[mythtv] Proposal for vsync.c rewrite

Doug Larrick doug at ties.org
Tue Jul 6 08:33:26 EDT 2004

I'm writing to the list for feedback on my plans to rewrite the current 
vsync.c as a set of C++ classes, before I write any code.

The biggest problem I see with the current A/V sync methods is that they 
mix two tasks:
1. video frame sync to the display device
2. video sync with audio

Here's a survey of what the existing A/V sync methods do:
* OldAVSync can sleep, or sleep as long as possible then busy-wait (if 
jitter reduction is on).  It warps the video display interval either a 
lot or a little to gain A/V sync.
* ExAVSync uses the hardware methods (nVidia device, DRI, or OpenGL) to 
find the vertical refresh.  If this is unavailable it does a poorer job 
of sleeping than does OldAVSync.  It warps the video 1/2 frame interval 
every 3 frames until back into A/V sync.
* VTAVSync uses the hardware methods and also tries to fall back to 
/dev/rtc before finally reverting to sleeping.  It stretches or trims 
audio frames (rather than video) to achieve A/V sync, unless things are 
really out of whack.

I'd like to move the entire job of video frame sync to a set of new 
classes.  A factory method will choose the best available subclass from 
the following list:
  nVidia device polling
  SGI OpenGL extension
  /dev/rtc - VTAVSync has this code
  usleep with busy wait (ReduceJitter).

The first three are preferable since they actually sync us to the 
vertical retrace.  /dev/rtc is available only on Linux, and the last 2 
methods are important as a fallback.  There may be other methods 
available on other platforms.

The A/V Sync methods are then simplified to compute & provide an 
adjustment factor to the frame sync methods.  In fact, at this point 
OldAVSync and ExAVSync become so similar it may be worth merging them.


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