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Re: filmscanners: Scratch the Gear Teeth Theory
On Wed, 18 Jul 2001, Robert Meier wrote:
> Unfortunately, I do not have the email with the scan
> anymore but it seemed to me that the banding happens
> at constant pixel spacing. Therefore, I do not believe
> that it is a problem with the CCD itself because it's
> quite unlikely that the sensors are bad in a equal
> spacing. One thing I could imagine is the amplifier.
> In order to reduce noise due to fast read-out times
> and to allow somewhat faster scanning there might be
> more then 1 amplifier per CCD line. Assuming they use
> 32 amplifiers, i.e. pixel x goes to amplifier 'x MOD
> 32' and assuming that the gain for one of these
> amplifiers is off then you would see such banding.
> That's a pretty wild guess, though.
> The original poster said that he saw the banding only
> when adjustment were done. Have all other parameters
> been the same? For example I have heard some issues
> with multi-scanning on Polaroid scanners which could
> lead to soft images. If I remember correctly Nikon
> scanners have some HW support for multi-scanning. So
> instead of soft images an artifact could be banding.
> Hint: Wild guess!!!
> One thing you could check is if the banding always
> happens at the same place. For example do a scan of a
> picture that has some clear sharp lines. Scan it and
> record where the banding happens relative to this
> line. Repeat it to check for consistency. Then move
> the picture to be scanned a little bit within the
> holder. If the banding does change relative to the
> line it is quite likely a HW issue. Otherwise it could
> be, but doesn't have to be, a SW issue.
Where I've seen it, it's a "venetian blind"
effect, and is uniform across the entire image.
The bands are around 30-35 pixels wide. Hard
to measure exactly.
I'm not sure I want to guess what's causing it.
Maybe mechanical, maybe electrical, who knows.
The software theories mentioned in other posts
this morning strike me as improbable, however.