Found this bit of at
www.dvddemystified.com/dvdfaq.html
"Picture dimensions are at maximum 720x480 (for 525/60 NTSC display) or 720x576 (for 625/50 PAL/SECAM display). Pictures are subsampled from 4:2:2 ITU-R BT.601 down to 4:2:0 before encoding, allocating an average of 12 bits/pixel in Y'CbCr format. (Color depth is 24 bits, since color samples are shared across 4 pixels.)"
Now if I only understood what it means...