[mythtv-users] Very slight jitter on high power machine

Brad Templeton brad+myth at templetons.com
Tue Dec 14 06:28:53 UTC 2004

On Mon, Dec 13, 2004 at 08:22:04PM -0500, Doug Larrick wrote:
> >	I am running at a native 1920 x 1080 resolution, but I have to
> >	tell Myth to use only about 1872 wide due to overscan, so xv is
> >	scaling the horizontal just a touch.  I don't think I want it to
> >	scale the vertical due to interlace issues.
> >
> >    No fancy filters or de-interlace set.  Though it seems I have the
> >	CPU for them.
> This sounds quite similar to my setup.  I'd say you're almost there.  If 
> it's any consolation... I ran into similar (if not worse) jitter 
> issues... the difference being that I wrote code to fix them!

Not familiar enough with the insides to do that yet.
> I would suggest running 1920x540p for video output, and turn on bob 
> deinterlacing.  You will need to run the GUI at 960x540 for your sanity 
> (otherwise the text is unreadable).  This is the electrical equivalent 
> to your HDTV of running at 1080i, but allows you to do scaling of the 
> frame (for overscan or for the zoom modes).

Actually, I find the text quite fine at either of those resolutions, or
at 1080i.  I do think that going to 540p is, in theory, quite a sacrifice,
throwing away half the info.   I mean cinematic stuff is shot at 24fps,
for example, so you gain nothing with 60hz progressive vs. interlace.
You're throwing away half the resolution

Well, not quite since my TV, like most, is only 720 lines, so I was never
using it all, but you get the idea.

When I have tried 540p it 

> As far as your occasional jitter, I'd suggest making sure OpenGL vsync 
> is being used (with 'mythfrontend --verbose playback), and make sure 
> you're installing mythfrontend as SUID root.

I am running mythfrontend suid root, though strangely it still issues
the diagnostic advising this.  Though again with 60% idle time -- the
P4 really is a lot better than the Athlon -- there should be CPU to spare.

Well, I did have a glx loading problem which I fixed, but there are
still a few skips.  But how do I test for opengl vsync?  In the verbose
log you describe, those terms don't appear, except as a substring in this:

    avsync_delay: -30, avsync_avg: -30, warpfactor: 0.9, warpfactor_avg: 0.0149401
'video_output' mean = '33338.09', std. dev. = '557.68', fps = '30.00'

However, I do notice something else that could have some bearing.

Two bursts of lines like this:

    2004-12-13 22:15:33 A/V diverged by 17.1295 frames, extending frame to keep audio in sync
2004-12-13 22:15:33 A/V diverged by 17.3877 frames, extending frame to keep audio in sync

The bursts, as you would expect, slowly reduce the out of sync.  However,
I presume this is the thing I always see when starting a program.  I presume
it's normal, but whenever I start play of a program it is badly out of
sync, and it adjusts (as these logs suggest) within a couple of seconds.
Annoying, but tolerable.  I'm spoiled by the Tivo's more instantaneous

I think it is better after I fixed the glx loading issue, but it's not
yet perfect, and I want perfect(*).   The skips are most clear when you have
things like slow panning, or the enterprise zooming off to warp, that sort
of thing, but they are/were on the order of 1 per second.  I need to record
some new tests.

(*)Why demand perfection?  For proper operation of a PVR, you want to feel
there is no sacrifice between watching live and watching recorded.

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