[opendtv] Re: RGB mania
- From: jeroen.stessen@xxxxxxxxxxx
- To: opendtv@xxxxxxxxxxxxx
- Date: Fri, 14 Jan 2005 12:51:09 +0100
Hello,
John Shutt wrote:
> Unless someone can show me otherwise, I don't think that negative values
of
> R, G, or B ever occur in this application of MPEG-2 delivery of video to
the
> home, and a SCART or VGA connection is wholly adequate for transferring
> decoded video material from an STB to a consumer display.
I can think of 3 _potential_ disadvantages of an RGB link:
1.
The color gamut is precisely defined by the signals being unable
to go negative. In principle a YUV (YPbPr, YCbCr, whatever) link
could make a larger gamut by making combinations that would lead
to "illegal" RGB values. This applies even more if the Y signal
is attenuated, so that the color signals are relatively stronger.
But I think that this is an invalid argument, given that most
applications are limited to R.709 color gamut anyway.
2.
Joe Kane mentioned this one in 1997: with analog RGB there is the
risk of offset or linearity errors that would affect the greyscale
tracking. With YUV you know at least that the greyscale is
carried by only the Y signal. But an offset error (clamp error)
on the U&V signals is at least as destructive to the greyscale
tracking, so this is not a very valid argument for YUV either.
3.
With digital RGB "graphics" signals (e.g. DVI out) it is common
to have black = 0, white = 255, but with digital YUV "video"
signals (e.g. ITU-R.656 or SDI) it is common to have black = 16,
white = 235. The Y signal has headroom that the RGB signals have
not. This headroom is reserved for the ringing due to anti-
aliasing filtering, and thus plays a role in the perfect recon-
struction of the original analog video signal. However, if the
target is a display with digital RGB inputs, and the signal must
be limited between black and white anyway, then it does not seem
to matter whether this limiting is applied before an RGB
interface or after a YUV interface. Only if such display contains
an up-scaler then it would be beneficial (for aliasing) to apply
the limiting after the scaler, and opt for the YUV interface.
Obviously, the 0..15 range on the Y signal is the equivalent
of a small negative range on the RGB signals. Also obviously,
sooner or later this part must be clipped off, we can not make
negative light. But it is better not to apply clipping while the
sample rate (resolution) is still low, as clipping causes aliasing.
Do it as late as possible, after any up-scaling, then it will cause
less aliasing. Maybe that is what Tom was hinting at.
Then again, the rejection of an RGB interface only applies if
it has no headroom and it is sampled at a low resolution, and
then it is implicitly digital. This condition does not apply
to any analog (Scart-like) interface... So John is right.
Beware: with analog VGA the limitation is not in the interface,
but in the implied 0..255 RGB levels to the DACs that feed it...
If the VGA carries a signal at the native resolution of the
display, and if the signal goes directly to the display without
any post-processing, then I see no point in postponing the
clipping (to black) to beyond the VGA interface. (Note the ifs.)
Tom, am I guessing you right ?
Greetings,
-- Jeroen
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