EVS XS - 4.4. Network Transfers

XS Series DISK RECORDER - Version 10.03 - Hardware
Technical Reference Manual
Issue
10.03.B
EVS Broadcast Equipment SA
–
March 2010
23
1.7.9 INTERPOLATION
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 by-side effect is the
alternation of original fields (perfectly focused) and interpolated fields
(unfocused), resulting in a "pumping" video signal.
4-LINE INTERPOLATOR
The 4-line interpolator uses a more sophisticated calculation based on the 4
neighboring lines. By using suitable coefficients for the weight of each line in the
resulting calculation, we apply this interpolation to all fields
. The final result is a
permanently, slightly unfocused picture. The advantage is a stable output signal
with no jitter and no "pumping", but the vertical bandwidth is even more reduced.
The interpolator is of course always
disabled at 100% playback speed, because
there is no parity violation.
EVS use the same techniques with the Super Slow Motion disk recorder, working
with all models of Super Motion cameras (150/180 Hz). The only difference