In YUV encoding, the chrominance components (U and V) are given equal bandwidth, although usually less than the luminance (Y), since the human visual cortex is more sensitive to brightness than to color. However, it has been shown that our eyes aren't equally sensitive to the different chroma components either - we detect differences in red-to-cyan transitions easier than we detect them in magenta-to-green.

YIQ encoding takes advantage of this by rotating the U and V components 123 degrees around the luminance axis, yielding the new I and Q components. Q can now be more severely filtered (or quantized, when dealing with digital media) than I, without being perceptible to the viewer.

YIQ encoding was widely used in the early days of NTSC, but modern broadcasting equipment usually encodes equiband U and V.

Source: Some MPEG glossary I had lying around.

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