ZDNet Definition for: Chroma Subsampling

A reduced color resolution in digital component video signals. To accommodate storage and bandwidth limitations, the two color components (Cb, Cr) in digital video signals are compressed by sampling them at a lower rate than the brightness (Y). Color information is actually discarded.

YCbCr is the YUV color space recorded digitally. Y is brightness (luma), and Cb and Cr are the U and V color difference signals (see YUV for details). In chroma subsampling, only the colors are compressed, not the luma because the eye is more sensitive to brightness than to the color components.

The 4:4:4, 4:2:2, 4:2:0...
YCbCr is designated as 4:n:n. The 4 represents a sampling rate of 13.5 MHz, which is the standard frequency (ITU-R BT.601) for digitizing analog NTSC, PAL and SECAM. The next two digits represent the Cb and Cr rate. Review the illustrations below for details. Each 8x8 matrix represents a "macroblock" of 64 pixels in a video frame. The pink squares are the pixel locations where the sample is taken. Sony's HDCAM uses a different notation because it compresses both the luma and the colors (see 3:1:1).




4:4:4 (Cb/Cr Same as Luma)
Cb and Cr are sampled at the same full rate as the luma. MPEG-2 supports 4:4:4 coding, but having the same number of color difference samples as the luma is considered overkill and not worth the additional bandwidth to transmit it. When video is converted from one color space to another, it is often resampled to 4:4:4 first.




4:2:2 (1/2 the Luma Samples)
Cb and Cr are sampled at half the horizontal resolution of Y. Co-sited means that Cb/Cr samples are taken at the same time as Y. 4:2:2 color sampling is widely used and considered very high quality. It is used for prosumer and professional digital video recording, including DV (at 50 Mbps), Digital Betacam and DVCPRO 50 and is an option in MPEG-2.




4:1:1 (1/4 the Luma Samples)
Cb and Cr are sampled at one quarter the horizontal resolution. Co-sited means that Cb/Cr samples are taken at the same time as the Y. Co-sited 4:1:1 is used in DV, DVCAM and DVCPRO formats.




4:2:0 (1/4 the Luma Samples)
The zero in 4:2:0 means that Cb and Cr are sampled at half the vertical resolution of Y. MPEG-1 and MPEG-2 use 4:2:0, but the samples are taken at different intervals. By the time MPEG-2 came along, it was known that 4:2:2 coding was often converted to 4:2:0, which is why MPEG-2 sampling more closely lines up with the 4:2:2 pattern. H.261/263 also uses 4:2:0.

Analog Too
Engineers use the 4:n:n nomenclature loosely to refer to relative bandwidths of analog signals. For example, if digital RGB signals (computer graphics) are converted to analog color difference signals (YPbPr), it starts off at 4:4:4 YPbPr. When appropriately filtered prior to sampling, it might be characterized as 4:2:2 YPbPr, which is not an accurate use of the terminology because it is not digital, but it is used. See YCbCr, ITU-R BT.601 and YUV.




Analog or Digital?
No wonder confusion reigns. These outputs on the back of this DVD player are analog, correctly identified as Y, Pb and Pr, not the digital Y, Cb and Cr.

Reproduced with permission from Computer Desktop Encyclopedia. Copyright (c) 1981-2009 The Computer Language Company Inc. All rights reserved.