[opendtv] Re: New Chips Improve Color TV Dramatically

  • From: jeroen.stessen@xxxxxxxxxxx
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Thu, 1 Jul 2004 10:58:27 +0200


Doug McDonald has said some very interesting things:
> All I know is that when one views the SMPTE bars
> from Digital Video Essentials, with the color matrix
> set the way I have it set, one gets pure Red, Green,
> and Blue ... no mixtures. There is also no mixture
> with a computer input to the RBG input.


> AH! but, as Jeroen so ellipically put, with primaries
> that are "purer" than the official HDTV standard,
> they (the bars) perhaps actually should. Since
> the green on my set is greener than the standard, and
> more saturated, a substantial amount of red and
> a bit of blue should be added to make it the
> official color. This is not done. (My set is not CRT,
> but the same logic applies.)

Whoa ! Do you know what this means ? It means that
the "Joe Kane method" for calibrating the saturation
can not be applied to wide-gamut displays like Doug's !

Let me explain.

With "Digital Video Essentials" you get 3 pieces of
colour filter: red, green and blue. If you follow the
calibration instructions, then when you are done you
should see from a colour bar picture: only the red,
magenta, yellow and white bars through the red filter,
the green, cyan, yellow and white bars through the
green filter, and the blue, cyan, magenta and white
bars through the blue filter. This means, I believe,
that you have adjusted your display for _cutoff_ of
each channel for the 3 colours that should not be
seen. There will be no red in the black, blue, green
and cyan bars, no green in the black, blue, red and
magenta bars, and no blue in the black, red, green
and yellow bars. And for a traditional display that
has native standard (EBU, RP.145, Rec.709) primaries,
this will be correct. So far so good.

But what about a wide-gamut display ?!
As Doug has correctly worded, if some of the colours
are better than standard then in order to display the
correct standard colours it is necessary to mix in a
fraction of the other colours. (An over-saturated
display is calibrated by a reduction of saturation.)
So, in his case there should be a bit of red and/or
blue mixed into his super green to give standard
green. The red and/or blue should not be cut off.
You can never do a correct colour calibration for a
wide-gamut display based on the cutoff method. You
can only do it with a professional colour analyzer !

If we may assume that Doug has calibrated his display
with the (D)VE disc and the filter(s), then he has
actually calibrated it wrong. He will be rewarded
with a slightly over-saturated picture. This may be
pleasing for him, but it is not "correct" !

And what about multi-primary displays ? How are you
going to "calibrate" a 5-primary display (e.g. red,
yellow, green, cyan, blue), or a 4-primary display
(e.g. red, green, blue, white) with a colour bar with
only 6 colours, and a maximum of 3 (RGB) filters ?
Answer: you can't. Not without a real colour analyzer.

Joe, be warned: you're going to mess up the high-end
display market if you advise your customers to do
their own calibration of wide-gamut displays !

Disclaimer: I may have a wrong interpretation of the
method of calibrating the saturation with the SMPTE
colour bar picture. This is where you view a blue
bar next to a white bar, through a blue filter.
You should adjust the 2 bars for equal brightness.
At over-saturation, the red and green will clip to
zero and the blue output will be too bright.
At under-saturation, there will be red and green
mixed in and the blue output will be too dark.
But you can not see the contribution from red and
green at all, only the brightness of the blue.
I think that it will go wrong then. Am I right ?

-- Jeroen.
| From:     Jeroen H. Stessen | E-mail:   Jeroen.Stessen@xxxxxxxxxxx  |
| Building: SFJ-5.22 Eindhoven| Philips Digital Systems Laboratories  |
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