I am wondering all the time why I see so many exactly 30 sec or 6o sec versions of songs in music libraries. Why should there be need, that the versions are exactly that long?
Also most TV stations require a certain amount silence at the start and end of a commercial, therefore a 30 sec spot would only have 29 sec approx sec of actual sound or music. This prevents commercials butting up against each other audio wise on transmission. I cant remember the exact EBU duration I think it is 10 frames @ 25 fps at either end.