YIQ

YIQ is the color space used in the NTSC television standard. I stands for intermodulation, while Q stands for quadrature. Its counterpart in PAL and other systems is YUV.

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.

Formula

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:

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.

See also

• Color space:
  • RGB color model commonly used for color monitors
  • CMYK color model for color printing
  • HSV color space
  • HLS color space
  • RYB color model the traditional color model used by artists.
  • YUV for PAL television