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Re: I think I got it now



Mikael Reichel (10 Jul 95):
>You must all be truly in love with Ranks, how else could you abuse its origin
>with such tenderness?
>
No further comment required. . .

>2. DV's colour corrector both has high/lowlight compression according to what
>you mention as chroma light & dark. In addition our primary has an adjustable
>high and lowlight knee point allowing you to gently roll off the top (or
>bottom). In addition it allows you to do a soft - linear transition fully or
>partially desaturating. Soft and feathery clouds is like walk in the park. A
>fully digital walk of course.
>
>Clipping in digital domain is baaad.
>
>3. Sure I would like to have 12 bits too!
>
The analog white compression circuit that Rob used in a previous life would
take a signal off the photocell and softly compress the top half to two
thirds of the signal into a final 5 to 10%. The desired scene contrast
elements could start out normal amplitude (and retain reasonable analog
signal to noise) while the high contrast elements were compressed by 2 or 3
to 1 and the result was never quantized. Yes, the circuit was very similar
to the knee circuit in analog cameras and was the model for the
modification. This circuit also operated before the film gamma correction
of the video channel. To duplicate this in the digital world, after
quantization, might require two or three more bits (4 to 8 times the
dynamic range.) The current state of the art (OK, the way it is done in the
URSA) gives an arguable 14 bits. Let me know when there are 18 to 20 bit
A/Ds that work at 27Msps and Rob's white compression may be possible in the
front end of the telecine.

Not that I make any claims of perfection for the circuit that Rob is
familiar with. It was mounted on an old MKIIIc, only was useful in
negative, may have added some noise of its own and I haven't decided if
compression before the afterglow correction was "proper". But hey, it was
suitable for many of Rob's requirements and apparently he can't find that
capability in his current tool box.

--- David Tosh  dlt at earthlink.net  (CIS 72167,1376)
--- Video Engineer, Complete Post, Hollywood