[mythtv-users] Why does playback only use 1 core on some BluRay movies?

Ronald Frazier ron at ronfrazier.net
Sun Nov 22 22:05:29 UTC 2009

> H264 can only utilize multithreaded decoding if it was encoded the
> right way. I don't know all of the details. But from what I understand
> H264 can be decoded with one thread per "slice". If it was encoded
> with a single slice then it can only use a single core. I am not sure
> how check for the number of slices to confirm if this is the case for
> you.

Interesting. I didn't know that. Using that as a starting point for
futher research, it appears that the slice thing isn't a limitation of
h264,  but merely of the current implementation of ffmpeg. Other
codecs (such as coreavc on windows) do not have this limitation.
Apparently the multithread branch of ffmpeg fixes this too, but it's
still got a lot of bugs at the moment (and because myth uses an
embedded copy of ffmpeg, even once those changes are complete we won't
see them until they get rolled into myth).

I haven't yet figured out how to detect stuff like from the ripped
file, but I'm assuming that's what goes on. Interestingly, by skimming
through the movie, I've discovered that as soon as it reaches chapter
4, it switches to dual core decoding and stays that way for the
remainder of the movie. Odd.

Anyway, thanks for the info. This helps me decide that most likely the
only real solution is to get a VDPAU capable card (or a faster CPU,
but that's probably not a better investment). For now, that's not an
option so I'll have to stick to transcoding it to less demanding file
size/resolution/whatever. Thanks

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