The Engineer's Guide to Standards Conversion - Snell

It is possible to identify the 3:2 field sequence in the input signal. The third
repeated field is discarded so that the remaining input consists of exactly two fields
for each film frame. The effective field rate is now 48 Hz, but as pairs of input fields
have come from the same film frame, they can be de-interlaced to recreate the
frames at 24 Hz. This forms the input to a standards conversion process which
outputs 50Hz interlaced video. Whilst the principle appears reasonably simple, there
is some additional complexity because video edits take place without regard to the
3:2 sequence on the tape. The converter must be able to reliably deduce what has
happened on edited material.
Field
repeats
60Hz field
30Hz frame
3
2
TV
frame
same different different same same same different
Fig 4.3.2
same different same same same different different
EDIT
breaks
3:2
sequence
In 3:2 pulldown video, there are two types of frame. One type
contains two fields from the same film frame. The other contains
fields from different frames. A video edit can break the 3:2
sequence and produce a tape with only a single field representing a
film frame.
Fig 4.3.2 shows that there are two types of input frame; one type contains fields
from the same film frame, the other contains fields from different frames. After
editing it is possible to have a film frame which is represented by a single field. In
order to follow what is happening in the input a large number of fields of storage
are required and this makes the converters expensive.
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