[opendtv] Re: 1080P Question

  • From: "Stessen, Jeroen" <jeroen.stessen@xxxxxxxxxxx>
  • To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
  • Date: Thu, 10 Sep 2009 09:09:06 +0200


Mike Tsinberg wrote:

Ø  No, common mistake created by marketing.

And you thought that you should add to the confusion ?  ;-)

Ø  Both current HDTV (rec 709) and xvYCC  are 8 bit systems.

Ø  The color gamut for HDTV, of each Red, Green and Blue colors is designed 
within BT.709 standard for 16 to 236 level systems or 220 quantization levels. 
16 to 236 is a legacy issue for component YCrCb.

It's 16..235 for Y' and 16..240 for Cb,Cr. And it's not a legacy issue, it is 
an inherent necessity
for proper sampling and reconstruction. These are the reference levels for 
nominal black and
white, i.e. for large areas. The other levels are used for the overshoots and 
undershoots that
occur during proper anti-aliasing filtering. A filter without ringing would 
have too little bandwidth.
You can't have proper (smooth) motion portrayal without anti-aliasing. If you 
cut off the over-
and undershoots in order to limit the range, then that clipping is a non-linear 
operation and it will
cause aliasing. There will be jittering and flickering on all sharp moving 

Of course somewhere in the video path we still have to get rid of these extra 
levels, particularly
the below-black levels. Traditionally this was done --after reconstruction-- in 
the analog time-
continuous domain, where clipping will not cause aliasing anymore. Clipping off 
the overshoot
was not necessary, as CRTs could handle the peak white. With digital pixelated 
displays this
becomes a different story. Then the only solution seems to be that the spectrum 
at the display
be not too full, i.e. an oversampled display or an underutilized transmission 
Neither seem to be attractive. It helps that full-HD is already a bit better 
than we really need,
so indeed we see much smoother motion with HDTV. I find this very pleasing.

Ø    A fully digital interface, such as HDMI, is capable of a full 0 to 255 
performance without increasing the 8 bits per color specification.

This is a common mistake created by the computer community. As in the beginning 
they were
dealing only with static images, or perhaps images that move only in discrete 
speeds of multiple
pixels per frame, aliasing was not yet an issue. Also, since most of the chain 
was still analog
(CRT !) it was common to calibrate the black and white levels in the analog 
domain (potmeters
in the CRT monitor). With an all-digital chain you need some headroom and 
footroom for
calibration. It's not wise to use the full (8-bits) range for your signal over 
the entire chain.

Ø  The xvYCC is a new color gamut standard that uses the full range of values 

Ø  0 to 255 or 256 quantization levels in an 8-bit space to represent colors. I 
included a

Ø  picture of color space comparison.

Technically the levels Y' = 0 and Y' = 255 are probably still illegal, as they 
are reserved for
synchronisation. Besides that, the illustration is plain wrong. Even the 
co-ordinates for the
Rec.709 primaries are very very wrong. And the color space of xvYCC can not 
even be
described as a simple triangle in (x,y) space. It depends on the luminance. If 
you combine a
low value for Y' with large absolute values for Cb,Cr then the color space is 
even infinite.
Or at least it goes way beyond the visible colors. Katoh and I have illustrated 
it nicely during
our presentations at the HPA Tech Retreat 2007. I suggest that you look it up.

Essentially the extension of the xvYCC color space relative to Rec.709 has two 

-          formally illegal combinations of legal Y'CbCr values that will lead 
to R'G'B' values
that are < 0 or > 1, which I like to call wide-gamut c.q. bright-gamut colors, 

-          all combinations of formerly illegal Y'CbCr values, that will lead 
to even more
extreme R'G'B' values.

It's only the latter colors, typically very bright colors, that eat into the 
space that should have
been reserved for ringing (anti-aliasing). Typically they represent fluorescent 
objects or
light sources. Yes, this does allow a bit more "sparkle" in the picture, but 
they are not so
relevant to the wide-gamut issue. They can be avoided by proper scaling of the 
signal range.

Ø  Deep color increases bit depth from 24 bits per pixel (8 bit per color) to 
36 bits
(12 bit per color). Either 709 or xvYCC can be used with deep color. Perhaps
"deep color" is misleading. It should be called probably "accurate or deep gray"
or something.

Correct about the meaning of deep color. It is my opinion that a video chain 
allows only
one 8-bit bottleneck, and then only if the original signal comes with enough 
(analog ?!)
noise to dither away the quantisation error of the 8-bit quantizer. If there 
are multiple
processing steps with multiple 8-bit bottleneck between them, then the 
quantisation noise
will soon be unacceptable. Therefore, every interface that can be upgraded to 
more than
8-bits is very welcome. Internal interfaces (i.e. on a chip or on a PCB) are 
easy. The
external HDMI interface was a logical candidate for an upgrade of the standerd.

It used to be that contrast control was in the analog domain (again: CRT), so 
the video
path before that had a constant amplitude and 8-bits would do. Now that 
everything has
been moved to the digital domain, we need more precision at the display input. 
picture is changing again with the introduction of backlight dimming, where 
extra bits
can be carried over the backlight intensity signal. And it will be changing 
again with the
introduction of High Dynamic Range signals and displays, where an entire 
intensity map
(of low resolution) will be carried in parallel with the video path. But most 
likely these
signals will not be separated over the HDMI interface, so it is there that a 
high precision
will remain essential for a combined HDR signal.

By the way... HDMI has always been capable of 12-bits precision. That's because 
re-uses the hardware of the DVI interface, which can do 8-bits R'G'B'. HDMI can 
(also) Y'CbCr 4:2:2, and with 12-bits components this also adds up to an 
average of 24
bits per pixel. It costs nothing ! The "deep color" extension means that this 
can also be
enabled for 4:4:4 components, but at the cost of more bits per second over the 

-- Jeroen

  Jeroen H. Stessen
  Specialist Picture Quality

  Philips Consumer Lifestyle
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