The following illustration shows how color subsampling by the DVD-Video standard 4:2:0 format works. Four pixels in the small quare shown in the below picture has the RGB values: pixel 1 (247, 166, 247), pixel 2 (239, 158, 239), pixel 3 (255, 158, 247), and pixel 4 (247, 150, 239). YcrCb values in 4:4:4 sampling format yields: pixel 1 (199, 162, 155), pixel 2 (191, 162, 155), pixel 3 (197, 169, 156), and pixel 4 (189, 169, 156). Averaging the RGB values of pixel 1 and 3 yields (251, 162, 247) which is equivalent to YCrCb (198, 165, 155). In MPEG-2 4:2:0 sampling format, the subsampling values of Cr=165 and Cb=155 are used with the original Y values of 199, 191, 197, and 189 for pixels 1, 2, 3, and 4, respectively.

This subsampling process causes loss of color information, but differences when compared to the original picture are not discernable by the human eye. The supplied DVD-Color 2.16 can be used to convert values between the RGB and YCrCb color space.

To represent a two-by-two square of 4 pixels in 4:4:4 sampling format requires (4 pixels) * (8 Y-bits/pixel + 8 Cr-bits/pixel + 8 Cb-bits/pixel) = 96 total bits

To represent a two-by-two square of 4 pixels in 4:2:0 sampling format requires (4 pixels) * (8 Y-bits/pixel) + 8 Cr-bits + 8 Cb-bits = 48 total bits

The 4:2:0 sampling format achieves a 50% reduction in the number of bits required to represent digital video with almost no noticeable loss of color information. This is an effective form of compression. Standard D1 resolution NTSC television displays 720 pixels per line and 480 lines per video frame. Each pixel is redrawn 30 (actually 29.97) times per second. The pixel display rate for D1 resolution is 10.4 million pixels per second as following:

720 pixels/line * 480 lines/frame * 30 frames/second = 10.4 million pixels/second
For PAL and SECAM video, the pixel rate for D1 resolution is also 10.4 million pixels per second as following:
720 pixels/line * 576 lines/frame * 25 frames/second = 10.4 million pel/second

A 2-hour movie requires the storage and transmission of the following
2 hour * 3600 seconds/hour * 10.4 million pixels/second = 75 billion pixels

Using a standard representation with 8 bits of luma data at full resolution and 8-bits of each of red and blue chroma difference components with a 4:2:0 chroma sampling format, 900 billion bits of data are required for a 2-hour movie as following:

(4 pixels) * (8 Y-bits/pixel) + 8 Cr-bits + 8 Cb-bits = 48 total bits/4 pixels or the equivalent of 12 bits/pixel.
12 bits/pixel * 75 billion pixels = 900 billion bits

This means that over 100 gigabytes of video data is required to store a typical 2-hour movie. A DVD disk holds about 9 gigabytes of data. It is obvious that additional compression steps need to be performed so that video data get reduced by more than a factor of 10 to fit a typical movie onto a DVD disc.