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UHD-BD bitrates
#1
Assuming the following:
  • UHD-BD uses HEVC for UHD while BD uses AVC (generally) for HD
  • UHD 10bit needs 1.25x bitrate than 8bit
  • HEVC needs down to 0.5x bitrate to get the same AVC (subjective) quality
  • I guess generally HEVC requires around 0.65x bitrate of AVC
  • so, for each 1mbps 8bit AVC = from 1x0.5x1.25 = 0.625mpbs to 1x0.65x1.25 = 0.8125mpbs HEVC 10bit
  • UHD = 4x HD = 4x0.625=2,5x up to 4x0.8125=3.25x
So, if we have a supa-dupa BD with a very (and I mean it!) high bitrate, of, let's say, 36mbps, the UHD-BD, with a superb HEVC encoder should have 36x2.5=90mbps at least; now, we know that HEVC is not always twice efficient, so the bitrate should be even higher, up to 36x3.25=117mbps;

Now, according to this post (https://forum.blu-ray.com/showthread.php?t=277330) there were some UHD-BD with bitrates as low as 37mbps - and as high as 88mbps. So, while the 88mbps figure is in line with the best BD discs around (thinking that the HEVC encoded used has a 2x efficiency), the 37mbps is rather low - it must be compared to 37/2.5= 14.8mbps AVC BD at best, and 37/3.25=11.4mbps at worst... not so high bitrates, if we talks about 1.78:1 films with grain.

According to the Kush Gauge (and the Kush Gauge advanced calculator I made), to get "optimal" quality for a 1.78:1 grainy film with a lot of action, AVC will require 20.9mbps. Now, not every film is very "active", so for some movies the bitrate could be reduced even in half - around 10.4mbps - and in that case 37mbps for HEVC 10bit 2160p is enough.

Let's say that one of us wants to release his project on UHD-BD format; a BD-25 should be not enough for a feature film, while a BD-50 should do. Now, TSM2UHD has an advice to use 50mbps for BD-50, and never more than 64mbps ( even if to be compliant we can use up to 82mbps), a bitrate that is 1.35x the lowest bitrate found on commercial UHD-BD (and up to 1.72x if 64mbps will be used); a quite healthy figure - if we hope the HEVC encoder used would be as efficient as the ones used by studio (but I bet it will be not...).

A BD-50 has 46.6GiB capacity; takin in account 7% (more or less) of .m2ts and UHD-BD structure overhead, will leave around 43.66GiB for the movie; it translate to 43660MiBx8/50mbps=6985s=116m (or 43660MiBx8/64=5457s=90m). So, we can fit most of the movies at acceptable bitrates.

Do remember that 50mbps and 64mbps include audio and subs, so the final bitrate for video will be less, depending on how much audio tracks we want to add, and their quality (subs size would be negligible in comparison); I think that 46/48mbps and 60/62mbps could be easily achieved.

Conclusion: if we think the Kush Gauge is more or less right, to get optimal quality for a 2.40:1 HEVC 10bit 2160p movie with a lot of motion, we need 49.7mbps - near the lowest max bitrate limit for a BD-50. So, if we have a 1h30m action movie with one DTS-HD MA and several AC-3 tracks, we could encode it in HEVC 10bit 2160p and it will safely fit on a BD-50 using a 64mbps bitrate (around 60mbps for video), while for action movies up to 1h56m, we should lower the bitrate to 50mbps (around 44mbps for video); movies with low motion could benefit of lower compression, or longer duration.
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