[vlc-devel] Another stutter issue

Juha Jeronen juha.jeronen at jyu.fi
Sat Mar 5 18:11:59 CET 2011

Hi all,

There is something similar, but unrelated to the stutter issue that was
just fixed by Laurent.

At least when reading from a DVD image, the playback sometimes stutters
in a regular pattern about twice a second. Usually pausing and resuming
playback, or seeking back in the stream (shift+arrow), fixes it
temporarily. The extracted test clips, again, do not exhibit this (or at
least not so often).

It's easy to see the issue using a framerate doubler. Especially
Phosphor, but Bob will do, too, if one keeps an eye out for sudden
changes in the perceived vertical resolution. (Phosphor is easier,
because the scanline visual effect immediately breaks down very visibly
if the output framerate is not stable.)

Possible explanations I can think of:

a) Maybe the processing chain is not aligned to vsync? Framerate doubler
= 60Hz, LCD display, also 60Hz. There should be exactly one output frame
rendered at each vsync, and there is no room for any misses. This may
set limits on acceptable times when the decoding of the next frame
should begin (so that all output frames can be rendered in time for the
next vsync).

b) Happens more often for DVD images than files => maybe related to the
subtitle renderer, like the other stutter issue was?

At least it is not a PTS issue, because it doesn't always occur at the
same position in the stream.

Any ideas? For this one, I don't have a known-good version, as the issue
has always been there in the versions which I've used for development.


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