 Sound on sound, which Les Paul invented, adds a new performance to an existing recording by placing a second playback head in front of the erase head to play back the existing track before erasing it and re-recording a new track.Multi-track recorders also differ from early stereo and three-track recorders that were available at the time in that they can record individual tracks while preserving the other tracks.The original multi-channel recorders could only record all tracks at once.