This Recommendation also covers the pixel characteristics that represent a 525- or 625-line interlace digital television image. This Recommendation specifies methods for digitally coding video signals. It includes a 13.5 MHz sampling rate for both 4:3 and 16:9 aspect ratio images with performance adequate for present transmission systems.
ITU-R BT.601-7-2011 history
2011ITU-R BT.601-7-2011 Studio encoding parameters of digital television for standard 4:3 and wide-screen 16:9 aspect ratios