The NTSC standard does not call for a resolution of 720x540.
In fact, the NTSC standard was adopted in 1952, and is purly analog.
There is no specified "resolution", there were no "pixels" when the NTSC
standard was adopted.
However, for what it's worth, the NTSC standard does use a 525-line scan
system (in the US) comprised of two fields, an even and an odd field, of
262.5 scan lines each (and yes, there really are half scan lines, and
you can actually see them on a good tv set with a vertical hold
control). However, about 20 lines are lost to the vertical blanking
interval (the black bar that you see if the pictures loses vertical hold
and starts "rolling"), so only a bit over 500 lines are available for
actual image display, and on real-world TV sets a number of these are
not visible on an actual TV set due to "overscanning". So while the
standard doesn't define "pixels" or resolution per se, from a vertical
standpoint the image has a viewable vertical resolution of about 480
pixels, interlaced into even and an odd fields of 240 viewable lines
each, from a total image component of 525 lines in two fields of 262.5
lines each.
Horizontally, the color subcarrier is at 3.58 MHz, so in "crude"
implementations, the monochrome bandwidth begins to fall off somewhere
around 3.2 MHz to make way for the color signal at 3.58 MHz and it's
sidebands extending upwards and downwards on either side of that
frequency. In fact, however, the color and monochrome information are
"interleaved", and sets with a high quality comb filter can extract
monochrme information up to about 4 MHz. A very, very rough rule of
thumb used by broadcast engineers is that you get about 80 lines of
horizontal resolution per MHz, give or take, and depending on the
quality of transmission and all of the equipment from signal generation
to the display CRT. So, for broadcast use, you can expect 240 to 320
lines of horizontal resolution, and it's tough to do much better than
that with a broadcast signal, limited by the color subcarrier, the sound
subcarrier and the nuances of vestigal-sideband AM modulation transmission.
However, if the chroma (color) and lumanence (monochrome) portions of
the signal can be kept separate -- truly separate, physically separate,
toally separate -- then there are no real limits on the monochome
resolution and it's possible to get up to 8 or even 10 MHz of monochrome
bandwidth, giving resolutions of up to between 640 and 800 lines. But
the only way to do that is to use S-Video connections between the
original signal source and the display (which may not be capable of
displaying that much resolution even if it is present).
Once the monochome and color signals are mixed, however (as they must be
in all broadcast and cable signals), the ability to achieve these higher
resolutions is lost forever, and no subsequent separation of chroma and
lumanence signals, or subsequent use of S-Video connections, will ever
restore them.
