[Tig] Re: variable density optical

Dominic Case cased at atlab.com.au
Mon Oct 23 01:56:32 BST 2006


Oh dear - I always go cross-eyed trying to explain cross-modulation.

In a nutshell, image spread is mainly because no film emulsion is perfect,
and the photochemical effects of exposing and developing a grain of silver
halide tend to spread to its neighbours. In electronic imaging you can apply
aperture correction to counteract a similar effect (which is caused quite
differently) - that's even done in film processing by some clever chemistry
in the emulsion, but it's not a perfect cure.

The effect of image spread on very high frequency wave forms is that the
"valleys" or "troughs" in the waves tend to get filled in a bit. As a high
frequency tone changes in amplitude, the average width of the line changes
as a result of this lopsided fill-in: this results in a spurious low
frequency signal modulating at the rate at which the amplitude of the high
frequncy tone varies.

The most obvious effect is of increased sibilance or "spitting" in "s"
sounds.

If a print is made of the negative, image spread strikes again - but as it's
a reversed image, you now get the peaks feathered, and this reverses the
change in average width of the line, cancelling out the effect.  Though
there is high-frequency loss as a result of both stages.

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-----Original Message-----
From: woz [mailto:warrenl at gmail.com]
Sent: Monday, 23 October 2006 11:45 AM
To: Dominic Case; tig at tig.colorist.org
Subject: Re: [Tig] Re: variable density optical


Sorry if this is blindingly obvious to all but in the spirit of back to 
basics stupid questions....

Why do you get 'image spread' (assuming lamp focus is set correctly?) and 
why does it cause cross mod distortion, and with what is it cross 
modulating?
Thanks
woz

----- Original Message ----- 
From: "Dominic Case" <cased at atlab.com.au>
To: "TIG" <tig at tig.colorist.org>
Sent: Monday, October 23, 2006 12:20 AM
Subject: RE: [Tig] Re: variable density optical


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>>If the film was negative, they'd just flip the polarity
>>and work with it that way.
>
> OK, that works for the clipping idea to mask out dirt (and film grain, 
> which
> is the biggest contributor to ground noise in an optical soundtrack).
>
> But it's not a total solution. Image spread in any optical soundtrack 
> leads
> to cross-mod distortion. When you are reading a print, the two generations
> of image spread (neg and print) tend to cancel each other out, minimising
> the distortion. That obviously won't happen if you are running the 
> negative.
>
> Though I guess you'd usually be reading the negative soundtrack because 
> you
> don't have a print - so there's not much that can be done about it. But
> remember "just flipping the polarity" doesn't solve everything.
> _________________________________
> Dominic Case
> Atlab Australia
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