Tuesday, February 10, 2009

Retelling Yet Another Myth

Last month I put together my second MythTV box. I've become tired of my VIA Chrome box... video acceleration was a joke. So I thought that an onboard GeForce 7050PV would work since it had XvMC support. Plus NVIDIA's (binary-only) drivers were usually pretty good. Right?

I ordered Shuttle's SN68PTG5 AM2 case, figuring I could re-use an old AMD64 CPU I had and stuff the hard drive in from the previous MythTV box. When I got the case in it was a bit larger... and louder... than I expected. I needed to push my TV forward to get the box to fit behind it. The sheer amount of real estate I was able to work with made the opportunity worthwhile however - there was plenty of space around the motherboard, even with the ginormous heatsink installed.

Maneuvering around the box wasn't bad - I was able to put in the old drive and fill both memory channels with two Corsair XMS2 512M PC2 6400 sticks without a single flesh wound. Putting in the CPU was another issue however - I was short a pin.

Yeah... my old, to-be-recycled CPU was for Socket 939. This was a AM2 socket board - meaning it took 940-pin CPUs. Gah.

I paused for a few days and ordered a 2.1GHz AMD Athlon X2. It was a 65nm Brisbane core and only consumed 45W, so the lower thermals would serve a Myth box fairly well. Plus AMD chips are cheaper than avocados right now... it was an easy sell.

I wrangled the case together, put on some new thermal paste, and re-attached Shuttle's massive (did I mention its massive?) heat pipe for the CPU. Installed my old pcHDTV tuner card to receive HDTV channels from my local cable provider, then sealed everything up. I attached the barebones StreamZap Ir receiver via USB then did a openSUSE install with the MythTV repositories.

lircd actually worked without a hitch... the StreamZap didn't have any issues. Neither did the pcHDTV card - it worked out of the box as well. Things were going well, so I shoved the machine behind the TV, connected it via HDMI and went along my way.

At first things appeared to work well. Thanks to NVIDIA's nvidia-settings application I was able to effortlessly setup my 1080p LCD to work over HDMI. X configuration was TONS easier than with VIA's chipset - there it took me nearly 20-30 hours to get the monitor configuration correct for SVGA out. Of course it doesn't hurt that HDMI is just a DVI formfactor... but I digress.

No, the big pain was XvMC. With acceleration enabled, video would playback (either live or recorded) for anywhere between ten seconds to five minutes, then send X into a complete CPU spin. I'd suddenly have 100% usage on a single core and a complete lockup of X11. X had to be SIGKILL'd - repeatedly - before things would stop spinning out of control.

Luckily I bought a dual-core CPU so I could easily regain control. No matter now hard I tried, however, video acceleration just wouldn't do anything but hard lock MythTV.

Finally I gave up and just told MythTV to use the CPU for decoding. This worked acceptably, even with 720p streams. The CPU was beefy enough - it's a dual core, dual channel rig that has the bandwidth. Just a shame that neither NVIDIA nor VIA were able to provide a chipset that would allow accelerated MPEG2 playback in Linux. C'mon - is it really that bad?

Now things are recording and playing back just fine. The vsync is disabled right now - so I do have image tearing during high-motion decoding. But hopefully NVIDIA's next round of Linux drivers will fix XvMC, give me accelerated video and take all my worries away.

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