| | Definition: | | The U and V signals above 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 it does for red-cyan. Thus, if signals I and Q are formed from a 123 degree rotation of U and V respectively [sic], 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. The first edition of W.K. Pratt "Digital Image Processing", and presumably other authors that follow that bible, has a matrix that erroneously omits the axis flip; the second edition corrects the error.
Since an analog NTSC decoder has no way of knowing whether the encoder was encoding yuv or YIQ, it cannot detect whether the encoder was running at 0 degree or 33 degree phase. 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.
The D-2 composite digital DVTR (and the associated interface standard) conveys ntsc modulated on the YIQ axes in the 525-line version and pal modulated on the YUV axes in the 625-line version. |
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