Audio/Video Glossary

A term used to describe the separate video signal components used in S-video based videotape formats such as Hi-8 and S-VHS.

YELLOW BOOK

(See CD-ROM)

YIQ (YC_bC_r - Luminance/In phase/Quadrature phase)

A bandwidth optimized, matrixed, composite video signal format used by NTSC video systems employing separate luminance and two derived blue and red chroma components derived from the YUV format.

The modulated carriers (U) and (V) (phased 90 degrees apart) are what carry the color information in an NTSC-encoded composite color picture. This is called “quadrature”. We call the modulated carriers “I” and “Q” in NTSC ("In phase" and "Quadrature phase"). I and Q are bandwidth-limited differently so they’ll fit into the NTSC transmission spectrum.

The human visual system has less spatial acuity for magenta-green transitions than it does for red-cyan. Thus, if signals I and Q are formed from a 123 degree rotation of U and V respectively, the “Q” signal can be more severely filtered than “I” without being perceptible to a viewer at typical TV viewing distance. YIQ is equivalent to YUV with a 33 degree rotation and an axis flip in the UV plane. The first edition of W.K. Pratt “Digital Image Processing”, and presumably other authors that follow that bible, has a matrix that erroneously omits the axis flip; the second edition corrects the error.

Since an analog NTSC decoder has no way of knowing whether the encoder was encoding YUV or YIQ, it cannot detect whether the encoder was running at 0 degree or 33 degree phase. In analog usage the terms YUV and YIQ are often used somewhat interchangeably. YIQ was important in the early days of NTSC but most broadcasting equipment now encodes equiband U and V.

The D-2 composite digital DVTR (and the associated interface standard) conveys NTSC modulated on the YIQ axes in the 525-line version and PAL modulated on the YUV axes in the 625-line version. (See YUV)

YUV (YC_bC_r - Luminance/Chroma R-Y/Chroma B-Y)

A bandwidth optimized, matrixed, composite video signal format employing separate luminance and two derived blue and red chroma signals. The Y (or luminance) component is derived from RGB signals by adding the Red, Green and Blue channels together in a specific formula to make a Black & White signal (the formula is 30% Red, 59% Green, and 11% Blue). R-Y is derived by taking the newly-created luminance signal and subtracting it from the raw Red signal. Since we already have a luminance signal (Y), we don’t need the Y portion of the red component so subtracting it leaves only the color difference component we call R-Y. We do the same thing with the Blue channel, subtracting Y from it leaving only the color component of blue or B-Y. We do not need to make a G-Y component because we have the others, so we can matrix them to get what’s left, which is Green.

(U) and (V) are the names of the color components used to modulate the color subcarrier. That is, Y, R-Y and B-Y are further matrixed into YUV in order to maximize the bandwidth to skintone colors (which are primarily red). One of them (I think it’s U) is used to modulate the color subcarrier at 57 degrees from the color burst reference (along the vector that is primarily red/orange). V is used to modulate the subcarrier at 90 degrees from U.

This professional composite video signal format requires special equipment capable of handling matrixed signals. Interface devices are used to link the various systems, i.e., RGB, Y/C, YUV and YIQ (a system similar to YUV).

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