DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

EBU Tech. R62, Recommended
dominant field for 625-line 50-Hz
video processing.
A piece of equipment that performs this film-to-video
conversion in realtime is called a telecine. (The term film
scanner ordinarily implies nonrealtime operation.)
In 480i29.97 systems, the film is run 0.1% slow, at
about 23.976 Hz, so that the 5 ⁄2 ratio of 2-3 pulldown
results in a field rate of exactly 59.94 Hz. Figure 36.1
sketches four film frames; beside the set of film frames
is the sequence of video fields produced by 2-3 pulldown.
The 1 and 2 labels at the right indicate the first
and second fields in an interlaced system.
This scheme is often called 3-2 pulldown. However,
SMPTE standards assign letters A, B, C, and D to sets of
four film frames; the A-frame is associated with the
frame without a duplicate (redundant) field, so the
sequence is best described as 2-3, not 3-2.
When a 2-3 sequence is associated with nondropframe
timecode, it is standard for the A-frames to take timecode
numbers ending in 0 and 5.
In a sequence containing 2-3 pulldown, cadence refers
to the temporal regularity of the A-frames. Careful
editing preserves cadence; careless editing disrupts it.
In an interlaced sequence, field dominance refers to the
field parity (first or second) where temporal coherence
is susceptible to interruption due to editing. In principle,
video edits can be made at any field; however, it
is poor practice to make edits anywhere except the
beginning of field one. (See page 403.)
In 480i consumer laserdiscs (now obsolete), video
sequences incorporating 2-3 pulldown were fully
recorded on the disc, despite the 20% waste of storage
capacity. Stepping through disc frames would result in
certain frames displaying content from two different
film frames, in the manner of the static lattice of
Figure 6.12 on page 61. A “white flag” was encoded on
the disc, in the vertical interval associated with each
new film frame; the flag allowed laserdisc players to
correctly weave fields together for playback of stills.
430 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES