[mythtv] vsync and frame timing adjustments

Doug Larrick doug at ties.org
Fri Aug 27 08:46:01 EDT 2004


Doug Larrick wrote:
> Thanks, will do.  I found enough time last night to get some 
> instrumentation code in place, and will try to run some experiments over 
> the next few days with all three variants of the code.

As promised... here is some data.  They're pretty big, so I've not 
attached them.

Gnumeric version: http://jekyl.ddts.net/doug/frame-logs.gnumeric
Excel version: http://jekyl.ddts.net/doug/frame-logs.xls
HTML version (big): http://jekyl.ddts.net/doug/frame-logs.html

I haven't tried the .xls file.

I have not spent a lot of time analyzing this data, but the interesting 
part is in the "frame-front-only.log" tab (sheet), where I've 
highlighted some sections in orange.  This is the code that's in CVS, 
without the adjustment I added a while back.  You'll note that there 
does appear to be a straddle problem that's fixed by this code; I bet 
that's the weirdness I was seeing.


The "frame-new-code.log" tab is not exactly what you proposed... I had 
to change the adjustment to 200 in order to keep up with my 60Hz refresh 
(on my development system); I was able to bump the threshold to roughly 
-8k as I thought I had remembered it, with no ill effect.  I also added 
a safety net: 500 usec from the negative threshhold, I do a +2000 
correction.

     if (m_delay > -500)
         OffsetTimeval(m_nexttrigger, -2000);
     else if (m_delay < -(m_refresh_interval-500))
         OffsetTimeval(m_nexttrigger, 2000);
     else if (m_delay < -(m_refresh_interval/2))
	OffsetTimeval(m_nexttrigger, 200);

I'll be happy to explain what the columns are when I'm not late for work 
:-), though I think they're self-explanatory if you look at the code.

Let me know what you think.

-Doug
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