separates the brightness signal (Y) and the color difference signals (R-Y, B-Y) from the
three primary color signals (RGB).
A process of dividing a signal along the time axis when converting from analog to digital.
Also called digitizing.
Abbreviation for Serial Digital Interface. It is a standard for the packet transmission of
digital video and audio signals. The SDI standard includes the SMPTE259M
specification for SDTV and the SMPTE292M specification for HDTV.
The brightness signal carries luminance information, and contains no color
information. It is also called the Y signal or luminance signal. Black-and-white TV signals
have only a brightness signal. When the brightness signal is combined with color signals,
color images are displayed. The brightness signal expresses the shape, contour and
gradation, while the color signals add color to them. Home VTRs divide video signals
into brightness signals and color signals for processing and recording. In home VTRs,
the brightness is frequency-modulated for recording, and the amplitude of the original
brightness signal is converted to the amplitude of the carrier frequency for transmission.
When the magnitudes of the three primary color image signals are indicated by R, G and
B, the brightness signal is expressed by the following equation: Y = 0.30R + 0.59G +
0.11B. Here, each value is multiplied by a factor in order to adjust it in accordance with
the unique sensitivity of the human eye to color. Unlike electronic imaging devices, the
human eye does not perceive color with uniform sensitivity. The human eye is most
sensitive to green, followed by red, and is least sensitive to blue. These color sensitivity
characteristics are compensated for by the multiplication factors.
Color difference signals:
after the brightness signal (Y) is removed from a component signal. When transmitting
color video signals, it is desirable to use all three color signals, R, G and B, since they
are the three primary colors of light, but this requires a wide bandwidth of about 6 MHz
for each signal. The color difference signal system was developed to improve the
transmission efficiency. For the NTSC signal system, the process of producing a
brightness signal from the R, G and B primary-color image signals is based on the
following equation: Y = 0.30R + 0.59G + 0.11B. This equation can be rewritten as "R - Y
= 0.70R - 0.59G - 0.11B and "B - Y = -0.30R + 0.41G - 0.11B." These equations yield
the following equation: G - Y = -0.51 (R-Y) - 0.19 (B-Y). Since the G-Y signal can be
produced by using 51% of the R-Y signal and 19% of the B-Y signal, only two color
difference signals and the brightness signal need to be actually transmitted. The color
difference signals are given a bandwidth of 0.5 MHz, applied with orthogonal modulation
at an angle of 90° in order to prevent interference with the color subcarrier, and then
placed at the same location for transmission using the same bandwidth as a
A method for superimposing an image on top of a portion of another image. It is called
chroma-key because it removes the chroma (color) level in reference to a standard key.
This image composing technique is commonly used to replace the actual background
with another image in order to make subjects (persons) appear to be at a different place.
Video signals in which the sampling frequencies for the brightness signal and color
difference signals (Y, R-Y, B-Y) have a ratio of 4:1:1.
Sampling is done with a frequency of 13.5 MHz for the brightness signal and a
frequency of 6.75 MHz for the R-Y and B-Y color difference signals. The R-Y and B-Y
color difference signals are sampled for every other scanning line. In other words, only
the brightness signal of some scanning lines is sampled (4:0:0), and the following
scanning lines are sampled at 4:2:2. This system is often used for the low-resolution
625-line format since it is efficient and inexpensive as compared to the 4:2:2 system.
The color difference signals are the components (R-Y, B-Y) that remain