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

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

> 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.

Hi Mike,

What about the zero crossing points that are exactly f separated, which will
not be detected at Nyquist...hence, why I say slightly more than Nyquist is
required to guarantee %100 detection???

But, one thing that is VERY important is there is a difference between
sampling sine waves and square waves.  These test patterns are square waves.
Though at Nyquist, you guarantee detecting the frequency, you do not
guarantee full amplitude.

To sample square waves, and GUARANTEE getting at least ONE "sensor" that
contains full amplitude, you MUST sample at 4x f (or 2x the line width, NOT
line pair width)...and in the case of an image, f really doesn't matter, but
the thickness of the line (which would be 2f).

Austin

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