What does 4:2:2, 4:2:0 or 4:4:4 sampling mean?

Q: Dear Q-ru,

When referencing certain kinds of video I sometimes hear people say things like:  4:2:2, 4:2:0 or 4:4:4. What are they referring to and what is the difference?

Thanks,
Brad

A:  Brad:

4:4:4, 4:2:2, 4:1:1, 4:2:0 -- You may have heard these numbers mentioned in various discussions of digital and HD video, but what do these numbers mean?  Where did they come from? 

Well, in the early 80’s tests were done to determine the sample formats and rates for the color space of digital video.  The eventual sample structure used for SDI video ended up being roughly 4 times the base sample rate chosen.

The first number in the sequence, “4”, is for luminance (Y) or the black and white information.  This is where most of the picture detail is.  Early tests in television human vision discovered a greater sensitivity to black and white information, while the color is filled in with less detailed areas of the human eye.  This means a reduction in
the color information isn’t as recognizable by your eye as a reduction in the black and white information.  This is what the 2nd and 3rd numbers in the sequence are for.  They refer to the red (CR) and blue (CB) channels of the video signal.  Green is not sent, as you can calculate green from red, blue and luminance information.  So in 4:2:2 systems the color information (CR, CB) is sampled at half the rate of the luminance information (Y).

4:4:4 video is similar, but in this format all the color information is sent. It can also be applied to RGB signals in which case RGB computer graphics are really 4:4:4.

For HD the sampling rates are 5.5 times greater but the “4-based” nomenclature has been extended to the world of HD video as well.

Here are some sampling structures along with several respective video formats that fall in these structures:

• 4:4:4 Systems – MPEG-4, HDCAM SR, P2 (select models)

• 4:2:2 Systems – D-1, D-5, DigitBeta, DVCPRO 50, DVCPRO HD, P2, HDCAM, XDCAM HD

• 4:1:1 Systems – NTSC DV, DVCAM, DVCPRO, HDV

• 4:2:0 Systems (Vertically interleaved & Co-sited) – PAL DV, DVD, mainprofile MPEG-2, HDV, XDCAM EX

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