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Revision as of 15:25, 16 February 2006 by Jdastrup (talk | contribs) (Prebuffering Pause or stutter with Nvidia)

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X-Video Motion Compensation, or XvMC, allows video programs to only send a graphics card differences from the last frame instead of sending a whole frame. XvMC can be used to reduce CPU utilization when playing back MPEG-2 video. In theory it should also reduce bus bandwidth requirements. Savings will be most dramatic with slow CPUs or when playing HD content. MythTV can be compiled to use XvMC for native playback. External players such as Xine and Mplayer can also be compiled to support XvMC. (Xine's plugin is oddly named xxmc.)



Unless otherwise noted, any mention of Nvidia hardware implies the use of the recommended Nvidia driver version for MythTV at the time of this writing: 1.0.7676

Supported Video Formats

Currently, only MPEG-2 is supported. This is usually a hardware limitation. As an exception, one of the Unichrome chips is capable of XvMC accelerated MPEG-4 playback, but the Unichrome driver doesn't support that functionality at the time of this writing.


XvMC is currently supported in XOrg and thus MythTV on certain Nvidia and Unichrome video card hardware. Sadly, XvMC is not currently supported by the ATI drivers, even though the hardware is capable.

Example CPU Savings

  • A 2.93Ghz Celeron D w/1Gb DDR 3200 SDRAM with an Nvidia FX 5200 playing SD content uses between 8-12% CPU with XvMC enabled. Between 10-14% without.
  • AMD Athlon XP 2400 w/512mb DDR 3200 SDRAM with an Nvidia FX 6200 playing HD content (1080i) uses between 20-25% CPU with XvMC/BOB enabled. 80% cpu without (of course XvMC still locks to lock on "prebuffer" in MythTV 0.18 a lot)
  • AMD Athlon XP 1600 w/1GB DDR with a Nvidia 6800LE playing SD content uses about 15% with XvMC enabled. About 25% without
  • AMD Athlon 800 w/768MB PC100 with Nvidia Geforce 2MX playing SD content uses about 15%-20% CPU with XvMC enabled, 45-50% CPU without.
  • VIA C3 Nehemiah 1GHz uses around 20% with viaXvMC, can hardly play mpeg2 without.
  • Intel P4 2.26 playing HD content can use 95%+ without XvMC and 50% with XvMC enabled.
  • (Add your example Hardware/Software/Content/CPU stats here)

Tips and Tricks

To get XvMC working with MPLAYER

mplayer -vo xvmc -vc ffmpeg12c

(assuming you are running mplayer with XvMC support compiled in)

Prebuffering Pause or stutter with Nvidia

Although XvMC sounds like a dream come true for those wanting to play HD content with a slower system, it's not perfect. Even with XvMC and low CPU usage, many still experience problems like prebuffering pauses, mild to moderate audio or video stuttering, problems while the OSD is displayed, and other issues. Many simply turn off XvMC if their CPU is fast enough, but if you need to use it, here's some suggestions that may help:

  • Try disabling OpenGL VSync.
  • Try setting this in your "Device" section in your xorg.conf
    • Option "NVAGP" "1"
  • Use BOB (2x framerate) interlacing

OSD Flicker

If you are using MythTV version 0.18 and you experience severe OSD stutter or flicker when XvMC is enabled, try upgrading to 0.19. The OSD flicker is fixed for the author in 0.19 with an Nvidia FX 5200.

Greyscale OSD

MythTV version 0.19 ships with a hack to enable full color OSD with XvMC enabled. However, the hack is disabled by default. The code change is described here.

To fix the problem and you're using 0.19, do this:

  • Make a change to your xorg.conf file. To your Devices section, add this:
Option "XvmcUsesTextures" "false" 
  • Make a change to the "settings" table in MythTV's MySQL database. You can access your MySQL mythconverg database from the command line like this:
mysql -u root -p mythconverg

You will be prompted for a password. If you don't remember setting one up, try just pressing the ENTER key.

Here is the SQL statement you should use to make the change:

INSERT INTO settings SET value='UseChromaKeyOSD',data='1',hostname='yourhost';

But change 'yourhost' to the hostname that your frontend is running on. If you're not sure what your hostname is, run `hostname` on the command line to find out.

  • Now restart X11 (for the xorg.conf change to take effect) and your frontend (for the SQL change to take effect) and see if it works!

External Links