Although audio tapes and video tapes look different, they’re based on the same principles. Each has a roll of magnetic material which slides past the so-called “playback head” in the VCR or cassette deck (cassette decks have one playback head, VCRs have two or more). In both machines, these heads detect changes in magnetism as the tape moves by. The changing magnetic signal is translated into music on your tape deck, or both video and sound on your VCR. Without a changing magnetic signal over a playback head, you’d get no sound, no picture.