[mythtv-users] Playback limiltations [was: Sssssllooooowwww watching TV]

[Bruce Markey]

Cedar McKay wrote:
...
> this is wildly off topic, but if you have worse output with mythtv than 
> tivo, you have something wrong. Check your encoding prefs, and also your 
> tv-out solution (hardware and configuration). If you want to continue 
> the discussion, it should be moved to myth users.

Cedar, I appreciate you enthusiasm on this in your nine
points earlier (actually ten but you used "3" twice ;-).
However, comparing your picture with the picture at your
friend's house introduces several variable. If you are
going to suggest that everyone should see MythTV's picture
as having better quality, I'd like to point out a few
limitations.

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 doesn't have access to this timing. Further, 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.

While there is nothing wrong with the story that you like
your picture more than your friend's, it is unrealistic at
this point for me to expect MythTV to have a better quality
picture than the custom hardware of the retail DVRs.

--  bjm