When the National Television Systems Committee (NTSC) prepared the standards for commercial television broadcasting in the United States, Canada, Japan, and parts of Central and South America, they decided on 525 lines of display at thirty frames per second. In most of Europe, Australia, and parts of Central and South America, however, the Phase Alteration Line (PAL) standard, using 625 lines at 25 frames per second, was used.
These two standards are not compatible, and DVD video has the same NTSC vs. PAL problem as its "predecessors" - videotape and laserdisc. The MPEG video that is on DVD discs may be stored in digital format, but it must be formatted for one of two mutually incompatible systems.
There are three differences between discs intended for playback on different systems: picture size and aspect ratio, display frame rate, and surround audio. Video from film is usually stored at 24 frames per second, but when transferred to DVD, it is usually preformatted for one of these standards. Movies formatted for PAL display are usually sped up by 4%, so the audio must be adjusted accordingly before being encoded.
Some players will only play NTSC discs, some players will only play PAL discs, and some will play both. Because of the amount of storage available on DVD discs, many title producers include additional video and audio tracks, so that all formats are available.