B-Y
B-Y – One of the color difference signals used in the NTSC system, obtained by subtracting luminance from the blue camera signal. This is the signal that drives the horizontal axis of a vectorscope. The human visual system has much less acuity for spatial variation of color than for brightness. Rather than conveying RGB, it is advantageous to convey luma in one channel, and color information that has had luma removed in the two other channels. In an analog system, the two color channels can have less bandwidth, typically one-third that of luma. In a digital system each of the two color channels can have considerably less data rate (or data capacity) than luma. Green dominates the luma channel: about 59% of the luma signal comprises green information. Therefore it is sensible, and advantageous for signal-to-noise reasons, to base the two color channels on blue and red. The simplest way to remove luma from each of these is to subtract it to form the difference between a primary color and luma. Hence, the basic video color-difference pair is (B-Y), (R-Y) [pronounced “B minus Y, R minus Y”]. The (B-Y) signal reaches its extreme values at blue (R=0, G=0, B=1, Y=0.114, B-Y=+0.886) and at yellow (R=1, G=1, B=0, Y=0.886, B-Y=-0.886). Similarly, the extreme of (R-Y), +-0.701, occur at red and cyan. These are inconvenient values for both digital and analog systems. The color spaces YPbPr, YCbCr, Photo YCC and YUV are simply scaled versions of (Y, B-Y, R-Y) that place the extreme of the color difference channels at more convenient values.
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