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
- (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
If you are using MythTV version 0.19 with an Nvidia card and experience these symptoms:
- mild to moderate playback stutter
- NVP: Prebuffering pause in your frontend log
- Try disabling OpenGL VSync.
- Try setting "NVAGP 1" in your xorg.conf under "device" settings
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.
MythTV version 0.19 ships with a hack to enable full color OSD with XvMC enabled. However, the hack is disabled by default. To enable it, read this.
The above link mentions:
- Making a change to your xorg.conf file
- And also making a change to your "settings table". The table referred to here is a MySQL table. Usually the database is named "mythconverg" and the table is named "settings". You can usually 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 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!