MythTV DVD Jukebox question

Brandon Beattie brandon+plug at beatties.us
Mon Nov 7 18:21:00 MST 2005


On Mon, Nov 07, 2005 at 05:17:13PM -0700, Grant Shipley wrote:
> On 11/7/05, Brandon Beattie <brandon+plug at beatties.us> wrote:
> > Now I'm not against Myth, but it's best to use the right tool for the
> > job.  Myth as a DVD player, or even dvd player and encoder, or a PC for
> > just DVD playback is a step back in quality.  The only reason I see to
> > do it is if you want to sacrifice a little bit of quality to never risk
> > scratching a DVD.
> 
> Do you know if this is a problem of the current crop of players on
> linux?  How about theater tek for windows?  People seem to rave and
> rave about the quality of the image using TTek and Powerstrip to 1x1
> pixel map to their pj.

1x1 mapping is only fully useful if your source(DVD), video output, and
display match.  Seeing how a DVD is 480P normally, if you're display is
not 640x480 (For 4:3 video) or 854x480 (for 16:9 video, but really most
dvd's are 720x480 which isn't a 16:9 true resolution anyway) 1x1 pixel
mapping doesn't buy you anything.  Chances are your display will not
have a true dvd resolution, so having 1x1 pixel mapping is worthless,
even if the computer could do it.  One main importance in preserving
quality is matching the clocks between source and display.  Each set is
different in it's native clock.  It could be 59.94Hz, 60Hz, 72Hz,
119.88Hz, or 120Hz (There are actually another half dozen frequencies,
but they're not found on most TV's, such as these others all will be
present in a selection of HDTV's and projectors at any retail store)

If your display is not an analog display (CRT) then it has a _set_
frequency and it drops or repeats frames based on when it gets inputs
from the source.  LCD/3LCD DLP/3DLP all have set frequencies that are
often multiples of 60Hz.  My projector's native Hz is 180Hz.  If your
computer is refreshing at 60.09hz then your projector will add or drop
frames every 11 seconds.  Now having a computer display refresh of 
60.09 is being fairly lucky.  The refresh rate is different on every 
resolution and dependson things such as the motherboard and the
frontside bus speed of the motherboard The next issue is when the
software gets the data to the video card.  If the software is not in
sync exactly with the video card then the video card will also repeat
and drop frames.  Mplayer has an option to sync to the video card, but
it only works for the matrox G400/G450 cards and uses framebuffer, but
even then it's a psuedo sync.  Other tricks to help are using opengl
Vsyncing, however, this only lets the software know when a frame gets
displayed -- this does not allow software to say when a frame gets
displayed.  So it's down to the software to see if it needs to add in an
extra frame, or delete, but this is guessing game to beat the graphics
card.  We've built a very complex playback system for Myth that does
pretty much every trick in the book to keep video in sync, but it runs
about 95% in sync, so every 3 seconds we'll add or skip a frame if we're
getting out of sync.

Now the only reason there's even an issue with syncing frames is that
DVD's are usually 24Hz, which is pretty dang slow for video.  The human
eye can see up to 70Hz depending on lighting and the eye, most people
see about 40Hz-50Hz in a typical situation.  (Side note: This is why
sports are filmed at 720P, it's 60Hz progressive, vs 1080i which is the
equivelant of 1080P at 30hz, motion looks smoother on full frames than
double speed of half frames.. fps matters).

So bottom line is anytime you're dealing with a source under what your
eye can see, refresh can really effect viewing pleasure.  HDTV for
example looks more "real" not just because of it's higher resolution,
but it's increase in frame rates.

DVD players that support 3:2 and internal de-interlacing (Or if your
display supports these) will make a bigger difference in viewing quality
than most anything else.  Software like Xine if you force 3:2 pulldown
does very little, as the display won't do 3:2:2:3:2:2:3:2:2:3, it will
do 3:2:4:2:5:1:2:3:1:2:3:1:4:2:3:2:3 -- and anything they're not
perfectly even, video looks stuttered.  The bigger the image, the more
you notice it (ie, on a small tv and a car moves 2 inches between frames
isn't as noticeable as a car that moves 1 foot on a projector screen per
frame.  

I spent about 50-60 hours perfecting a modeline to get 60.01Hz, used
color calibration software/dvd's, and tuning the projector and use
monsters inc (The opening of the sushi restaurant) and the opening scene
of LORT.  In what I could do to perfect a PC, it never did look as good
as what my $300 DVD player does, with only spending 20 minutes tuning
the projector to it (Using component from the DVD player, and DVI from
the computer, FX5200 video card).

Windows is somewhat a different story.  Theater Tek, ATI, and Windows
all have been built specifically to support syncing audio and video, and
do a better job than Linux.  They have fixed some problems (Like
syncing, gamma, brightness, contrast, and color correction, and it can
come close and even beat many DVD players, but they're not 100% perfect
and there's a $600 DVD player that can beat any PC hands down.  Linux is
lightyears behind when it comes to handling AV well, and Windows is
decently struggling. 

I'm somewhat a videophile, it's often annoying for me to go to a theater
and see a movie there (I see every frame because my eyes have been
trained to notice the flaws).  I still enjoy my lower quality home
theater with 1024x576 images because it's functional though, and I use
it for more than just DVD's.  But, if I were considering building a PC
vs using a DVD player, I'd get the DVD player.

--Brandon



More information about the PLUG mailing list