4.5.
Video Interpolation
Introduction
The playing back of smooth slow motion pictures carries specific issues: since some
fields must be repeated at regular interval to provide the video at the playback speed
required by the operator, parity violation appears regularly on the output video signal. This
issue is specific to interlaced formats (525i, 625i and 1080i) and does not concern
progressive formats (720p and 1080p).
If O and E represent respectively the odd and even fields of a standard video signal
(50/60 Hz), we have:
The original video signal:
◦
O E O E O E O E O E O E O E O E
The output video signal at 50% speed:
◦
O
O E
E O
O E
E O
O E
E O
O E
E
The output video signal at 33% speed:
◦
O
O
O E
E
E O
O
O E
E
E O
O
O E
The output video signal at 25% speed :
◦
O
O
O
O E
E
E
E O
O
O
O E
E
E
E
Fields with parity violation are shown in bold, underlined letters. As it appears from the
above table, whatever the playback speed (with the exception of the normal 100%
playback speed), a number of fields violate the normal parity of the output signal. This
parity violation induces a 1-line shift of the field, resulting in a vertical jitter of the picture.
The jitter frequency depends upon the chosen playback speed.
To avoid this phenomenon and provide a stable output picture, EVS developed 2 types of
line interpolator: 2-line and 4-line interpolators. The interpolation process can be enabled or
disabled by the operator on all EVS slow motion systems.
2-Line Interpolator
The 2-line interpolator actually generates a new field, when the original field is in parity
violation. Each line of this new field is calculated by a weighted average of the 2
neighboring lines. This process solves the problem of parity violation and vertical jitter, but
the drawback is a reduction of the vertical resolution on the interpolated fields, that appear
unfocused. Another side effect is the alternation of original fields (perfectly focused) and
interpolated fields (unfocused), resulting in a "pumping" video signal.
44
4. Software Specifications
EVS Broadcast Equipment SA
Issue 12.05.C - November 2014
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