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A few weeks ago, I picked up a 2.0GHz Core 2 Duo Mini to playback HDTV video, and there are some questions I'd like to answer that I don't see answered anywhere else.
Will it play .mkv files? Yes, with Perian (the QuickTime plugin), or more commonly VLC or MPlayer. QT Player / Front Row is probably what you'll use least often though, despite its convenience, because it is noticeably more demanding of system resources.
Yes, but will it play 1080P .mkv files? You just had to go there, didn't you. With all three of the above solutions, the answer is a solid 'no'. 720p and 1080i appear to be fine, but it just won't push 1080p x264 decoding from .mkv containers with one core, and that's the problem. No x264 decoders appear to be multithreaded yet; when they are, I'd imagine this will become possible.
What's the video hookup? DVI-to-HDMI cable into the Mitsubishi WD-73733 DLP rear projector, resulting in 1080p60 video (but remember that we only push 1080i or 720p meaning the Mini does scaling and pulldown because this 1080p60 rez never changes; despite the pulldown, I notice no jitter on playback). With overscan enabled, the menu bar disappears off the top of the screen. Otherwise I've no objections. This probably would not happen with LCD or plasma.
What's the audio hookup? Optical mini-to-toslink cable into the amplifier. No problems whatsoever so far, even beyond 2 channels.
Will it play WMV? No, at least not the hi-def ones you're thinking of, still gotta use the XBox 360 for that and endure the resulting fan noise, until such time as one of those three players implements useable support for what is unfortunately a proprietary format.
Also worth noting, the bandwidth requirements of streaming compressed HD video, even at 1080P, appears to top out around 30 megabits (note that I'm not referring to raw video bandwidth here, which I'm aware is much much greater). I mention this to make it clear that 802.11g networks or USB2 disks are not going to be bottlenecks. I stash the media on a USB2 disk myself.
Would you do it all over again? I could go either way, really. Even though some other .mkv-playing devices are starting to emerge on the market, they have their problems too, and this is just a really immature corner of the A/V world right now. I'd expect significant movement on this in 2008. The Mini does a wonderful job of playing 720p and 1080i .mkv's, but admit it, you'd rather push fancy 1080p into the TV set (the argument that 1080i = 1080p when your TV set has a good line doubler does not apply here, because the Mini is doing all the scaling and pulldown to deliver its own 1080p60 signal). I would also like the ability to switch output resolution to 1080i or 720p, or even 1080p24, but it isn't an option in OS X display preferences, and I'm frankly unsure if that limitation is from OS X, the video chipset, or even the DVI cable.
Should I do it? Maybe, but beware of the fact that everyone's an experimenter here (and there's no guarantee that 1080P decoding will ever work, at least on the 2.0GHz mini). Still, it's probably one of the best ways to play .mkv in your living room. And it beats the optical format wars.
-Chris