GRASS VALLEY zodiak, User Manual

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48 Zodiak User Manual
Section 2 — Concepts
summed with the background. Key fill video must be shaped using the key
cut signal actually used to create the hole in the background. Video shaped
with a different key cut signal will not key correctly.
The Zodiak system, as well as some external devices like DPMs, can also
divide a shaped video signal by its own key signal. The result is called
unshaped video. For optimum image quality, shaping and unshaping oper-
ations should be minimized. In general, devices should be installed and
configured in a manner that prevents them from having to unshape
incoming video just to reshape it again afterwards.
Specifying whether source signals are shaped or unshaped is part of the
Zodiak system configuration process (source definition). Properly shaped
video is an important aspect of keying. See
Properly and Improperly Shaped
Video on page 58 for more information.
Note that an unshaped signal viewed directly will show harsh edge arti-
facts due to dividing by a small number. This is normal and expected.
Keying this signal will clean up its appearance.
Key Control Signal Adjustment
During keying, the selected key cut signal can be converted into a key
control signal. It is the key control signal that actually cuts the hole in the
background video. Adjusting the key control signal (Clip and Gain) is
essential in the keying process. The art of setting up a good key is to use just
enough Gain to suppress any imperfections in the incoming key signals.
Setting Gain too high can cause ragged key edges.
The Zodiak system provides two methods for adjusting the key control
signal, Clip and Gain, and Clip Hi and Clip Lo. Note that the same basic
keying process is controlled by either of these methods.