[mythtv] MythTv with Via XvMc MPEG Decoding: Motion jitter

Terry Barnaby terry1 at beam.ltd.uk
Fri Sep 17 04:26:03 EDT 2004

Hi All,

I have just had a chance to look at the motion "artifacts" problem
with using the latest Via XvMc MPEG Decoding with MythTv.

There is a large amount of motion jitter on the display. I have
narrowed this down to some problem with MythTv 0.16 CVS.

My System:
M10000 Via system running Fedora Core 2.
Kernel 2.6.7 with DRM from CVS date 2004-9-12.
Unichrome Via X driver from CVS 2004-9-16, AGP-DMA enabled
Unichrome Via libXvMC library from CVS 2004-9-16
MythTv from CVS + XvMC QMatrix mods.

My Tests:
1. Using MythTv 0.16 CVS there is a large amount of jitter both using
	the Via XvMc MPEG Decoding and the internal software decode
	with Xv display output.
	I am unable to turn on Vertical sync mode with 0.16, it seems
	to have been removed (experemental AV sync).

2. Using MythTv 0.15.1 the display looks good with both the Via XvMc
	MPEG Decoding and the internal software decode with Xv display
	output. Using Via XvMc MPEG decoding CPU usage is about 12%.
	If I enable Vertical sync mode the picture quality is still
	very good, but additionally when a jump forward in the video
	is performed (cursor right key) the display restarts quickly
	and cleanly. Without Vertical sync mode there is a period of
	about 3 seconds of display sluggishness.

So the motion jitter problem seems to be an issue with a new
synchronisation scheme in MythTv. Note I am running MythTv as root
still so the Real-time mode is enabled.

The Chroma QMatrix code does not seem to have made much difference
that I can see as yet, but I have been focassing on the motion problems ....

I will investigate further today.

Dr Terry Barnaby                     BEAM Ltd
Phone: +44 1454 324512               Northavon Business Center, Dean Rd
Fax:   +44 1454 313172               Yate, Bristol, BS37 5NH, UK
Email: terry at beam.ltd.uk             Web: www.beam.ltd.uk
BEAM for: Visually Impaired X-Terminals, Parallel Processing, Software
                       "Tandems are twice the fun !"

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