There ARE Non-square samples, and their shape IS NOT dependent on the # of scanning lines. There are also Non-Square picture elements in CCD and CMOS image sensors, but their electrical charge is never converted to digital in an analog CCTV camera.

If you are aware of a standard that defines the shape of 480i (NTSC) or 576i (PAL) non-square samples differently than through the use of the luminance sampling frequencies, please bring that to my attention. Thank you.