AGP + PCIe = Bad news

While the title seems gloomy, this post is actually very good news for my TV addiction.

So, in an attempt to redistribute video capabilities in the house to accommodate the new TV, I decided to reprovision my second-best MythTV frontend box and use a MediaMVP (with mvpmc of course) in it’s place for the time being.  This will only be a temporary solution, but since we only have a standard-def TV in the bedroom, it seemed like the best move.

So, I took the box upstairs, VGA cable in hand, excited about finally having a VGA port that can take 1080p input.  I plug in the box and boot it up, and it detects the correct resolution automatically (thanks nVidia!) and presents me with a MythTV menu.  Before I go on, I better give a little background on my current HD Myth setup.

Before Sunday morning, the only HD set in the house was my 40″ JVC LCD.  Connected to it, I have a 2.8GHz Pentium4 with a 256MB nVidia 6200 AGP card in it.  I use a DVI-HDMI converter to drive the TV.  Until today, this box has been just a little too slow to play HD content without the help of XvMC (or so I thought).  XvMC is nice if your system is too slow, because the GPU can do some of the MPEG decoding for you.  However, it brings some other issues, like monochrome On-Screen Display and really choppy playback while the OSD is on the screen.  The result was that I had an ugly black-and-white OSD while watching HD, and I always had to hit ‘exit’ immediately after invoking any function that brought up the OSD.  However, it worked and I just planned to upgrade the box later.

So, back to the “second-best” box upstairs.  I plug it in, it drives the new TV to 1080p, and I go to watch an HD program just to see how underpowered it is.  What, you ask, happened?  It played the HD content perfectly, with no XvMC, and with a full-color OSD.  Huh?  What is going on here?  This box is clearly slower than the other in every respect.

So, I started thinking about what is faster about this box than the other.  Both had Intel processors, both were Intel chipsets, both were using similar AGP cards, with the same driver version.  Then something I read a few weeks ago popped into my head.  I read somewhere that motherboards were not supposed to support AGP and PCIe slots because of some quirk about how they would be presented to the system.  I remembered that the (ultra-cheesy) motherboard that I had in the “fast” box actually had both types of slots, since it was purchased just as PCIe was gaining traction.  I figured that maybe the AGP slot was crippled (I think the article mentioned that the AGP slot was hung off of a slower-than-AGP PCIe drop) and that was causing me not to be able to push enough data to the card to support HD content.

So, I ripped a PCIe 256MB nVidia 6800 from another box and swapped the AGP card out.  I booted up, turned off XvMC and pulled up an HD program.  It played perfectly!  I’m now kicking myself for not thinking to try this earlier, instead of dealing with a monochrome, jumpy OSD for a year.  The moral of the story is: Don’t buy a combo AGP+PCIe motherboard, and if you do, don’t expect AGP to work well.

Category(s): Linux

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