VIDEO LEVELS AND CHROMA KEY
50 SuperView3000 User Manual May9, 2002
B
Mostusers will use one of the six full amplitude saturated colors, or black or
white, fort he key color. (See Ta ble xx ).
Thesituation is more complex if you wish to key on a color other than a full
amplitudesaturated color, or if your host computer outputs non-standard signal
levels.You must then understand how chroma key levels are handled by the
system.
Thechroma key circuitry in the SuperView is designed to operate with a variety
offrame buffer output levels. The key color acceptance range you specify may
notbe centered on the color value you wrote to the host computer frame buffer
(seethe following discussion).
high-resolution frame buffers generally output vid eow hich falls into one of
threedifferent voltage ranges depending on the design. These voltage ranges
are:
•0.700Volt p-p (black level of 0 V)
•0.714Volt p-p (black level of 54 mV)
•1.000Volt p-p (black level of 0 V)
Probably98% of computers fall into the first two categories. The voltage levels
aresufficiently close that, in most cases, the same key range will work for both.
Onlythe peak-to-peak video amplitude is of concern; the SuperView contains
circuitryto cancel any DC offsets.The upper and lower thresholds for the
chromak ey arede termined byeight bit digital-to-an alogco nverters.The
convertersoutput a voltage in the range -270 mV to +1250 mV.
Consider a computer frame buffer with a 700 mV output voltage range. We
wantto key on full amplitude saturated cyan, i.e. no red, full amplitude green
andfull amplitude blue. To accomplish this, let’s choose ranges such that we are
lookingfor 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
largeranges, but they reserve only 1.6% of the available color gamut for the
TABLE9. ChromaKeyColor Values–Saturated Colors
Color Red Green Blue ChromaKeyColor
White 100% 100% 100% 525 1250 525 1250 525 1250
Yellow 100% 100% 0% 5251250 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% 5251250 -270 175 5 25 1250
Red 100% 0% 0% 525 1250 -270 175 -270 175
Blue 0% 0% 100% -270175 -270 175 525 1250
Black 0% 0% 0% -270175 -270 175 -270 175