What are 4:2:2, 4:1:1, and 4:2:0?

These are all shorthand notations for different sampling structures for digital video. They are also used for CIF and QSIF and suchlike MPEG frame sizes, but in the discussion that follows, I focus on the numbers for SDTV (standard-definition TV) digitized to the ITU-R-BT601 standards: 13.5 MHz sample frequency and 720 pixels per line.

The first number refers to the 13.5 MHz sampling rate of the luminance: "4" because (a) it's nominally almost approximately sort of four times the NTSC and/or PAL color subcarrier frequencies, and (b) because if it's "4" the other numbers can be integers whereas if it were "1" the formats would be "1:0.5:0.5", "1:0.25:0.25", and "1:0.5:0" respectively, and which would you rather try to read off in a hurry? The 13.5 MHz sampling yields 720 pixels per scanline in both 525/59.94 and 625/50 systems (NTSC and PAL/SECAM). This number applies to D-1, D-5, Digital Betacam, BetaSX, Digital-S, and all the DV formats just the same.

The other two numbers refer to the sampling rates of the color difference signals R-Y and B-Y.

In 4:2:2 systems (D-1, D-5, DigiBeta, BetaSX, Digital-S, DVCPRO50) the color is sampled at half the rate of the luminance, with both color-difference samples co-sited (located at the same place) as the alternate luminance samples. Thus you have 360 color samples (in each of R-Y and B-Y) per scanline.

In 4:1:1 systems (NTSC DV & DVCAM, DVCPRO) the color data are sampled half as frequently as in 4:2:2, resulting in 180 color samples per scanline. The U and V samples are considered to be co-sited with every fourth luminance sample. Yes, this sounds horrible -- but it's still enough for a color bandwidth extending to around 1.5 MHz, about the same color bandwidth as Betacam SP (which, were it a digital format, would be characterized as a 3:1:1 format).

So where does 4:2:0 (PAL DV, DVD, main-profile MPEG-2) fit in? 4 x Y, 2 x R-Y, and 0 x B-Y? Fortunately not! 4:2:0 is the non-intuitive notation for half-luminance-rate sampling of color in both the horizontal and vertical dimensions. Chroma is sampled 360 times per line, but only on every other line. The theory here is that by evenly subsampling chroma in both H and V dimensions, you get a better image than the seemingly unbalanced 4:1:1, where the vertical color resolution appears to be four times the horizontal color resolution. Alas, it ain't so: ITU-R-601 pixels are taller than they are wide, and interlace already diminishes vertical resolution; as a result, multigeneration work in 4:2:0 is much more subject to visible degradation than multigeneration work in 4:1:1.

"Now how much would you pay? But wait, there's more!" In US implementations of 4:2:0, the color samples are supposed to be vertically interleaved with luminance, whereas in European 4:2:0 they're supposed to be co-sited. Practically speaking, this is a headache for developers of codecs, encoders, and DVEs, but for DV purposes it's not especially exciting, since only European DV is 4:2:0.