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VIDEO LEVELS AND CHROMA KEY
50 SuperView 3000 User Manual May 9, 2002
B
Most users will use one of the six full amplitude saturated colors, or black or
white, for the key color. (See Table xx).
The situation is more complex if you wish to key on a color other than a full
amplitude saturated color, or if your host computer outputs non-standard signal
levels. You must then understand how chroma key levels are handled by the
system.
The chroma key circuitry in the SuperView is designed to operate with a variety
of frame buffer output levels. The key color acceptance range you specify may
not be centered on the color value you wrote to the host computer frame buffer
(see the following discussion).
high-resolution frame buffers generally output video which falls into one of
three different voltage ranges depending on the design. These voltage ranges
are:
• 0.700 Volt p-p (black level of 0 V)
• 0.714 Volt p-p (black level of 54 mV)
• 1.000 Volt p-p (black level of 0 V)
Probably 98% of computers fall into the first two categories. The voltage levels
are sufficiently close that, in most cases, the same key range will work for both.
Only the peak-to-peak video amplitude is of concern; the SuperView contains
circuitry to cancel any DC offsets. The upper and lower thresholds for the
chroma key are determined by eight bit digital-to-analog converters. The
converters output a voltage in the range -270 mV to +1250 mV.
Consider a computer frame buffer with a 700 mV output voltage range. We
want to key on full amplitude saturated cyan, i.e. no red, full amplitude green
and full amplitude blue. To accomplish this, let’s choose ranges such that we are
looking for a red value less than 25% of maximum, and green and blue values
which are greater than 75% of maximum. These may appear to be relatively
large ranges, but they reserve only 1.6% of the available color gamut for the
TABLE 9. ChromaKeyColor Values–Saturated Colors
Color Red Green Blue ChromaKeyColor
White 100% 100% 100% 525 1250 525 1250 525 1250
Yellow 100% 100% 0% 525 1250 525 1250 -270 175
Cyan 0% 100% 100% -270 175 525 1250 525 1250
Green 0% 100% 0% -270 175 525 1250 -270 175
Magenta 100% 0% 100% 525 1250 -270 175 525 1250
Red 100% 0% 0% 525 1250 -270 175 -270 175
Blue 0% 0% 100% -270 175 -270 175 525 1250
Black 0% 0% 0% -270 175 -270 175 -270 175