[mythtv-users] Slow/choppy playback problem (not the usual ones, either!)

Yan-Fa Li yanfali at best.com
Fri Oct 15 05:20:13 UTC 2004

Hey Joshua,

Well, I have an old server based machine running an ATI Radeon 7K on 
PCI, but it's a 64/66 Slot so it runs at AGP 1 clock 66MHz.  The problem 
with Video cards on PCI is they swamp the bus.  There used to be a 
setting for old PCI Video Cards which enabled retries, thankfully with 
AGP those days are past.

On a side note the PVR350 isn't doing regular video.  It's doing 
compressed 1MB/s max MPEG2 streams which it's decoding into 30 fps/60 
fields per second (NTSC), video internally.  Now I could have this wrong 
because I believe it also supports Frame Buffers, but as I understood it 
they decode and re-encode non-mpeg video streams back to mpeg2 using 
ffmpeg, which places the burden on the memory and cpu subsystems rather 
than PCI.

Let's do a bit of math:

	30Hz * 800 pixels * 600 pixels * 3 Bytes Per Pixel = 41.19MB/s

If it were 2D windowing you could cheat and use accelerated 2D drawing 
commands in the GPU to reduce traffic.  However, I believe this is not 
the case with Video, since we actually need to transfer complete decoded 
frames across the PCI bus to be rendered on the card.  Someone who 
know's the code side can probably tell us better.

This huge bandwidth consumption, even for 800x600 resolutions, is why 
old Unified Memory Architecture systems like Intel's "extreme" video are 
so crappy.  They steal so much bandwidth from the system they hobble it. 
Nvidia's NForce 2 Dual Banked Memory IGP based systems which have excess 
bandwidth to spare prove this as they actually perform on par with a 
discrete video card of the same class.

PCI33/32 pretty much starts to crap out at around 45MB/s in a shared 
environment and maxes out at around 80-90MB/s if it completely owns the 
bus, no where near the claimed 133MB/s specification.  This is why the 
incentive to go to AGP is so high; dedicated bus, double the bandwidth 
even for the x1 version.

I would guess adding an AGP video card is probably the way to go.


Joshua M. Thompson wrote:
> On Thu, 2004-10-14 at 19:27 -0700, Yan-Fa Li wrote:
>>Looks like you're not running nvidia's X server driver...  Wait a 
>>minute.  GeForce PCI!!!!!  You're running the video off the PCI BUS ?
>>Say it ain't so.
> The card previously came from my old Shuttle machine which had no AGP,
> so yes it is PCI and yes it's running the nvidia v61.11 drivers.
> Although I could be wrong the fact that it runs great with mplayer seems
> to indicate that there is some other issue. Before getting the PVR350 I
> used this machine to play movies off my NFS server, and those are
> custom-compressed high-bandwidth, full-resolution .ogm files (including
> multiple AC-3 audio tracks) ripped right from my original DVDs. If it
> can handle *those* I would think MythTV should not have a problem. Plus
> lots of people use the 350 as an unaccelerated framebuffer just fine and
> that's PCI too.
> I can try picking up a cheap AGP card with video out over the weekend if
> you really think that will help, but I strongly suspect the issue is
> elsewhere based on my observations.

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