7.5. MPEG-2 Compression using YCrCb color subsampling.
The following section is a brief explanation on why the format of video data using the YCrCb color system contribute to the reduction of storage for video stream. Most if not all commercial DVDs employ MPEG-2 as the compression scheme for encoding video stream. Therefore, knowledge on YCrCb color subsampling can be beneficial or at least a prerequisite to DVD authors who want to explore the details of this technical area further.

7.5.1. Color Space.
A color space is a mathematical representation of a set of possible colors for a given color model. There are three main color models:
RGB (Red-Green-Blue, used in computer graphics and color television),
YIQ, YUV, and YCrCb (used in broadcast and television systems),
CMYK (Cyan-Magenta-Yellow-Black, used in color printing).

When working with computer graphics or digital video, RGB and YCrCb are the most common color models. Computer monitors display RGB (red-green-blue) color; every color displayed on screen is a mixture of different shades of red, green, and blue. A 16-million color display has 256 shades for each color component.

YCrCb is the standard for DVD video and is somewhat more abstract to visualize. YCrCb is the color space defined by Recommendation ITU-R BT.601 (formerly know as CCIR-601) standard, where Y is the luminance component and Cb and Cr are the chrominance (color difference) components. YCrCb is the method of color encoding for transmitting color video images while maintaining compatibility with black- and-white video. This type of encoding has traditionally been used in composite television. This method uses less bandwidth than the three separate video signals in an RGB video transmission in computers. Another reason for using YCrCb signal is that the human eye is less sensitive to chrominance than luminance. Compression algorithms can take advantage of this phenomenon and subsample the values of Cb and Cr without significant visual degradation of the original color signal and achieve very high degree of compression.

YCrCb consists of two major components: luminance (Y) and chrominance (CrCb or also known as UV). Luminance describes the black-and-white component of a video signal (pixel color). The amount of luminance or Y-component of a video signal indicates its brightness (the amount of light intensity). Certain amount of luminance is required to make a color pixel visible to the human eyes. Chrominance (or chroma) describes the color portion of the video signal which includes the hue and saturation information. Hue indicates the color tint of an image. Saturation describes the condition where the output color is constant, regardless of changes in the input parameters. The chrominance components of YCrCb include the color-red (Cr) component and the color-blue (Cb) of the color.

Digital coding in the form of YCrCb signals can represent a substantially greater gamut of signal values than can be supported by the corresponding ranges of RGB signals. Therefore, it is possible to produce YCrCb signals which would result in out-of-range values when converted to RGB. To prevent overflowing conditions, clipping is applied to the YCrCb signals before performing the RGB conversion. Clipping can be applied in a way that maintains the luminance and hue values and minimize the subjective impairment by sacrificing only saturation.