[mythtv-users] XvMC unusable with HDTV, Xv won't keep up

Joe Barnhart joebarnhart at yahoo.com
Fri Oct 8 17:26:22 UTC 2004

I'm caught here.  I'm trying to watch HDTV programming
(1080i and 720p sources) and I'm not able to reliably
watch anything.

My system includes a GeForce4 MX card, latest nVidia
6111 drivers, and KnoppMyth on an Athlon 2800.  All
drives are DMA, memory is sufficient (1G), etc.   I'm
using S/PDIF output for my sound using the alsa
1.0.5rc1 libs.  All output is done at 1080i to the VGA
connector, which is hooked to a rear-projection TV

XvMC with 1080i source material:
Playback is fairly smooth, but the system hangs if I
skip forward or backwards.  Mythfrontend log shows:

2004-10-08 02:39:55 prebuffer wait timed out..
2004-10-08 02:39:55 waiting for prebuffer...

Over and over.  Nothing exits this mode except
Ctl-Alt-Backspace (reboot X server).  Video appears to
be down-resolution (maybe 540?) unless I enable "bob"
deinterlace.  (Deinterlace should not be needed since
both the output device and input are interlaced.) 
Severe artifacts on fine-grained textures when panning
the camera.  It appears to be scaled from the

2004-10-08 02:39:06 detectInterlace(Ignore Scan,
Interlaced Scan, 29.97, 1088) - >Interlaced Scan
2004-10-08 02:39:06 Interlaced: Interlaced Scan 
video_height: 1088  fps: -1


2004-10-08 02:39:06 Image size. dispxoff 0, dispyoff:
0, dispwoff: 1920, disphoff: 1079
2004-10-08 02:39:06 Image size. imgx 0, imgy: 0, imgw:
1920, imgh: 1080

Xv with 1080i source material:
Playback stutters because the system can't decode the
MPEG fast enough to keep up with the source material.
The deinterlace filter is not needed for high-quality
playback, but the artifacts on fine-grained texture is
still there.

XvMC with 720p source material:
The image stutters and jerks back and forth in time
rapidly.  CPU utilization is only 5% or so.  Sometimes
goes into "slow motion" effect with halting sound.

Xv with 720p source material:
Playback is relatively smooth.  Some dropped frames
causing a bit of jerkiness here and there.  Strong
aliasing effects with near-horizontal lines exhibiting
severe jaggies.  Some sources (e.g. Fox major-league
baseball games) exhibit the same slow-motion effect as
XvMC.  I suspect the stream was broadcast as 480p
widescreen (from the moire effects and apparent
resolution), but the mythfrontend saw it as 720p.

I'm very close to a working system, but I can't yet
watch my recorded programs without major troubles. 
Some folks in myth-dev had similar problems, but
they've updated their alsa drivers and are all now
happy, leaving me to search again for solutions to my problem.

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