Background Information and Techniques - Computer Animation: Algorithms and Techniques - page 491

Table B.6 Video Format Comparison [ 10 ]
Standard Lines Scan Pattern Field Rate (Hz) Aspect Ratio

NTSC 525 2:1 interlaced 59.94 4:3

PAL 625 2:1 interlaced 50 4:3

SECAM 625 2:1 interlaced 50 4:3

go from the end of one scanline to the beginning of the next and the time it takes for the beam to go from
the bottom of the image to the top. The aspect ratio of a 525-line television picture is 4:3, so equal
vertical and horizontal resolutions are obtained at a horizontal resolution of 480 times 4/3, or 640 pixels
per scanline. PAL and SECAM are the other two standards in use around the world ( Table B.6 ). They
differ from NTSC in specifics like the number of scanlines per frame, the field rate, and the frequency
of the color subcarrier, but both are interlaced raster formats. One of the reasons that television tech-
nology uses interlaced scanning is that, when a camera is providing the image, the motion is updated
every field, thus producing smoother motion.

Black-and-white signal
A black-and-white video signal is basically a single line that has the sync information and intensity
signal (luminance) superimposed on one signal. The vertical and horizontal sync pulses are negative
with respect to a reference level, with vertical sync a much longer pulse than horizontal sync. On either
side of the sync pulses are reference levels called the front porch and the back porch. The active scan-
line interval is the period between horizontal sync pulses. During the active scanline interval, the inten-
sity of the signal controls the intensity of the electron beam of the monitor as it scans out the image.

Incorporating color into the black-and-white signal
When color came on the scene in broadcast television, engineers were faced with incorporating the
color information in such a way that black-and-white television sets could still display a color signal
and color sets could still display black-and-white signals. The solution is to encode color into a high-
frequency component that is superimposed on the intensity signal of the black-and-white video.
A reference signal for the color component, called the color burst , is added to the back porch of each
horizontal sync pulse, with a frequency of 3.58 MHz. The color is encoded as an amplitude and phase
shift with respect to this reference signal. A signal that has separate lines for the color signals is referred
to as a component signal. A signal such as the color TV signal with all of the information superimposed
on one line is referred to as a composite signal.
Because of the limited room for information in the color signal of the composite signal, the TV
engineers optimized the color information for a particular hue they considered most important: Cau-
casian skin tone. Because of that, the RGB information has to be converted into a different color space:
YUV. Refer to Figure B.59 . Y is luminance and is essentially the intensity information found in the
black-and-white signal. It is computed by Equation B.170 .

Y ¼ 0 : 299 R þ 0 : 587 G þ 0 : 114 B (B.170)

The U and V of television are scaled and filtered versions of B-Y and R-Y, respectively. U and V are
used to modulate the amplitude and phase shift of the 3.58 MHz color frequency reference signal. The
phase of this chroma signal, C , conveys a quantity related to hue, and its amplitude conveys a quantity
related to color saturation. In fact, the I and Q stand for “in phase” and “quadrature,” respectively. The