|
B-Y
|
A color-difference signal that results after subtracting Y from B. One of the
receiver's chroma detectors outputs B-Y. Afterward, B-Y combines with Y to
reproduce blue details on the TV screen. On the TV color wheel, B-Y occupies the
location at 180°. Some vector scopes display this same position as 0°.
In either case, the position is 3 o'clock. On a Cartesian graph, this location is
in Quadrant 4. The formula for B-Y is this: 0.11B - (0.30R + 0.59G + 0.11B)
|
|
Burst
|
Or Color burst: The phase reference signal from the TV station. This signal
occurs during the horizontal blanking interval. Burst includes about eight cycles of
the color subcarrier frequency, for U.S. NTSC, 3.579545 MHz. The TV receiver locks
its color reference oscillator to the phase of this signal. The lock persists for
one video line, assuring that incoming chroma reproduces the correct colors. Inter-
ference with the burst signal can cause NTSC phase errors. Modern circuits, such as
ATC (automatic tint controls) and VIR (vertical interval reference)
technology can correct such errors.
|
|
Chroma
|
The color saturation and phase components of the television
signal. During the horizontal blanking interval, the screen is momentarily
black. Meanwhile, the burst signal briefly replaces saturation and phase. The
amplitude of the radio signal represents saturation. The phase (0 to
360°) represents hue.
|
|
CIE
|
International Commission on Illumination (CIE). This is the international organization that
sets standards for color and light. The headquarters are in France. The CIE's Standard
Illuminant D65 determines the color reproduction proportions for most television receivers.
|
|
CCW
|
Counterclockwise: For instance, a vector scope that reads 360° starting at 3 o'clock
(burst) and proceeding toward 2 o'clock.
|
|
CW
|
Clockwise: For instance, a vector scope that reads 360° starting at 9 o'clock
(burst) and proceeding toward 10 o'clock. Also stands for continuous
wave, which is the type of signals that your TV generates to denote color
reference signals. One color reference signal detects R-Y color difference signals. The
other color reference signal detects B-Y color difference signals. The receiver derives
these reference signals from the burst signal.
|
|
Contour, TV Luminance
|
NTSC and other world TV systems assign a gray value to each color. This method of color reproduction is part of
HSB (hue, saturation and brightness) color theory. On NTSC black and white receivers, maximum red (30%)
appears as a medium gray. Meanwhile, maximum green (59%)appears as a light gray. Maximum blue (11%)appears
at as very dark gray. The brightness of any color is the same as its gray or luminance value. The usual
standard for TV luminance is CIE Illuminant D65. By mixing maximum values for each of the three primary colors
and superimposing them, one achieves Illuminant D65. This illuminant value appears as white, and it contains
no color. Web colors don't adhere to the NTSC contour.
|
|
Gamut
|
The palette of available colors. The three-color, NTSC gamut is broader than any
two-color gamut. (A three color system allows the choice of nearly every point on
the color wheel. A two-color system allows the choice of any point on the axis between the
two primaries.) Yet full-color reproduction is available in either system. Naturalistic
pictures are only possible through the careful choice of primary colors. For example,
in two-color systems, red and cyan as primaries. For special effects, a colorist or
artist might instead choose unnatural colors.
|
|
G-Y
|
A color-difference signal that results after subtracting Y from G. The TV station
doesn't transmit G-Y. Instead, the receiver reconstructs G-Y by
inverting and proportionally summing R-Y and B-Y. Afterward, G-Y combines with Y to
reproduce green details on the TV screen. On the TV color wheel,
G-Y occupies the location at 300°. Some vector scopes display this same
position as 240°. In either case, the position is 7 o'clock. On a
Cartesian graph, this location is in Quadrant 3. The formula for G-Y is this:
0.59G - (0.30R + 0.59G + 0.11B)
|
|
HSB
|
Hue, saturation and brightness, a means for defining colors. This is the system that
most television systems use. You'll also find HSB values in many computer "paint" programs,
including Adobe PhotoShop and PC Paint. The hue value is a polar
measurement in degrees. There are 360 degrees. They indicate a position on the rim
of the color wheel. For example, in television, red occurs at about 11 o'clock.
(HTML introduces a 103-degree twist. HTML red appears at 3 o'clock.) In TV and HTML,
saturation is color intensity. For example, red is a saturated color. Pink is
the same color at a lower saturation. That is, pink is red with added white.
