[mythtv-users] Refresh Rate reported for interlaced output (was: New nVidia 169.07 and interlace changes)

[Alex Halovanic]

My understanding was that for most purposes X doesn't really distinguish 
between a progressive and interlaced display; it outputs a 1920x1080 picture 
at ~ 60fps and then the hardware device handles sending the interlaced 
picture to the tv, presumably by simply discarding half the field lines for 
each frame.

Most of the time when you just display an interlaced video in myth, the fields 
being displayed for the video don't match up with those fields getting sent 
to the device.  Applying interlacing to a 60fps video gives you about 
effectively 30fps for moving objects; applying another round of interlacing 
introduced by the unsynced display makes it more like 15fps and very juddery 
for motion.

Bob2x deinterlaces the picture by showing all 1080 fields at 60fps.  It does 
this by showing just the odd fields for 1 frame, which measure 1920x540, 
stretching it out vertically to 1920x1080 (making each field 2 pixels high 
instead of 1).  This causes some very slight bounciness in thin elements or 
the edge of motion, as some pixels are rapidly switching between showing an 
edge and showing the space adjacent.

Even with just displaying static elements, you still necessarily get this 
bounciness on a 1080i display (check out some of the menu elements in the 
myth main menus for example).  I think that any bounciness introduced by 
bobbing is covered up by the inherent bounciness in the 1080i display, even 
when the fields are being displayed in reverse order, and you're basically 
now just interlacing a progressive 60fps video, instead of reinterlacing an 
already interlaced video.  Therefore: a perfect 1080i display at the expense 
of a lot of apparently unneccessary cpu work.

Incidentally, I finally figured out how I got a perfect 1080i picture without 
bob.  It absolutely required the double refresh rate patch, otherwise the 
video drifts in and out of sync every few seconds even with everything else 
identical.  I've watched the CBS broadcast of the Buffalo-Cleveland NFL game 
for over 30 minutes and 100,000 interlaced frames now and it's never once 
lost sync.  I'd appreciate if others could test this, especially with display 
devices other than the onboard component out (such as DVI with the predefined 
1920x1080_60 tv modeline)

My set-up:
-XV video sync enabled in nvidia-settings
-OpenGL video sync enabled (I require both to stop tearing, ymmv)
-Use Video As Timebase enabled (this is crucial or else the fps drifts all 
over the place)
-No zoom or overscan in Mythfrontend's playback settings, at least vertically 
(there's no point trying to sync things if their sizes aren't identical)
-The double refresh rate patch applied to SVN (0.20 will probably do fine as 
well)

Obviously you can't be dropping frames from a bad recording or a loaded CPU or 
hard disk, and I'm not sure if it will work if you're stuck with a station 
that is broadcasting a combination of interlaced and progressive frames.  
When you start playing the recording, there's a good chance it won't be in 
sync.  If it's not, pause and unpause the video, check any motion for judder, 
and keep pausing and unpausing until it starts synced up.  I think it's about 
a 50% chance of it being synced whenever it starts or restarts playback so it 
shouldn't take more than 5 pause-unpauses to get it right.  Now, sit back and 
watch the perfect 1080i picture, resisting the urge to pause or skip around 
and throw things back out of sync ;)


-Alex