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YIQ (Y for luminance, I for in-phase and Q for quadrature), like YUV, is a color space used in television signals. YIQ is used predominantly by the NTSC television standard for encoding color information. The Y component, like in YUV, is used to encode luminance information, and is the only component used by black-and-white television receivers. The I and Q signals contain the actual color information. The YIQ color space is actually exactly the same as YUV, except the I-Q plane differs from the U-V plane by a simple 33-degree rotation and axis-swap. This rotation puts the I color axis in the orange region of the color space, which is where flesh tones are found. Since the human eye notices incorrect flesh tones more easily than other color defects, I can then be given a higher bandwidth than Q in a severely limited-bandwidth video signal such as NTSC by modifing the circle-based quadrature modulation into an ellipse-based variant, where the I axis is the major axis of the ellipse and the Q axis is the minor axis. Most newer systems from PAL onward, that don't suffer from such tight bandwidth limits, prefer the technically easier-to-use YUV color space.
A formula exists for converting colors from the RGB color space to YIQ. This formula, where R, G, and B are defined on a scale from zero to one, is shown below.
The approximate value of the formula is:
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 + 0.311135BThe approximate value of the matrix is:
The exact value of the formula is:
Y = + 0.299R + 0.587G + 0.114B I = + 0.877(R - Y) cos 33 - 0.492(B - Y) sin 33 = + [(0.877 cos 33)(1 - 0.299) - (0.492 sin 33)(-0.299)]R + [(0.877 cos 33)(-0.587) - (0.492 sin 33)(-0.587)]G + [(0.877 cos 33)(-0.114) - (0.492 sin 33)(1 - 0.114)]B Q = + 0.877(R - Y) sin 33 + 0.492(B - Y) cos 33 = + [(0.877 sin 33)(1 - 0.299) + (0.492 cos 33)(-0.299)]R + [(0.877 sin 33)(-0.587) + (0.492 cos 33)(-0.587)]G + [(0.877 sin 33)(-0.114) + (0.492 cos 33)(1 - 0.114)]BThe exact value of the matrix is:
The approximate value of the inverse matrix is:
The exact value of the inverse matrix is:
True I and Q decoding has, more than likely, been rare in color TVs; it requires more-complex circuits. Some TV engineers say you can see the improvement when true I and Q decoding is used, compared to the simpler compromise R-Y / B-Y decoding.
Color space