Brightness (or luminance, Y) is the value of a color that appears on a
monochrome TV. For example, in television, red makes up 30 percent of luminance. Green
is 59 percent. Blue is only 11 percent. In black-and-white, maximum red appears as medium
gray. Maximum green is light gray. Maximum blue is very dark gray. HTML colors don't
follow this brightness contour. A maximum value for any HTML color is 255 (Hex
value: ff). Translating that into percents, each HTML color accounts for 33 percent
of brightness.
|
|
I
|
The in-phase chroma signal. Signal I describes the saturation values of all
hues on the axis or line between orange and blue-green. Human eyes are particularly
sensitive to hues in this range. The most important color is flesh-tone orange
at 57°. Note: This hue is correct for all human flesh tones. Only the
saturation or lightness of the tone changes between people. Uniquely in NTSC,
the I signal band is three times as wide as the Q signal. (Q is the other chroma
hue signal). The I and Q signals are still narrowband signals. Together, they
equal about one-eighth the definition of the Y signal.
|
|
-I
|
The negative phase of the in-phase chroma signal. Given sufficient luminance, the I signal
would appear as a blue-green hue. On a clockwise vectorscope, this hue occurs at 237°. On a
counterclockwise vectorscope, -I appears at 303°.
|
|
Luminance
|
(Sometimes Y, Luma.) The brightness of the TV picture, or of a pixel. See: Y,
pixel, HSB.
|
|
Luminance Contour, TV
|
See Contour, TV Luminance.
|
|
Mooncam, Apollo TV
|
On the moon, Apollo missions 12 through 17 deployed two slightly different color mooncam
technologies. Westinghouse engineered one system and RCA designed the other. The cameras
produced field-sequential color signals. In both technologies, a three-color wheel with six
segments scanned a hardened video camera tube. Neither camera type was compatible with Earth
TV standards. After reception at earthbound downlink stations, picture signals underwent
conversion to color NTSC, PAL and SECAM.
At a downlink station, the distorted sync was normalized for Doppler shift. Then an analog
recorder copied the signal onto a magnetic disc. The disc built up enough video data to reproduce
one video frame by combining six neighboring video fields: Each Earth frame consists of six lunar
fields: Even red, blue and green fields, and then odd fields of the same colors. A delay line shifted
some moon fields from odd to even or even to odd. After all these changes, one field left the disc
machine as the machine simulataneously began processing the next video frame. After this step, but
before transmission, a proc amp added Y, I and Q channels. Further circuitry modulated video
carriers with the Y, I, Q and sound signals. Possibly a two-color system could have eliminated the
complex operations on the video disc. See our Color Mooncam Page.
|
|
NTSC
|
The television industry organization that set standards for television in the U.S.A. NTSC stands
for National Television System Committee. The NTSC prepared a color standard for analog color TV that
is compatible with black-and-white TV. The new system operates at nearly the same frame and field
rate as monochrome TV requires: (Black and white sets ignore the very slight differences.)
NTSC color also has the same number of definition lines (525). NTSC color even fits into the same bandwidth
as black-and-white TV requires. The FCC accepted this standard in 1953. The first NTSC color shows went on the
air in December 1953 and early 1954. RCA carried on much of the research and legal support for NTSC color. For
example, RCA invented the shadow-mask TV tube. Interestingly, in 1938, Werner Flechsig demonstrated his very
similar tube. An operational Flechsig tube appeared at the International radio exhibition Berlin in 1939. (This was
years before John Baird claimed to have invented his cumbersome, three-neck Telechrome tube.) Yet RCA engineers
never saw Flechsig's invention. Apparently RCA designed its tube independently. Other prominent research
contributions to NTSC color came from Hazeltine and Philco. CBS Hytron engineers invented the first CRT with color
phosphors on the back of the tube envelope. The CBS color tube was the first its kind.
