Using different refresh rates
This is a brief description of the various refresh rates that the NMT can output.
European interlaced content like MPEG2/H.264, SD and HD off-air recordings, DVDs and the odd bit of European 50Hz progressive broadcasting (Swedish SVT HD for example).
May be useful for film/25p progressive video sourced content - but probably has little benefit over 50p. Not very clear how many displays who actually support it. (It doesn't have the same advantages over 50p that 23 has over 60p)
24 Hz (aka 23.97 or 23.976 Hz)
Should be used for most HD movie content - particularly those extracted from HD-DVD or BluRay - and media derived from US HD broadcasts and US/Japanese movie DVDs
60 Hz (or ideally 59.94 Hz)
Should be used for US/Japanese interlaced content (MPEG2/H264 SD and HD off-air recordings), 720/60p progressive content (such as off-air recordings of US 720p broadcasters when in 60p) and when playing US/Japanese DVDs.
If you don't have a display that will show 23.97 Hz at a 2:2->5:5 pulldown then you are best off keeping to 50 Hz and 60 Hz.
Many US displays won't cope with 50 Hz output - in that case 60 Hz only is the rule.
There is not that much pure 24 Hz (as opposed to 23.97 Hz) content out there - though some European HD-DVD/BluRay releases may have been in this mode.
When you play a file in it's native refresh rate, the screen times the refresh rate exactly correct and the movie will have a more even flow. You can use a program like AVInaptic or Mediainfo to investigate your media.
If you choose for example 60 Hz over a correct rate in the media file, your TV probably runs pulldown to correct the framerate, which may causes small stuttering.
To test your video output settings and autoframerate use these Frame Rate Test Files. The white line that travels from right to left should travel smoothly if your NMT and TV are set to the correct matching settings for the file frame rate.
Thank's to the users Noggin and HeedZ for providing the information in the NMT forum.