98 How does MPEG handle 3:2 pulldown?
Description
This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
98 How does MPEG handle 3:2 pulldown?
MPEG-1 video decoders had to decide for themselves when to perform 3:2
pulldown if it was not indicated in the presentation time stamps (PTS)
of the Systems layer bitstream. MPEG-2 provides two flags
(repeat_first_field, and top_field_first) which explicitly describe
whether a frame or field is to be repeated. In progressive sequences,
frames can be repeated 2 or 3 times. Simple and Main Profile limit are
limited to repeated fields only. It is a general syntactic restriction
that repeat_first_field can only be signaled (value ==1) in a frame
structured picture. It makes little sense to repeat field pictures in
an interlaced video signal since the whole process of 3:2 pulldown
conversion was meant to convert progressive, film sequences to the
display frame rate of interlaced television.
In the most common scenario, a film sequence will contain 24 frames
every second. The bit_rate element in the sequence header will
indicate 30 frames/sec, however. On average, every other coded frame
will signal a repeat field (repeat_first_field==1) to pad the frame
rate from 24 Hz to 30 Hz:
(24 coded frames/sec)*(2 fields/coded frame)*(5 display fields/4 coded
fields) = 30 display frames/sec
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