[mythtv-users] Another performance question...

[Clyde Stubbs]

On Fri, Dec 05, 2003 at 10:14:50PM -0800, Mark Frey wrote:
> If I try and run with the default live-TV settings (480x480, RTJpeg), I'll be fine for "talking-head" type shows (News, etc.), but sports programming (basketball for example) will cause more-or-less constant stuttering (quarter second lurches every 1 or 2 seconds). Processor usage stays near but below 50%.

> NVidia GeforceFX 5200 video card with open source drivers

That's the problem (I have the same problem). The open-source drivers
do not provide the vertical-blank synch that MythTV needs to
use the "experimental A/V Sync". The original A/V sync code
doesn't work very well. Fortunately the nVidia hardware does
synchronize the frame flipping with the vertical blank.

I'm told that with the binary driver (4363??) and the "experimental"
A/V sync it all works well. I can't run the binary driver because
I need interlaced PAL output, which is broken in the binary driver.

I'm doing some work at the moment to improve this - I have been able
to reduce, but not eliminate the problem. Essentially, there are
two issues that I can see.

1) Myth demuxes the audio and video from the ringbuffer - it then feeds
the audio out to the audio device, and assumes that it keeps proper time.
I have no idea at this time if that's a good assumption.

Then, to keep the video in synch with the audio, the video output
code compares the video output times with the audio output times
and tries to slew the video output rate to keep them in sync.
This is what causes the jerkiness - because of audio buffering
and general scheduler non-determinism, this compensation cycles
about the ideal value.

I'm working on fixing this, basically by looking at what other
viewers (xine, mplayer) do (because they work rather well on the
same hardware) and hacking the Myth output stuff. Stay tuned.

2) The whole idea of using X to output the video seems like using
a sledgehammer to crack a nut. X is great in that it's networkable - but
we don't need that flexibility for Mythtv. There don't seem to be any
good alternatives right now, the directfb stuff seems most promising,
but I'm not sure what would be needed to get it going with Myth.

Clyde



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Clyde Stubbs                     |            HI-TECH Software
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