How HDTV handles 24 hz films, NTSC origin material...

Archived from groups: alt.tv.tech.hdtv (More info?)

ATSC Guide to the Use of the ATSC Digital Television Standard 4 Oct 95

5.2.4 Film mode

When a large fraction of pixels do not change from one frame in the image
sequence to the next, a video encoder may automatically recognize that the
input was film with an underlying frame rate less than 60 fps. In the case
of 24 fps film material that is sent at 60 Hz using a 3:2 pull-down
operation, the processor may detect the sequences of three nearly identical
pictures followed by two nearly identical pictures, and only encode the 24
unique pictures per second that existed in the original film sequence. When
24 fps film is detected by observation of the 3:2 pull-down pattern, the
input signal is converted back to a progressively scanned sequence of 24
frames per second prior to compression. This avoids sending redundant
information, and allows the encoder to provide an improved quality of
compression. The encoder indicates to the decoder that the film mode is
active. In the case of 30 fps film material that is sent at 60 Hz, the
processor may detect the sequences of two nearly identical pictures followed
by two nearly identical pictures. In that case, the input signal is
converted back to a progressively scanned sequence of 30 frames per second.