What are the NTSC variants?
There are three varieties of NTSC:
Unlike PAL, with its many varied underlying broadcast television systems in use throughout the world, NTSC color encoding is invariably used with broadcast system M, giving NTSC-M.
Only Japan's variant “NTSC-J” is slightly different: in Japan, black level and blanking level of the signal are identical (at 0 IRE), as they are in PAL, while in American NTSC, black level is slightly higher (7.5 IRE) than blanking level. Since the difference is quite small, a slight turn of the brightness knob is all that is required to enjoy the “other” variant of NTSC on any set as it is supposed to be; most watchers might not even notice the difference in the first place.
This is used in Paraguay and Bolivia (though Paraguay has recently switched to NTSC-M from PAL-N). This is very similar to PAL-M (used in Brazil). It is also closely related to PAL-Nc (used in Argentina) and PAL-N (used in Uruguay).