[mythtv-users] RE:Tivo output vs MythTV-PC output question

[Bruce Markey]

Michael Hill wrote:
> I was wondering if any of these issues are fixed by switching from a "dumb"
> analog capture card to the pvr-250 card...

Possibly. The hardware encoder is not subject to dropping
frames when other processes are temporarily busy. Also, the
timecodes should be more exact. The timecodes for software
encoding can be a little erratic when the systems is busy
which can contribute to jitter.

...
> Previous post [ bjm at lvcm.com and others ]:
> 
> Jitter: If you watch the crawler on CNN, it is smooth on 
> commercial products but has nervous twitches on myth. The 
> frame buffer should be updated during vertical synch but 
> myth hasn't had access to this timing. However, there is 
> work in progress to address this for video cards that report 
> vblank timing from their driver. Also, the frame buffer 
> is updated about every 1/30 of a second but because the 
> update is subject to a multitasking scheduler, the timing is 
> subject to when the process get the CPU. The "jitter reduction" 
> option can improve the timing but if the update straddles 
> the vertical refresh, there will be two updates during one 
> display frame and no update on the next causing this twitch. 
> You'll often see several seconds of smooth crawler followed 
> by a few seconds of twitches. Dedicated hardware can know the 
> refresh timing and not be subject to a process scheduler. 

Update =). Isaac did find some code that can check supported
cards to see when the refresh cycle completes. I've worked
out most of the to details to get good results for all of the
configurations that I can test and this is checked into CVS.
This gives the best results for nVidia cards and is somewhat
better for other cards as well. However, Isaac and I are seeing
very different results for live TV with software encoding.
Therefore, this needs more testing and will not be the official
video timing for 0.9.

To use the new timing, go to Setup->Tv Settings->Playback then
check "Jitter reduction" and "Experimental A/V Sync".

For nVidia, I see less CPU time for playback and virtually no
jitter for prerecorded and record in progress (provided the
recording isn't damaged, of course). For live TV I see very
little jitter. Isaac sees a lot when software encoding live TV
but it is nearly perfect with his PVR-250. I set up a PVR-250
last night and playback with the new timing looked very smooth
with each frontend systems that I tried.

--  bjm