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Join Date: Jun 2005
Location: Northern, VA - Washington, DC
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Film is shot (traditionally) at 24 frames per second.
Most digital displays, lets call it a 1080p display, show video at 60 frames per second (60hz).
With HD discs, if film is sent to the display at 1080i, it is actually 24 frames per second, that is broken in half (48 interlaced fields) then one field is repeated to get up to 60 fields (no longer called frames) per second. Now, a 1080p display ONLY can display at 1080p, so it must deinterlace the fields and get it back to 1080p/60. If done correctly, this will properly put the 24 frames back into a progressive format that should look darn near the original on the disc.
Now, if you have a film that is shot at 60 frames per second, progressive. Than that should NEVER be interlaced because you would lose film data.
As well, if you have a display that can properly show a 24hz source at 24hz (frames per second) then you probably shouldn't interlace that either. But, even if it was interlaced, as long as proper deinterlacing takes place, the original 24fps with full resolution should be able to be restored.
The hard part, is that properly identifying interlaced vs. progressive frame sequences can be difficult for some displays to do on their own, so it is typically better for the processing to be handled by the disc player.
This information is an 'as far as I know' and some generalizations were made - additional info/corrections is appreciated.
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