What did Ultra HD forum brought ?
Well, there is a fundamental difference between Ultra HD Forum’s specifications and UHD Alliance’s certification: whereas UHD Alliance had a very marked goal of selling stuff, namely brand new TV sets and UHD/HDR Blurays, Ultra HD Forum aims to specify the whole content chain, from production to consumption, for both live and on demand content, including all the intermediate stages: live capture, post processing, compression, distribution and consumption.
How does both standards stack against each other ?
| UHD Alliance | Ultra HD Forum | |
|---|---|---|
| Resolution | 3840×2160 square pixels | |
| Transfer Function | SMPTE ST 2084 | SMPTE ST 2084 BT.2100 |
| Colour Space | BT.2100 | |
| Maximum Production Brightness | N/A | 1000 nits |
| Metadata transfer method | N/A | H.265 VUI |
The relevant differences are marked in bold, respectively the transfer function and the maximum production brightness. But why are those parameters relevant ?
Starting with the one which creates less problems to users but not so to UHD Alliance: the Maximum Production Brightness. This parameter addresses not TV set manufacturers, but content producers and limits the maximum intended brightness for the content, thus creating a standardised limited on the maximum brightness the TV set shall display. This has two fundamental consequences:
Content producers understand that for optional viewing experience the TV shall not be placed on a very bright environment. Actually, Ultra HD Forum have recommendations on the minimum and maximum environmental brightness for optimal viewing experience. This represents a direct attack to Ultra HD Forum and specifically at Samsung. As we discussed on our original article, Ultra HD Forum allowed for TVs to be branded as HDR if the darkest black is darker than 0.05 nits and support a minimum brightness of 1000 nits. As UHD Alliance places a hard limit on the maximum brightness, it also limits the ability of LCD manufacturers to increase the contrast ratio just by increasing the power of the backlight, as Samsung has been doing for years. Instead, it requires manufacturers to start focusing on having darker darks shades, something which is structurally extremely difficult for LCDs, but trivial to other technologies such as OLED (and it’s most important supporter: LG).
Not compatible with non HDR TVs
The most import difference between both standards is not related to LCDs vs OLED, but on the color representation method. Ultra HD Forum recommends the usage of SMPTE ST 2084 on which Dolby lobbied heavily, and is mostly targeted at big theatre cinemas and BluRay. SMPTE ST 2084 has two main design characteristics:
- up to 10.000 nits maximum brightness
- Not compatible with non HDR TVs
On theaters, not being backwards compatible with non HDR setups is a complete non issue (as you would need updated projectors anyways), but on Ultra HD Bluray discs care needs to be taken to make sure that UHD Bluray discs work on both HDR and non HDR capable 4K TV sets. This has only two solutions: either burn two copies of the movie into the dish, the HDR version and the non HDR version, or use Dolby Vision. The former significantly increase the capacity requirement for a movie, which may not be that significant on a 100GB disc, and the latter is simply not compatible with the current Bluray Players currently on the market.