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YIQ colour-encoding system
A colour-encoding system similar to YUV. The U and
V signals in YUV must be carried with equal bandwidth, albeit less than that of luma. However, the human visual
system has less spatial acuity for magenta-green transitions than for red-cyan. Thus, if signals I and Q are formed
from a 123 degree rotation of U and V respectively, the Q signal can be more severely filtered than I (to about
600 KHz, compared to about 1.3 MHz) without being perceptible to a viewer at typical TV viewing distance. YIQ is
equivalent to YUV with a 33 degree rotation and an axis flip in the UV plane.
Because an analog NTSC decoder has no way of knowing whether an encoder was encoding YUV or YIQ, the decoder cannot
detect whether the encoder was running at 0 degree or 33 degree phase. Thus, in analog usage the terms YUV and
YIQ are often used somewhat interchangeably.
YIQ was important in the early days of NTSC, but most broadcasting equipment now encodes equiband U and V.
YIQ is a color model used in television broadcast systems (North American video standard - NTSC). Colors are split
into a luminance value (Y) and two chromaticity values (I and Q). On a color monitor, all three components are
visible. On a monochrome monitor, only the Y component is visible. The square, two-dimensional visual selector
defines the I and Q values, and the vertical visual selector defines the Y value. All values are scaled from 0
to 255.
back to Y-letter
YUV colour system
A colour-encoding scheme for natural pictures in which
the luminance and chrominance are separate. The human eye is less sensitive to color variations than to intensity
variations, so YUV allows the encoding of luminance (Y) information at full bandwidth and chrominance (UV) information
at half bandwidth.
back to Y-letter
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