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 wil
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