Wednesday, June 12, 2013

After 'Beyond HD Masters 2013' - some notes, 4k TV colourimetry

I had a day at BAFTA listening to various speakers from the industry talking about 4K (quad-HD, UltraHD, etc etc) and the surrounding standards.
ITU Rec.2020 is the document that covers 4k TV (in fact it defines two resolutions - 3840 × 2160 and 7680 × 4320 - which I'll refer to as 4k and 8k television, but these aren't the same as the 4096 pixels and 8192 wide resolutions used in digital film).

  • The colour space is monstrous! The 2020 triangle is even bigger than the P3 colour space (as defined by the DCI) - take that Mr. Dolby! It'll be a while before ANY displace device can faithfully reproduce that gamut. Thankfully we stay with D65 for white (well, 6504k to be strictly correct - Planck's was re-calculated in the 70s) and the primaries are;
  • red: 0.708, 0.292
  • green: 0.170, 0.797
  • blue: 0.131, 0.046
  • white: 0.3127, 0.3290
  • The new luma transfer function is: Y'= 0.2627 R = 0.6780 G + 0.0593 B and for the first time ever in television an allowance for constant luminance has been allowed. There is an almost philosophical argument by Charles Poynton and others that constant luminance is the way to go. Essentially the gamma response should be applied only to the derived luminance rather than the three colour components. I suppose your feeling on that comes down on whether you think gamma is correcting for the camera response (that's what I was always taught at the Beeb in analogue SD days) OR if gamma is a tool to give better dynamic range in the dark areas of the picture. I expect that constant luminance (proper Y as opposed to Y' / "luma") should best be constant in the case of 12-bit video (where you have so much more dynamic range anyway) but remain pre-corrected RGB in the case of 10 and 8 bit 4k.
  • Frame rates are defined up to 120FPS with no interlaced framerates - unfortunately non-integer (23.98, 29.97, 59.94) are still hanging around like bad smell! The Beeb's Richard Salmon showed a very convincing argument for >100FPS for sports footage. Essentially as you have more resolution the difference between detail in static pictures and moving scenes become objectionable. The problem is that currently HDMI 1.4 only supports a maximum of 30 FPS at 4k and so we're waiting for HDMI 2.0.

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