Progressive vs. Interlaced

Unlike NTSC video, which is natively interlaced, film is a progressive medium. When working with any video source, under optimal conditions you should always keep the original frame or field structure, and under real life conditions you should at least try to do this. That means keeping film in its original frames. At the same time, it's necessary to be able to send it to an NTSC display in the format it expects.

Why IVTC?

Since telecined video is already in both a proper format for display on an NTSC TV, and in a format that can be read by an HDTV, it may not seem worth the effort to recover the original film frames. If you're already processing the video in some way, such as encoding it to MPEG-2, MPEG-4, or some other format, it's pretty much always in your best interest to IVTC whenever possible. Not only does it ensure that the original frames are restored and available for playback on progressive displays, it also improves quality at any given bitrate. Even aside from the difference between encoding only 23.976 frames per second instead of 29.97, the bitrate required for any given quality level will be higher for interlaced video than progressive.

RFF Flags

Repeat First Frame flags tell an MPEG decoder to repeat certain fields in a video stream. Like telecine, these flags, also referred to as Pulldown flags, produce duplicated fields, but only when the file is played. Headers written with the video tell the decoder when each field is to be read. The decoder already has to convert half of each progressive frame into a field for interlaced display. Reading pulldown flags, it simply delivers every fourth field twice. Standard pulldown from 23.976fps to 29.97fps is called 2:3 pulldown, but may also be referred to as 3:2 pulldown. Other pulldown patterns can be used for other framerate conversions, but this is the only one that applies to IVTC.

Film Encoded as NTSC

Although it's always preferable to capture film in the original progressive frames instead of with telecine applied, it's not always done this way. For a variety of reasons, film is sometimes stored digitally as NTSC video, using a telecine pattern of duplicated fields. While this plays fine on a standard analog television, if you want to convert it to another format or process it for an advanced display like HDTV quality can suffer. When working with video encoded this way, you can usually re-create the original film content through the Inverse Telecine or IVTC process.