IVTC Explained
INVERSE TELECINE (IVTC)
I think I'm correct in saying that there is no such thing as an Inverse Telecine machine :). But, as the name suggests, inverse telecine is a process that turns a 30 fps movie back into a 24 fps movie. Basically what it does is take out all those extra fields that were added to the movie to make it 30fps. Its about now that I start spluttering because this is an awkward subject and I can't find any information on exactly how Inverse Telecine is performed! So instead I will describe what "looks" like should be done based on how it was telecined in the first place.
Lets go back to our picture! As you can see from the second row down, to turn the 24fps movie into 30fps we have to separated the pictures into 10 single fields (or half frames) by adding two fields that shouldn't normally be there. Counting from left to right, all we would need to do to turn or 10 fields back into 8 fields (to turn 30 fps into 24fps) is to delete fields 3 and 8. Remember we are talking fields here not frames.
But taking out fields 3 and 8 would produce a movie that had a field order of: top, bottom, bottom, top, bottom, top, top, bottom! Since you cannot weave together two bottom fields or two top fields we would need to swap them around. So imagine the order of the numbers as:
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1, 2
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3, 4
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5, 6
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7, 8
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T, B
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B, T
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B, T
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T, B
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To get the correct order we must change them to:
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1, 2
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4, 3
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6, 5
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7, 8
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T, B
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T, B
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T, B
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T, B
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Which gives us an order of: 1, 2, 4, 3, 6, 5, 7, 8 which should theoretically fix everything.
The Framerate Mystery Unraveled! 23.976 / 24 / 29.970 / 30
If the only framerates we use are 24, 25 and 29.97 then why to people speak of using 23.976? This is to do with how the movie has been created. A 25 fps movie still has the same amount of frames as a 24 fps movie because none have been added. But nevertheless a PAL television chooses to play them back at 25 fps. This makes the PAL movie play back at a slightly shorter length and means the audio will be out of synch slightly. To compensate for this, when a movie is telecined they apply to it what is called a 'pitch-correction' which speeds up the audio to match the playback speed, in the case of PAL this means they perform a pitch correction of about 4%.
The amount of frames a 3:2 pulldown telecined movie has is 30 fps. But an NTSC television will play them back slower at 29.970 fps (59.94Hz). The amount of actual frames hasn't changed, none have been added or taken out! Here is where the 23.976 part comes in! If we inverse telecine a 30 fps movie we would end up with 24 fps. But if we inverse telecined a 29.970 fps movie, because it has a slightly slower speed, instead of getting 24 fps as we should, we will end up with the slightly slower rate of 23.976 fps.