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

Michael Hill mikehillny at yahoo.com
Thu Jun 5 10:54:56 EDT 2003


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

The "scheduler jitter" is my only reason I have not retired my tivo box now
that I have the remote/ir blaster situation at bay.  I would assume that the
pvr-250 captures an interlaced picture as well and the playback might have some
jitter introduced ??  Anyone with pvr-250 chime in on this.

-mike

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. 

Deinterlace: Again using a crawler, when myth is paused the 
characters look smooth but are jagged in motion. On commercial 
products the characters are smooth in motion but have 'stair 
steps' when paused. I believe this is because they are doing 
something smart about updating the two interlaced fields so 
that scan lines are updated like the original interlaced signal. 
Myth has a frame of both fields updated once per full frame. 
The default linear blend deinterlace code waters down the data 
so it is neither a true reproduction of the interlace fields 
nor is it as crisp as the original sampling. 

Color reproduction: In a side by side comparison, my myth 
picture doesn't look as life-like because the color and 
intensity ranges are distorted by the bttv device while 
recording. One would hope that the default contrast and 
color settings would be 'line level' but apparently they 
are not. Brightness and hue are close to neutral but contrast 
and color at 50% are clearly distorted on each bttv card I've 
tried. For contrast, white and anything near white it is fully 
saturated while medium intensities are dull. For color, when 
solids are bright but not over-driven, medium tones look washed 
out. 

Setting the v4l contrast to something below 40% (usually 36-38%) 
greatly improves the picture. Color has greater variation 
from one card to the next but is usually best between 25-40%. 
Even after making these adjustments and adjusting the TV set 
to match, the color and intensity is not as close to the 
direct TV image as the commercial products. 


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