[mythtv-users] Very slight jitter on high power machine

Brad Templeton brad+myth at templetons.com
Mon Dec 13 23:36:22 UTC 2004

I finally got things working at 1080i on my HDTV.  Unfortunately, there
is a slight, but annoying jitter.  I've read a number of the articles
on jitter but I don't seem to have the causes of that.

By jitter I refer to what appear to be dropped frames, slight skips in
smooth motion during periods of high motion.   Not the "Sound is freaking
out" constant jitter or actual slow playback.

Sound remains in sync with the video generally all the time.

The jitter is less in mplayer, but I don't think mplayer is perfect.

Here's the rundown:

    System is fast -- DMA definitely on.  Frontend: 3ghz Pentium 4,
    100mbit network, disks easily capable of 30 megabytes/second read on
    the backend where video is stored.     Backend is Athlon XP 3000.

    Idle times are good -- 50 to 60% idle time on the P4 when playing
    the HDTV video.   Not a shortage of CPU.

    Running FC3, so alsa is the only sound driver.   However, driving
	the oss emulating /dev/dsp since my attempts to drive ALSA
	dircectly with entries like "ALSA:default" caused too much troubles.
	Running 2.6.9-1.667smp kernel.  Don't know if hyperthreading gets
	you anything here, but idle times say it's not hurting.

    Drivers say snd_intel8x0
	gnome-alsamixer says a Realtek ALC650F.
	It's an AOpen Xcube/65

    Video card is FX 5200 from Gigabyte.  Running Nvidia
	drivers version 6111.  (Had too many troubles with 6629)
	In any event, xvideo is working fine, though xvmc does not work.

	I am running at a native 1920 x 1080 resolution, but I have to
	tell Myth to use only about 1872 wide due to overscan, so xv is
	scaling the horizontal just a touch.  I don't think I want it to
	scale the vertical due to interlace issues.

    No fancy filters or de-interlace set.  Though it seems I have the
	CPU for them.

It seems to me this is a fairly ideal, high-powered setting, and it
should provide smooth as silk video playback.

I also notice inferior mpeg decoding to the TV when I watch it live over
the ATSC tuner.   Unfortunately, I can't get DVI to work, I am using
vga to component video.

Any thoughts?  My path to Myth HDTV has been a hard one, and I've been
pleased to get close to the finish line and then have this show up in
the way.  I would also like to resolve my DVI problems (and my independent
problem of making the frontend semi-diskless -- I can get FC3 to boot but
starting X with kde dies somewhere in that process.  I'll report more on
that later as I think a lot of people would like a diskless frontend.
I actually am going semi-diskless which is simpler.  By this I mean take
an old whiney hard drive you don't use any more and use it as the boot
device, then spin it down after booting.  Saves having to get ethernet
boot working.)

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