[mythtv] Idea for interlaced playback.

Cory Papenfuss papenfuss at juneau.me.vt.edu
Thu Nov 10 16:16:48 EST 2005


> Three things are needed for proper interlaced playback:
> 1) You need perfect line matching between the video framebuffer and
>   the tv-raster-lines. This can (at least on my nvidia card) be solved
>   with a proper modeline and overscan-settings. No form of rescaling must
>   be allowed as this will blend the fields together.
>
> 2) You need to synch at the field frequency.
>   With xv or the OpenGL synch function, this is no problem.
>   I'm not sure if OpenGL or XV will blur odd/even lines together. This
>   might be a problem.
>
> 3) You need to update half the lines (one field) at one refresh, and the
>   other half at the next (but not removing the previous field)
>   rinse, repeat.
>
> Some people seem to think you need to know when the TV-out is doing a
> odd-line or even-line refresh, which is not easy at all. But this is
> incorrect, all you need to do is make sure odd-field lines are only in odd 
> lines in the framebuffer and the same for the even fields and update the 
> fields in order. (This is known as "bob and weave")
>
 	I'm quite interested in this, because I use homebuilt hardware to 
directly connect VGA to s-vid on my TV.  I drive the VGA at NTSC 
frequencies directly and have always had great quality.  Trouble is that 
the VSYNC seems to be ignored (as you say).  The result is a tearing that 
can only be band-aided by enabling something like kerneldeint.

 	I was unaware that plain ol' XV had the ability to sync, but now I 
see (with xvinfo on my main machine, not my mythtv machine):

       "XV_SYNC_TO_VBLANK" (range 0 to 1)
               client settable attribute
               client gettable attribute (current value is 1)

Interesting...
-Cory

-- 

*************************************************************************
* Cory Papenfuss                                                        *
* Electrical Engineering candidate Ph.D. graduate student               *
* Virginia Polytechnic Institute and State University                   *
*************************************************************************



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