[mythtv] DVD playback issues, patch

John P Poet jppoet at gmail.com
Wed Nov 25 04:35:49 UTC 2009

On Tue, Nov 24, 2009 at 7:57 PM, David Engel <david at istwok.net> wrote:
> On Tue, Nov 24, 2009 at 06:36:21PM -0700, John P Poet wrote:
>> ...and that happens even when doing this with "simple" 720p mpeg2.  My
>> system should be able to decode 100 frames per second pretty easily.
>> So, the video being +3 frames behind the audio does not make sense.
>> There is no excuse for my system no being able to "keep up", unless it
>> is not being allowed to.  I assume the problem is that there can only
>> be one-frame-per-refresh, and the refresh is 60Hz.  I don't know what
>> the solution is, but I would love it if Davin or Mark could come up
>> with it ;-)
> I haven't followed any of this thread until this message.  Please
> pardon the interruption if I'm way off base.
> John, I believe you've touched on the reason I have my HD-PVR attached
> STBs set to 1080i instead of 720p -- interlaced content stretches much
> more smoothly than progressive.  With the fallback deinerlacing
> options limited to 1x, you never have more deinterlaced frames than
> refresh intervals (60, in my case) so you never have to drop any
> frames.  With progressive content, you always have more frames than
> refresh intervals so you always have to drop frames and the result is
> jerky.
> I have an idea that might remedy this.  We detect when we have more
> video frames than refresh intervals similarly to how we detect when to
> use the fallback deinterlacers.  When we detect this condition, what
> if we discard every other video frame?  In a sense, we fallback to a
> "0.5x deinterlacer."  Since we discard frames at a regular rate
> instead of somewhat randomly as currently done, the result might be as
> smooth current the stretching of interlaced content.

This is getting off topic for the original purpose of this thread, but...

That seems reasonable to me.  Although, you would only want to discard
every other frame if playback is at 2.0x.  For 1.5x, it would be every
3rd frame, for 1.25x it would be every 5th frame, and for 1.3x it
would be every 4.333333 frames, right?  Those fractions would still
cause issues, but would make the avsync easier.

frame index to discard = (refresh / ((speed * refresh) - refresh)) + 1

A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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