phenomena.
Having designed them that way, and having reviewed many other designs, I've
got a pretty good idea that what I said is terminologically and technically
accurate.
> > and again, the choice of number of bits used in the A/D matched to the
> > noise (by a factor of two, but it's still matched to the noise). If you
> > have more bits, fine, but you can't have less, or you lose good data.
> Specific matching by a factor of two would be bad design IMHO, and "not
the
> best" according to handbook criteria for quality signal recovery with low
> distortion.
Specifically WHAT handbook? I know adding more bits gives you nothing.
I've done thousands of hours of testing and analysing different designs, and
my statements are conclusive with my personal experience.
> According to HP's and other tech lit, that matching for film noise would
> occur at about 9 bits of A/D. We're not connecting on this point somehow.
You're right. For some reason, you think film noise has something to do
with this, and it doesn't (at least in what I was discussing). If you could
explain why you think that, perhaps we can find out where the
misunderstaning is...
Regards,
Austin
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