The three differences between NTSC discs and PAL discs are: 1) Picture size and pixel aspect ratio (720x480 vs 720x576), 2) Display frame rate (29.97 vs 25), 3) surround audio options (Dolby Digital vs MPEG audio). Video from film is usually encoded at 24 frames/sec. but is preformatted for one of the two display rates. Movies formatted for PAL display are usually sped up by 4% at playback, so the audio must be adjusted accordingly before being encoded. All PAL DVD players can play Dolby Digital audio but not all NTSC players can play MPEG audio. PAL and SECAM share the same scanning format, so discs are the same for both systems. The only difference is that SECAM players output the color signal in the format required by SECAM TV's. Note that modern TV's in most SECAM countries can also read PAL signals, so you can use a player that only has a PAL output.
from
http://www.microcinema.com/index/ntsc