The principle of MPEG2 is relatively simple. Figure 1 on the right shows how each frame of a movie is stored using MPEG2. Instead of storing each frame of a movie, MPEG2 stores the transitional changes between each frame. It starts with a key frame. Every frames after the key frame are the changes between each frame. Periodically, a new key frame will be generated; in case there's a bad spot on the DVD or the DVD drive skips a track during playback.