Vegas 12 preview screen

[Adam Stanislav]

Not to be nitpicking, but what’s a nit? I’m assuming you are not talking about head lice...

[Marco Ba]

Nit is a unit of luminance and it equals cd/m2.

[Christopher Young]

Phil ~

This is my take on this ever circulating issue on levels.

Deep color and xvYCC color space are designed to show the extended range of 0-255 over HDMI. We have receivers, TV sets and some projectors that can display this range. This is under the proviso that ALL connected components in the display chain from source to final display support these protocols. Not that I have delved too deeply into the matter but the only pieces of output source equipment I am aware of that supports any of these protocols, and that’s xvYCC, are the PlayStation and some of the newer camcorders on the market. Currently as it stands all video discs, including BD, DVD and Video CDs, are encoded as YCbCr. YCbCr is purely a 16-235 color space spec.

To the best of my knowledge there is no move to add xvYCC expanded color capability to the BD-ROM specification. The BD standard engineered around the broadcast 16-235 8-bit 4:2:0 ‘Standard' range output over HDMI. To go to an ‘Extended’ 0-255 8-bit range would mean a whole new BD standard. What would be more likely and the preferred path I would surmise would be to ratify a whole new 10-bit 4:2:2 range 0-1023 spec that would be backward compatible with the existing 8-bit 0-255 spec. We can only hope.

The upshot is regardless of what levels you cram onto a BD it will be outputting 16-235 levels on a BD player. If you output 0-255 to your display over HDMI it will clip your black levels to a dark grey which is the result of losing those bottom 16 steps of your full range 0-255 video that fall outside the 'studio' gamut of 16-235. To see a BD displayed to its best it should be connected to an HDMI display that is expecting a 16-235 level range coming in.

You can shoot and edit in full range 0-255 if you wish and if it’s going purely to the web that’s exactly what you would probably do. A big but here though! If the same material is then going to be broadcast it needs to be ‘scaled’ correctly, not just clipped top and bottom. Just scaled to 16-235, all the information is there, you don’t lose any, it’s just that it’s scaled to fit the ‘studio’ gamut. Subsequently it will look way better than any truncated 0-255 material displayed incorrectly.

I notice the new Pana GH4 has three working levels, 16-235, 16-255 and full range 0-255. A flavour for all desires and requirements... nice.

Chris Young
CYV Productions
Sydney

[Adam Stanislav]

Originally Posted by Marco Ba

Nit is a unit of luminance and it equals cd/m2.

Thanks, Marco.

[Marco Ba]

"Currently as it stands all video discs, including BD, DVD and Video CDs, are encoded as YCbCr. YCbCr is purely a 16-235 color space spec."

That again implies the mixing of reference levels and peak levels. Any of these devices including DVI, HDMI, LCD displays must be able to use the range of 1 - 254 for video data including headroom (and footroom).
It's the reference values which are set to 16 and 235 but this says nothing yet about the peak levels available and allowed. Even in digital broadcasting the wall of limiting white peak levels to 235 is obsolete meanwhile (there is a nice reading at Poynton's blog about this certain topic).

What's special about xvYCC is another thing because there it does widen the gamut which is not the case by just using the range of 16 - 255 within BT. 709. The latter does not alter the gamut in any way because the BT. 709 color space's gamut refers to the reference values, not the (allowed) peak values.

[Juris Lielpeteris]

Read carefully ITU-R BT.709:

6.10 Quantizarion Levels 8 bit
Black level R, G, B, Y 16
Nominal peak R, G, B, Y 235

6.11 Quantization level assignment 8 bit
Video data 1 through 254

It means that all video data including apperture correction peaks in encoders must fit in interval 1 through 254, they do not cause disorders of transmission path,
but useful interval including nominal video peaks must fit in 16-235, they must be visible on TV.

[Marco Ba]

Correct.

Unfortunately specs like this suffer from unsufficient descriptions. "Nominal peak" describes how the analog source peak is digitally represented and quantized. And analog peak is 1,0 V and in analog world it equals reference white without headroom.
So the analog source peak level – which is 1,0 V – will be represented digitally at value 235 in R', G', B', Y' (another flaw of this spec: it does not distinguish between RGB and R'G'B') but the digital signal offers further headroom.

Now the main infos (besides the gamut information) are:
The values specified for reference black and white (16 and 235), the fact the range of 1 - 254 is valid for video information and the fact BT. 709 says nothing about the way a display behaves (!).

It is critical to see how display specs connect to this and there are several specs out there which describes exactly this. Actually the display site is even more important because once a video signal is digitized everything which happens in systems like NLEs is based on display-refered video processing (exceptions are certain RAW workflows and compositings which usually are scene-refered)!

I recommend the reading of specs like ITU-R BT. 1886 (which is more or less the BT. 709-twin meant for transfer functions of flat panel displays), EBU – TECH 3320 (which – amongst others – describe the luminance ranges and gamma of displays) and EBU – TECH 3321 (which is a guideline for consumer flat panel displays).
Thus you'll see whereas reference white is meant to be displayed with 100 - 120 nits (tech 3320), peaks should be adjustable to least 200 nits without excessive flare (tech 3321). This is kind of the display representation of the headroom offered by BT. 709.

If you stand even more reading see the specs for the main digital interfaces.
ITU-R BT.1120 (for HD SDI) says, the quantization level assignment for videodata is 1 - 254 (for 8 bit). And the HDMI specs 13a (which is identical to BT. 709 in the matter of black and white reference and nominal peak) also says valid range is 1 - 254 (and it's easy to proof this).

So if you pass a digital hd video which uses headroom data via a digital interface to a flat panel display you could be rather sure the display will be capable to display even the headroom data. But it is up to you to ensure this headroom data is used with sense (because it should not contain color information – the given gamut is already exploited).