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[filmscanners] RE: 8bits vs. 16bits/channel: can the eye seethe difference



> > I agree with that, but that's not really dithering.  That's merely a
> > property of averaging, and how your eye works.  Dithering is
> deliberately
> > adding random noise to a signal.  What you are describing is called
> > aliasing.  They are entirely different.
>
> No, it's dithering, not aliasing. Aliasing is the translation of
> frequencies
> by sampling.

Paul, that IS what you described, the translation of frequencies by
sampling!  You said:

"If you have an area of blue sky whose actual analog color levels are, say,
R=85, G=110 and B=182.75"

Whether the 182.75 gets picked up as 182 or 183 is a form of aliasing, not
dithering.  If in fact, the original image has that kind of tonal
resolution, then it is not being sampled at a high enough frequency to
maintain that level of detail.  It's being undersampled, and as such, the
result of the sampling of the higher frequency components is aliasing.

> Whether the noise is deliberate or accidental is unimportant. The blue sky
> has no appreciable noise, but the film grain or CCD adds some,
> resulting in
> a signal that can be truncated to eight bits and yet still retain
> more than
> eight bits of useful information at lower spatial frequencies.

But it does NOT contain "more than 8 bits of useful information".  You can
not consider noise valid data.  I understand the effect you are talking
about, and it is not dithering.  The effect is caused by post data filtering
as done by your optical system, which is sampling at a lower frequency.

> > I don't believe that is true.  That is only used in the lower and some
> > intermediate level CD players.  It's called delta-sigma
> conversion, and I
> > was one of the original developers of this technique.  The high
> > end units I
> > am aware of still use very high end 20 bit A/D converters.  But,
> > that's not
> > got anything to do with dithering either ;-)  See my other post for a
> > Digital Signal Processing definition of "dither".
>
> I don't know what to make of this.

Simply "make" that I know exactly how these converters work, having
researched and designed them using discrete components prior to any
monolithic implementations, and that some of the monolithic implementations
are based on my work and designs.

> You claim to be one of the original
> developers of delta-sigma conversion, yet you then say it has
> nothing to do
> with dithering?

Not with a DSP definition of dithering.  With a misapplied definition of it,
perhaps...but the correct term is aliasing.  Dithering, in a signal
processing definition, is ADDING random noise (there are other uses of the
word dithering, as used in astronomy, but that is different that we are
discussing here).  There is always noise inherent in sampling that is
+-1/2LSB, which is the result of what is called aliasing.

Regards,

Austin

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