An update based on my latest fiddling and testing:

a) Right drivers for card with your kernel
b) Proper AGP bus speed and AGP interface driver in use
c) Proper user of openGL sync for retrace
d) Proper settings for deinterlace and, if used, xvmc
e) Proper sound settings and handling of 5.1 sound and multiple streams
f) Real time scheduling of playback thread
g) Making sure disk or network speed is adequate
h) Nothing else sucking up CPU on the machine

a) Right drivers for card with your kernel

Check /var/log/<xservername>.0.log.  Excerpt from my
/var/log/Xorg.0.log which shows nVidia driver version 7174:

                (II) LoadModule: "nvidia"
                (II) Loading /usr/X11R6/lib/modules/drivers/nvidia_drv.o
                (II) Module nvidia: vendor="NVIDIA Corporation"
                        compiled for 4.0.2, module version = 1.0.7174
                        Module class: XFree86 Video Driver

b) Proper AGP bus speed and AGP interface driver in use

        [EMAIL PROTECTED] ~]$ cat /proc/driver/nvidia/agp/status
        Status:          Enabled
        Driver:          AGPGART
        AGP Rate:        8x
        Fast Writes:     Disabled
        SBA:             Enabled

c) Proper user of openGL sync for retrace

        Start mythfrontend with the "--verbose playback" option and redirect
        output to a file.  Check for Open GL Sync messages (SGI OpenGL)
        when a recording or live tv is played back.

        mythfrontend.log:
        
                2005-04-25 22:29:36.515 Video timing method: SGI OpenGL

d) Proper settings for deinterlace and, if used, xvmc

        These are set via mythtv in the settings->tv->playback screens
        (deinterlace on the first and xvmc on the fourth or fifth, IIRC).  If
        xvmc is set properly, the OSD will be black and white.  If xvmc
        doesn't show up on the options screen, then it wasn't compiled into
        the program.  If xvmc is used, the "filters" field (which can also be
        used to set deinterlace) isn't used.  My personal opinion is that
        kernel deint appears to give a better picture (in terms of motion
        artifacts) than the others (including BOB).

e) Proper sound settings and handling of 5.1 sound and multiple streams

        I am using ALSA:spdif for sound and I can confirm that it
        works since my DD receiver switches to digital mode.  Not sure what
        exactly to check here, but you can use "aplay -L" to get a listing
        of the ALSA devices set up for your card / .asoundrc file.  Testing
        individual devices can be done with "aplay -D <device> <soundfile>".

f) Real time scheduling of playback thread

        Need to do a couple of things to get this working:
        
        1. chmod u+s mythfrontend
        2. Need to modify /etc/sudoers to add your mythtv user
                (use ALL option and NOPASSWORD; example in sudoers file)
        3. Start mythfrontend with sudo: "sudo mythfrontend &"
        
        At this point the logs should say something like "using realtime 
priority"

h) Nothing else sucking up CPU on the machine

        Check using "top" and confirm that mythfrontend and X are the top two
        items (X may be lower if XvMC is used).  Remaining items should be
        substantially less (not sure how to define "substantial"...).

Any other suggestions for things to check?
phlepper
-- 
I hear and I forget. I see and I remember. I do and I understand.  --  Confucius
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