Science Fair Project Encyclopedia
The Y component represents the luminance information, and is the only component used by black-and-white television receivers. I and Q represent the chrominance information. In YUV the U and V components can be thought of as x and y coordinates within the colorspace. I and Q can be thought of as a second pair of axes on the same graph, rotated 33° from V and U, respectively. Thus, I and Q are simply another way of locating a point in the U and V plane.
The reason for doing this is to take advantage of a trait of the human eye. It is more sensitive to changes in the orange-blue (I) range than in the purple-green range (Q). Thus less bandwidth is required for Q than for I. Broadcast NTSC limits I to 1.3 MHz and Q to 0.5 MHz, which keeps the bandwidth of the overall signal down to 4.2 MHz. In YUV systems, since U and V both contain information in the orange-blue range, both components must be given the same amount of bandwidth as I to achieve similar color fidelity.
True I and Q decoding in television receivers is rare, because of the costs of implementation.
This formula approximates the conversion from the RGB color space to YIQ. R, G and B are defined on a scale from zero to one:
|Y||= 0.299R + 0.587G + 0.114B|
|I||= 0.735514(R - Y) - 0.267962(B - Y)|
|= 0.595716R - 0.274453G - 0.321263B|
|Q||= 0.477648(R - Y) + 0.412626(B - Y)|
|= 0.211456R - 0.522591G + 0.311135B|
or using matrices
Two things to note:
- The top row is identical to that of the YUV color space
- If then . In other words, the top row coefficients sum to unity and the last two rows sum to zero.
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