CPP100
SECTION 6
CPP1OPS
25/10/06
www.snellwilcox.com
Version 1 Issue 6
6.11
Repeat Field Flag for 525 film originated material
Film material, which is scanned by telecine to produce a 525 line output, uses the well known 3:2 pulldown
technique to insert an additional field for each pair of film frames. This has the effect of increasing the output
field rate to 60Hz required for 525 distribution.
The output video signal has a distinctive pattern of 2 fields followed by 3 fields followed by 2 field’s etc... The
five field sequence can be identified by indicating the position of the repeat field as shown below.
Sequence count for 525 film originated material
The five field sequence can be further identified by a specific code for each field in the sequence. In the
example shown below a 3-bit code running from 0 to 4 is used to explicitly identify each field in the 3:2
sequence.
Post produced electronic editing
Although the process of detecting the 3:2 sequence is relatively straight forward, complications arise whenever
material is electronically edited. This causes temporary discontinuities in the 3:2 sequence at the edit point.
Downstream equipment which relies on the accuracy of this flag could be affected if the sequence counter
does not correspond exactly to the video at the edit point. Prefix uses a correlation algorithm to:
•
detect the edit
•
analyse the 3:2 sequence following the edit
•
provide accurate co-timed sequence count at Prefix output