YUV

YUV

The color encoding system used for analog television worldwide (NTSC, PAL and SECAM). The YUV color space (color model) differs from RGB, which is what the camera captures and what humans view. When color signals were developed in the 1950s, it was decided to allow black and white TVs to continue to receive and decode monochrome signals, while color sets would decode both monochrome and color signals.

Luma and Color Difference Signals
The Y in YUV stands for "luma," which is brightness, or lightness, and black and white TVs decode only the Y part of the signal. U and V provide color information and are "color difference" signals of blue minus luma (B-Y) and red minus luma (R-Y). Through a process called "color space conversion," the video camera converts the RGB data captured by its sensors into either composite analog signals (YUV) or component versions (analog YPbPr or digital YCbCr). For rendering on screen, all these color spaces must be converted back again to RGB by the TV or display system.

Mathematically Equivalent to RGB
YUV also saves transmission bandwidth compared to RGB, because the chroma channels (B-Y and R-Y) carry only half the resolution of the luma. YUV is not compressed RGB; rather, Y, B-Y and R-Y are the mathematical equivalent of RGB. See color space conversion and YUV/RGB conversion formulas.

Composite Video and S-video
The original TV standard combined luma (Y) and both color signals (B-Y, R-Y) into one channel, which uses one cable and is known as "composite video." An option known as "S-video" or "Y/C video" keeps the luma separate from the color signals, using one cable, but with separate wires internally. S-video is a bit sharper than composite video.

Component Video
When luma and each of the color signals (B-Y and R-Y) are maintained in separate channels, it is called "component video," designated as YPbPr when in the analog domain and YCbCr when it is digital. Component video is the sharpest of all.

The Term Is Generic
In practice, YUV refers to the color difference encoding system whether composite or component, and "YUV," "Y, B-Y, R-Y" and "YPbPr" are used interchangeably for analog signals. Sometimes, "YCbCr," which is digital, is used interchangeably as well, which does not help to clarify the subject. See YPbPr, YCbCr, luma, ITU-R BT.601 and YIQ.

Video Connections

Analog video can be transmitted as composite video, S-video or component video. High-end consumer and professional equipment uses component analog video (YPbPr) and three separate cables.

YUV Out

The top diagram shows how YUV signals are mixed and distributed to outside connectors, and the device on the bottom shows the actual ports from an NVIDIA display adapter. (Bottom image courtesy of NVIDIA Corporation.)

Analog or Digital?

The outputs on the back of this DVD player are really analog (YPbPr), but designated as Pb/Cb and Pr/Cr just to cover all bases. YPbPr and YCbCr are often used interchangeably, and erroneously.