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Re: Minolta Scan Multi Pro: An Initial Impression



In article <3C9AAEF1.3060608@shaw.ca>, Arthur Entlich 
<artistic-1@shaw.ca> writes
>If you don't mind, I'd like to advance this matter further.
>
>Some optical 2400-2800 dpi scanners don't seem to have this problem to 
>any degree.  Is it possible that is due to their having poor enough 
>optics to "neutralize" this aliasing by actually scanning well below 
>the 2400 dpi "minimum"?  As an example, I noticed a lot of other 
>problems with the HP S-10 and S-20 scanners, but not those discussed below.
>Even the Canon FS 2710 has considerably less of these specific problems.
>
>I have heard from one source that the true resolution of the S-10 was 
>only about 1000 dpi due to optical path issues.
>
Limiting the resolution of the optical system is certainly one, if an 
inefficient way, of preventing aliasing with a low resolution scanner. 
The downside is that a lot of information is lost in the "pass band" 
where the scanner can operate without aliasing, because it is not easy 
to build a  spatial brick wall cut-off filter in optics - such as was 
typically used with early digital audio systems.  Another, more 
efficient method is the oversampling-CCD, or the Hyper-CCD, as used on 
the Epson range of scanners.  Here the detector is structured such that 
it physically forms the spatial filter itself at exactly the correct 
spatial frequency for its optical resolution.

You also must remember that a digital image which is properly filtered 
to eliminate aliasing will look softer than an unfiltered image - the 
presence of all of that aliasing actually makes the image look sharp, 
but its really "sharp noise" that is present rather than genuine 
resolution.  Post sampling you can sharpen up a properly filtered scan 
with relative impunity compared to the unfiltered scan, due to the 
presence of this high frequency noise in the latter.

>The Nikons also might be more vulnerable to exaggerated grain due to 
>the nature of the light source, which is pretty unforgiving, while the 
>Minolta use a diffused cold cathode type.
>
That is effectively the same thing - the optical system in the Nikon 
(including the partially collimated nature of the light source) results 
in a much sharper image being presented to the CCD, an image which 
contains much higher spatial frequencies than would be present from the 
same optics used with a diffuse source.

>Can sharpening take place in electronic circuit design, as it can, via 
>filters, in effect with audio signals?
>
Yes - but generally only along the axis of the CCD.

Of course the manufacturer could have built a digital hardware filter 
into the unit as well, which would permit both axes to be sharpened, but 
that would hardly be cost effective in a unit designed to operate linked 
to a computer which is more than capable of implementing such a filter 
in software.
-- 
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers
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