[filmscanners] RE: Film resolution - was: Re: 3 year wait

["Austin Franklin" <darkroom@ix.netcom.com>]

> Austin Franklin wrote:
> >
> > Hi Mike,
> > > When sampling a square wave at the nyquist frequency, the
> square wave can
> > > be perfectly reconstructed from the resulting samples.
> >
> > As I've pointed out, at Nyquist, when the line is not entirely
> "seen" by ONE
> > sensors, two sensors will only see a reduced part of the line,
> and therefore
> > the detected amplitude of the line will be decreased.  That is NOT
> > "perfectly reconstructed", as you say, since you are not
> reconstructing the
> > amplitude accurately.  The ONLY way to guarantee acquiring full
> amplitude is
> > for the sensor spacing to be at least 4f, which guarantees that
> a line of
> > width f will be fully detected by at least one sensor.
> >
> > Try drawing this out on paper, or do something to visualize
> it...or heck,
> > design a scanner ;-)
>
> Sorry, but what you say isn't true.

Mike,

But I know what I say IS true as it applies to scanners!  I design digital
imaging systems, and have been for 25 years.  I have designed about a half
dozen scanners, and aside from designing them, I also have done extensive
testing and analysis on how these devices work.

> As I've mentioned numerous times
> (and hopefully this time will be the last), for black and white
> stripes, the Nyquist rate is twice infinity -- so to scan at the
> Nyquist rate in your example, the spacing between samples is
> half of zero distance apart.  When the samples are half of zero
> distance apart, it WILL sample correctly.

I believe the part that is important here is that scanners to not sample
points in time, they sample areas in space.  That is why 4f is sufficient to
 %100 acquire the amplitude and frequency of a square wave of width f (1/2f
being the width of the actual black line) with a scanner.  Why not try to
put that down on paper and understand what it is I am saying.

> Mike K.
>
> P.S. - Go get a book on Nyquist and go through the mathematical
>        derivations.

I have probably a dozen or more books on this subject, as well as may others
on many related topics, such as optics, digital audio etc., and yes, I've
read (and understand) them.  This is somewhat mandatory for anyone who
designs these kinds of systems, as I do.

> P.P.S. - What I've been doing for the last month is "Signal Integrity
>          Analysis" for a design of mine that's now being converted into
>          circuit boards by our circuit board dept.

Not meant at all to be a "mine is bigger" thing, but I am an engineer, and
have been for 25 years.  As I said above, I design digital imaging systems
(primarily) for a living.  For the past year, I've been running a project
(and the lead designer) for a digital video output system to simulate the
acquisition of "flat mail" for address recognition.  It's the second system
like this I've designed for the same client.  I've also designed a number of
pre-press scanning systems (some included built-in halftone processing), as
well as an optical saw blade measurement system that was accurate to
.0005"...and a vision system for seam tracking for a welding robotic arm,
and a vision system for a parts assembly robotic system, and a vision system
for a parts inspection system...and...quite a few acquisition boards for
different camera systems...and probably another 20 or more projects, some
related to digital imaging, some not.  Amongst some of those projects was
having designed a number of digital audio systems for studio recording and
playback applications.

> I'm analyzing
> those very kind of
>          edges where my "black and white" (equivalent) transitions are
>          signals that switch state in a few hundred pico-seconds
> -- and I'm
>          trying to make sure they'll do so reliably -- so when my
> circuit board
>          is made it'll work reliably. Even a signal that goes "black and
>          white" once a second still has frequency components in the multi
>          Giga-hertz range when that once per second change occurs.  So
>          in a vague way, "been there, done that", I'm reporting to you
>          what happens.

But what you are "reporting/analyzing" applies differently to scanners.  We
aren't talking about switching thresholds here, or ringing, or overshoot, or
undershoot, or knees.  The scanner sensors see the AVERAGED luminosity in
their FOV, no matter what the components are in the FOV.

Austin

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