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Old 1st May 2020, 05:18   #17  |  Link
Join Date: Jan 2006
Location: Portland, OR
Posts: 3,646
Originally Posted by excellentswordfight View Post
I dont really follow how this can be an issue. Isnt the decoder validated for levels and profiles? If a decoder is specified to handle main10 main teir level 4, how could it not handle MaxDpbSize and vbv-limits within the spec? As long as the buffer is not higher then the maximum maxrate of that level/tier and that it can feed data in that rate shoudlnt it be all good? Or is this specifically an issue for some DRM solutions?
The thing is that the first NAL unit is allowed to be bigger than the following NAL units, per spec. Most decoders do this right, but there are a couple common SoCs found in living room decoders that require ALL NAL units to be no bigger than the after-the-first size. Of course, almost always the first NAL unit is still under that side. But every now and then one doesn't. That was at least six months to root cause and get a fix for rolled out...

Or some older devices that do progressive download, but only support a legacy 32-bit data fields in .mp4 files. Today everything uses the 64-bit extention, and we don't really think about it. But at even a bitrate of 2.5 Mbps, after about four hours the 32-bit offsets run out of room and some older players fail. The secret? Turn off B-frames for long titles! That raises the limit to about 12 hours. Why? I can't even remember anymore.

I wish we could say all decoders are fully spec compliant, but there are all kinds of little things that don't work right. Or even if it is compliant, when doing DRM there are finicky ways things need to be structured to get adequate performance.

Mind expanding on this? Why would it use the peakrate as decision for selection this way? Shouldn't the decision be made on buffer status and chunk sizes? Would that really be an issue with ABR with MPEG-DASH and HLS?
At scale, a one-in-a-million problem where a fragments causes the player to crash will happen 0.1% of the time each hour of playback. That's many thousands of customers suffering from player crashes a day.

Someday I'll tell the story about how 1% of all bandwidth in North America was spent trying to keep my ex-wife happy...
Ben Waggoner
Principal Video Specialist, Amazon Prime Video

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