AVS Special Member
Join Date: May 2008
Location: San Francisco - East Bay area
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Blu-ray has 8-bit color because when the format was released, there were only single-layer 25GB discs. When you reserve some space for special features, reserve some space for a DTS HD MA or TrueHD soundtrack (and maybe a few other languages in DD), there's only enough space left on a single-layer Blu-ray disc for a 2.5 hour movie if the video is in decimated (i.e. 4:2:0) 8-bit video. The committee probably knew that adding a second layer was inevitable, but at the time the standard was set, there were only single-layer discs. For a 4K disc format, there's really nowhere to go to make the pit size smaller on the discs... the blue laser has the smallest diameter laser unless something like an ultraviolet laser is developed. So optical disc layers are going to be stuck pretty close to 25 GB for a while. With 4-times more pixels for UHD (home video is not 4K and it appears it will never be 4K, so it shouldn't be called 4K, 4K is a digital cinema format though), you'd need either 100 GB for a single sided disc (with no extras, probably) or you'd need 4 layers for the movie and additional layers for other features. Disc manufacturing now allows more than 4 layers so a new disc format would have to be adopted in order for us to have a movie format with more than 8-bit color. Call it Ultra-Blu-Ray (to go with Ultra HD) for the time being. Not only could it support more bits per color, but it could also make use of a much larger color space than the Rec. 709 space used for HDTV and Blu-ray.
More bits per color will FINALLY allow us to have video without contouring. Well... hypothetically anyway. 8-bit color using 16-235 has 219 levels. Human vision has been measured as being able to see 200 shades of gray plus or minus a few depending on which study you look at. So hypothetically, 8-bits should be enough to avoid contouring. But that doesn't seem translate to real-world HD video, at least not very often. It may be possible to master a disc in 8-bit space without visible contouring, but each component that touches the video from the disc player to AVR/processor, to video display likely does SOMETHING to the video even in bypass mode and if that isn't controlled perfectly (and typically video processing is done in 10 bit space with some exceptions at 12 bits and Samsung at 18-bits internally... but those processing steps have to be down-converted back to 8-bits and that's another point where it has to be done perfectly or you end up with contouring again. So 10-bit color SHOULD be enough to avoid visible contouring entirely, but with all the vagarities of video processing, I think it's possible that people could still screw-up 10-bit video so it's likely that it will take 12-bit video to really make all contouring issues a thing of the past. But the leap from 8-bits to 12-bits takes a LOT more storage space on the disc also... so now you want UHD on disc PLUS you want it in 12-bits so now you're up against the limits of what can be manufactured with current technology. I think I read something that indicated 8 layers was about the limit foreseen for economical production of discs and players. That just might be enough to accommodate HD at 12-bits but may not be much room for extras (additional languages, special features, etc.).
So we're on the threshold of moving on to a completely new video standard... HDMI 2.0 is just the first piece of the puzzle. We also need a new standard for the UHD format, presumably with more bits and a larger color space. Then we need a disc format that will support the new standard, and probably be backward compatible. After we get the new disc format, we then need new disc players and discs that support the new format. There are many steps to getting to the next video standard and things are just getting started that will let us get to this new standard.
Someone mentioned that 3D was already a 4K format... no it is not. 3D is achieved by sending a "regular" HD frame plus about 20%-25% more data that describes the differences seen by the "other" eye so that second frame can be "built" with very little additional data. 3D is an HDTV format with 1920x1080 resolution. But we are seeing 48 frames per second from a 24 frame per second Blu-ray disc. The right eye image is displayed for 1/48 of a second, and the left eye image is displayed for 1/48 of a second so you get the same 24 fps/Hz you get from a 2D Blu-ray movie. UHD 3D will require some sort of specification for 3D as well as 2D... that could be something similar to the HDTV frame-packed 3D format or it could be achieved some different way.
"Movies is magic..." Van Dyke Parks
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