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


  • To: lexa@lexa.ru
  • Subject: [filmscanners] Re: Film resolution - was: Re: 3 year wait
  • From: "Mike Kersenbrock" <michaelk@aracnet.com>
  • Date: Tue, 14 May 2002 08:49:16 -0700
  • Unsubscribe: mailto:listserver@halftone.co.uk

Austin Franklin wrote:
> It's actually required to sample at LEAST at slightly more than 2x (not
> simply 2x) what you are trying to sample to reliably DETECT the signal,
> since, as you said above, you can not guarantee phase coherency.  If you
> sample at exactly 2x, you can end up not detecting the signal at all (as
> shown by the transition point in the 1x Nyquist sample), since you can not
> guarantee at what point along the signal you are sampling, and technically
> could sample at the zero crossing for each of the two samples!  This is why
> CDs sample at slightly more than 2x 20kHz, or 44.1kHz.  Just because you
> sample at Nyquist, does not mean you don't get distortion...in fact lots of

If you sample at Nyquist, the full signal (in this case image) can theoretically
be reconstructed.  Unfortunately the author of that article doesn't seem
to grok (understand) what the Nyquist rate is.

The Nyquist rate is indeed "double", but it's double the highest
frequency component in that sampled.  If that being sampled are ideal
black and white bars, the frequency component is "infinite", so it
would take two times infinite samples per inch to record them.

If the image is one that has a sinusoidal pattern (and the output
device used is able to have the proper reconstruction filter limiting
the output frequency to half-nyquist), the result should be the same
as that recorded.  I think neither of these conditions for properly using
Nyquist are being used and so on a practical basis, a much higher
sampling is indeed required, but not because Nyquist is a failure,
it's because Nyquist is being misapplied and then being called a
failure.

More or less. :-)

Mike K.

P.S. - This discussion also is applying a two dimensional idea (for
       nyquist I recall it was signal vs time) in a three dimensional
       application (light intensity vs horizontal vs vertical) so
       all above would apply in both the intensity vs vertical and
       intensity vs horizontal "stripe" directions simultaneously.

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