GRASS VALLEY 1200, Installation And Service

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3-19
Frame Modules
Key generation is done by scaling the signal from the Background Hue
Correlation circuit using a variable gain comparator. The scaled output is
now a suitable multiplier control signal, is used for coring, and sent to the
key size, position, and softness circuit before going to the keyer module.
The Foreground Hue Correlation circuit output also goes through a force
mask multiplier. The force mask multiplier can be controlled
independently for foreground and background processing by way of
switches. The foreground output signal then goes to a difference circuit
that prevents suppression in the foreground from duplicating suppression
in the background.
The coring circuit removes noise in the video path by forcing the signal to
black when the signal is less than a set level. This process is further
qualified by the key signal, which must be non-zero to cause noise
suppression.
The AutoCKey circuit works in conjunction with the crosshair generator
and the cursor insertion switches. These circuits control the autosetup of
the chroma keyer. The crosshair generator inserts a cursor signal into the
video path. The cursor is positioned by the control panel joystick. The
cursor generator also produces a strobe used by the AutoCKey circuit to
capture luminance and chrominance data at the location identified by the
cursor. The captured data is read by the 1200 CPU for each field and used
to calculate the reference hue, luma suppression level, VGC clip and gain,
and shadow clip level. The operator can select an area of the scene using
the cursor and cause the system to set up a chroma key based on the video
in that area.
The 1200 chroma keyer also has the ability to add a luminance-based
shadow to the chroma key. The luminance Y signal is processed in a
variable gain comparator to generate a shadow control signal that is
combined with the key signal in the NAM. The video outside the key is
suppressed to black causing the shadow part of the new background signal
to be attenuated.
The key size, position, and softness circuit uses vertical and horizontal
filters to reposition, resize and soften key edges. This circuit is composed
of field and line delays, filter circuits, and motion detection. When vertical
processing is not used, most of the circuitry is bypassed.
The outputs of the chroma keyer are chroma key Y, chroma key C, and the
chroma key signal. The output signals then return to the crosspoint module
to be routed to one of the three keyers in the system.