Tonight I took an episode from Alias, Season 2, under the knife (MPressionist.X Pro) on a fact gathering mission for something I was researching.

As with many Prime Time TV shows, the principal photography for Alias is done with 35mm film, not with video. From the looks of things, it would appear that the show is edited in 24p, with the 2:3 telecine being applied when printing to the finished 29.97i broadcast master (which I imagine is Digital BETACAM or something similar).

I was intrigued by the breaks in the 2:3 pulldown flags and started studying them. It would appear that the 2:3 pulldown flags in the MPEG-2 video stream are being produced by an MPEG-2 encoder with inverse telecine on the front end which is determining the pulldown cadence by comparing the actual lines of pixels. The breaks in the 2:3 pulldown flag cadence always occur during "cuts through black" (which includes the title sequences that Alias employs when a change in locale has occurred; Paris, Rome, Istanbul, Yawn.) Presumably, upon encountering rows of identical black pixels, the inverse telecine function throws up its hands and reverts to a straight 29.97i cadence.

Which brings me to another byproduct of having breaks in the 2:3 pulldown flags when you publish MPEG-2 on DVD or DTV; the function of progressive display is impaired. When auditioning content on a high-resolution progressive display, a sequence of frames containing fields doesn't just look bad: to the non-professional, it looks bizarre. My guess is that most consumer DVD players which contain "progressive" DVD output are simply delivering progressive frames for coded pictures which include the "progressive_frame" flag in the picture coding extension. I doubt if most players have the circuitry for frame-buffer based de-interlacing algorithms - perhaps some high-end units do - but most would not.

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