chroma subsampling

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chroma subsampling

The reduction of color resolution in digital component video signals in order to save storage and bandwidth. The color components are compressed by sampling them at a lower rate than the brightness (luma). Although color information is discarded, human eyes are less sensitive to color than to brightness.

YCbCr Is the Digital Equivalent of YUV
The YUV color space is encoded digitally as YCbCr. The luma uses the same Y designation, but U and V become Cb and Cr (for details, see YUV).

YCbCr Is Designated as 4:n:n (Y:Cb:Cr)
The 4 is the sampling rate of the luma, and n:n represents Cb:Cr. Review the illustrations below. 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).


Analog or Digital?
Slightly confusing because the outputs on the back of this DVD player are analog, correctly identified as Y, Pb and Pr, not the digital Y, Cb and Cr.


4:4:4 and 4:4:4:4 (No Color Reduction)
The Cb and Cr colors are sampled at the same rate as the luma (Y). MPEG-2 supports 4:4:4 coding, but this is often considered overkill and not needed. However, when video is converted from one color space to another, it is often resampled first to 4:4:4. Four fours (4:4:4:4) adds a transparency channel (see alpha channel).


Analog or Digital?
Slightly confusing because the outputs on the back of this DVD player are analog, correctly identified as Y, Pb and Pr, not the digital Y, Cb and Cr.


4:2:2 (1/2 the Luma Samples)
Cb and Cr are sampled at half the horizontal resolution of the luma (Y). Co-sited means that Cb/Cr samples are taken at the same time as Y. Considered high quality, 4:2:2 is used in professional recording, including DV (at 50 Mbps), Digital Betacam, DVCPRO 50 and MPEG-2.


Analog or Digital?
Slightly confusing because the outputs on the back of this DVD player are analog, correctly identified as Y, Pb and Pr, not the digital Y, Cb and Cr.


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, and co-sited 4:1:1 is used in DV, DVCAM and DVCPRO formats.


Analog or Digital?
Slightly confusing because the outputs on the back of this DVD player are analog, correctly identified as Y, Pb and Pr, not the digital Y, Cb and Cr.


4:2:0 (1/4 the Luma Samples)
The zero 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 in the computer are converted to analog color difference signals (YPbPr), they start out at 4:4:4 YPbPr. When filtered prior to sampling, they might be characterized as 4:2:2 YPbPr. See YPbPr, YCbCr and YUV.


Analog or Digital?
Slightly confusing because the outputs on the back of this DVD player are analog, correctly identified as Y, Pb and Pr, not the digital Y, Cb and Cr.
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