3-18
Section 3 Ñ Functional Description
Chroma Keyer Submodule
The crosspoint module sources four buses to the chroma keyer (see
Figure 3-7 on page 3Ð20). Two of these carry Y and C signals in standard
decoded 4:2:2 format. The other two carry odd and even chroma samples
from a full bandwidth 4:4:4 chroma key source. For normal operation of the
switcher, the full bandwidth buses are not usedÑthe 4:2:2 chrominance
signal is upsampled and substituted for the full bandwidth buses. The
upsampling or interpolation is followed by a switch that allows normal or
full bandwidth operation. The signal after the switch has U and V samples
occurring at 13.5 MHz rate.
These U and V signals describe the chroma content of the input signal. The
Hue Correlation circuits compare the chroma content to a reference hue to
produce a signal that describes how closely the input signal color is to the
reference hue. The closer the hue in the input signal is to the reference hue,
the larger the response is at the output of the Hue Correlation circuit (if the
reference hue is blue, the output signal will be greatest in the blue areas of
the input signal).
Other functions associated with the Hue Correlation circuit are the
Aperture Control and the Fringe Control. The Aperture Control
determines how sensitive the circuit is to color matching. As aperture is
decreased, the circuit requires tighter matching in the hue for the same
response. The Fringe Control causes the Hue Correlation circuit to respond
to areas in the input scene that have little or no chroma.
The two Hue Correlation circuits on the 1200 chroma keyer are called
background and foreground. The Background Hue Correlation circuit is
used for background color suppression and for generation of the chroma
key signal. The Foreground Hue Correlation circuit affects colors in the
foreground that have been polluted by the background. The signal output
from the foreground circuit is used only for foreground color suppression
and has no effect on the generation of the key signal.
The output of the Background Hue Correlation circuit goes to a multiplier
where it can be multiplied to zero by the force mask signal coming from the
soft box mask generator. A zero level signal represents no response and
results in no suppression or key generation, a ÓblueÒ part of the scene is
preserved. After the multiplier, the signal can be processed for noise
suppression. The noise suppression is done using a circuit with a coring
function that measures the level and if the level does not exceed a set level
the output does not respond. Low level noise does not cause suppression
or key generation.
The signal then goes to background suppression. This is done by shaping
a matte signal with the signal from the correlation circuit. The shaped
matte signal (generally the same color as the background) is subtracted
from the chroma keyer input signal. The color background is suppressed to
black.
Summary of Contents for 1200
Page 10: ...x Contents ...
Page 16: ...xvi Safeguards and Notices ...
Page 74: ...2 46 Section 2 Ñ Installation ...