Some NTSC Standards
|
Type
|
Lines / Frame (vertical)
|
Fields / Frame
|
Frames / second
|
Fields / second
|
Lines / second
|
Colors, Transmitted
|
Colors, Reproduced
|
|
Monochrome
|
525 (486 active)
|
2
|
30
|
60
|
15,750
|
None
|
None
|
|
Color
|
525
|
2
|
29.994
|
59.9390
|
15,734
|
2 (I & Q)
|
3 (RGB)
|
|
|
NTSC contour
|
See Contour, TV Luminance.
|
|
NTSC Three-Color Mode
|
An NTSC station that uses the YIQ color model transmits I and Q chroma signals. Through phase
and amplitude variations, these chroma signals describe relationships between colors on the I and Q axes of
the color wheel. The I axis indicates the strength (amplitude) of hues in the range between orange (+I) and
blue-green (-I), which is the opposite of orange. The Q axis indicates the strength of hues in the range between
magenta (+Q) and green-yellow (-Q), which is the opposite of magenta. The receiver converts I and Q signals to
color difference signals: R-Y, G-Y and B-Y. These signals describe large (and in very few sets medium)
details. The luminance signal (Y) provides fine details in monochrome. After the set combines color-difference hues
with the luminance signal (Y), a full-color picture results.
|
|
NTSC Two-Color Mode
|
For medium picture details, NTSC portrays color in 2-color I-Mode instead of
3-color IQ-Mode. The high frequencies of the I signal convey these medium details. These high frequencies run
between 500 kHz and 1.5 MHz. (The corresponding Q signal has no frequency content above 500 kHz and would not be
operative.) Positive excursions of the I signal correspond to orange-red hues, that is, flesh tones. Negative
excursions of the I signal (the -I vector) correspond to blue-green hues. Typically, these hues are useful in
portraying vegetation, sky and marine scenes. The gamut of colors from the I signal is narrower than
the gamut of the combined I and Q signals. Yet for in medium details, the human eye can't perceive any difference
between IQ and I-only color. NTSC receivers that can detect the higher I-frequencies can reproduce both two and
three-color picture content. This content is unavailable on PAL and SECAM sets. These sets use a different color
model.
|
|
Pixel
|
(Sometimes pel.) The smallest resolvable part of a television picture. A contraction of
picture element.
|
|
Q
|
The quadrature-phase chroma signal. Signal Q describes the saturation values of all
hues on the axis or line between green and magenta. Human eyes are not as
sensitive to these hues as to the I-hues. Uniquely in NTSC,
the Q signal band is one-third as wide as the I signal. (I is the other chroma
hue signal). The I and Q signals are narrowband signals. In typical receivers, either I or Q
equals about one-eighth the definition of the Y signal. On a clockwise vectorscope, Q
occurs at 147°. On a counterclockwise vectorscope, Q appears at 33°.
|
|
-Q
|
The negative phase of the quadrature-phase chroma signal. Given sufficient luminance, the Q signal
would appear as a green-yellow hue. On a clockwise vectorscope, this hue occurs at 327°. On a
counterclockwise vectorscope, -I appears at 213°.
|
|
QAM
|
Quadrature amplitude modulation, the transmission mode for analog TV color data
(chroma). In this method, two signals represent two primary hues. The two
signals increase or decrease (amplitude modulate) the intensity of a 3.58
MHz RF subcarrier. The signal intensity changes represent saturation values for
each transmitted color. One signal lags the other one by 90°
(quadrature). The 90-degree Orthagonal) separation prevents
interference between the signals. Before transmission, the station suppresses the
subcarrier. While demodulating the signal, the receiver must reconstruct this
subcarrier. To do that, the receiver uses the burst signal from the station.
|
|
R-Y
|
A color-difference signal that results after subtracting Y from R. One of the
receiver's chroma detectors outputs R-Y. Afterward, R-Y combines with Y to reproduce
red details on the TV screen. On the TV color wheel,
R-Y occupies the location at 90°. CW and CCW vector scopes display this
position the same way, as 90°. The position translates to 12 o'clock.
On a Cartesian graph, this location is on the border between Quadrant 1 and
Quadrant 2. The formula for R-Y is this: 0.30R - (0.30R + 0.59G + 0.11B)
|
|
Y
|
The brightness of the picture. As a brightness standard, many TV manufacturers refer to an industry
reference, typically Illuminant D65: At full Illuminant D65 brightness, displays as apparent white.
In mathematical terms, Y = (0.30R + 0.59G + 0.11B). When Y ascends to 100%, the picture appears
white, and no color displays. When Y descends to 0%, the picture blacks out. Again, no color
displays.
|
|
|