#Wish list
Not so. When JPEG performs subsampling, it looks at "u" and "v" pixels of
chrominance information for every "y" pixels of luminance info and considers
them ONE pixel of chrominance (averaged from the "u" and "v" values of those
pixels). The format for specifying subsampling is "y:u:v:" so "4:4:4
subsampling" basically says, "For each 4 pixels of luminance, take the
corresponding 4 pixels of chrominance and treat them as 4 pixels." In other
words, don't subsample at all. The chrominance values of the four pixels are
treated as individual pixels.
But the human eye is more sensitive to abrupt changes in luminance than in
chrominance, so long ago the broadcast experts who designed the NTSC system
figured out that they could save bandwith by treating every two pixels' worth
of chrominance as if it were only one. (Actually, they didn't call them
"pixels" back then — being analog, it was more along the lines of cycles of
the 3.58MHz color burst reference sine wave attached to the front porch of NTSC
scan lines.) Thus we have 4:2:2 subsampling in NTSC broadcast signals. This
basically means that the luminance can change every pixel, but the hue and
saturation can only change every other pixel. Due to human visual perception
limits, this is an acceptable compromise for the bandwidth gained. (Actually,
it's not hue and saturation per se, but "redness" and "blueness," but that's a
LOT more complicated and it's much easier to think in terms of hue and
saturation. The basic point I'm trying to make is the same either way.)
So if JPEG subsamples at 4:2:2, the chrominance averaging is no more than
that performed by NTSC anyway. Nothing is really lost (except some possible
Nyquist artifacting, but that's not likely to be noticed at 30 frames per
second), and quite a bit of extra compressibility at a given quality level can
be gained.
But when JPEG is set to 4:1:1, then we have FOUR pixels of chroma info
averaged into one, over the space of four pixels of luminance info! So hue and
saturation are basically held to the same value over FOUR pixels of changable
luminance! This is TWICE the subsampling that NTSC itself does, so the
chrominance detail of the image IS noticeably reduced when brought to NTSC.
The three various values of subsampling for JPEG exist because there are
different things people do with JPEG. 4:1:1 may be just fine for CD-ROM
animations for games, for instance. It would save more room and reduce
luminance artifacting from JPEG at the expense of less-noticeable chrominance
detail, since CD-ROM animations have to be compressed quite heavily to fit in
the 300kB/sec. bit rate that MPC2-spec double-speed CD-ROMs require. For
output to prepress for glossy magazines, 4:4:4 may be the only acceptable
value, since NTSC isn't involved here and there is no maximum data rate or file
size, and JPEG would be used at all only to save a little disk space over other
lossless compression schemes.
But for true broadcast-quality NTSC, 4:1:1 is unacceptable. 4:2:2 is
acceptable but may result in Nyquist artifacting in the chrominance data.
4:4:4 may be overkill for most situations, but would not permit Nyquist
artifacting to occur in the chrominance data (not that any but the most
highly-trained eye would notice if it did).