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I don't know of a ready-made tutorial but I am sure I can give you one fairly quickly.

1. Install the CCCP if you haven't already in Windows.
2. Install MeGUI and all the updates and profiles (these should be downloaded and installed automatically).
3. Go to Tools Menu>AviSynth Script Creator.
4. In the video input tab, browse to the MKV file.
5. It will then load the film. Go to the Resize box and resize the film to the resolution of your choice (probably 1280*720 or 1280*536 depending on the aspect ratio of the film).
6. Don't mess with the other settings and save the avisynth script.
7. Now back in the MeGui window load the saved avisynth script.
8. Go to encoder settings and look for the AppleTV profile, I think it is called x264: PD AppleTV. Then go to configure and change the bitrate to a level that is appropriate for HD, something like 5000 for 720p.
9. Now click Enqueue and you should have two jobs in the queue for the two passes, click start on the first one and then wait until it is finished.

I just did that from the top of my head so it might not be all right. I haven't actually used that profile but I have used MeGUI to encode 30-40 films for the PS3 and it has always produced very good results.
 
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