Answer - There are several different tv. systems in the world PAL, SECAM, MESECAM & NTSC. The scan lines and picture sizes are different, this causes a tape or DVD that has come from the PAL system & is being viewed on say the NTSC system, to play but not be recognised by tv. it is being feed to.  If you have a DVD from say England in the PAL system it can be viewed on most computer DVD players as the monitor on a PC or Apple computer is the same world wide. Win DVD can recognise the different regions if set up to play them all.