Sel-Sync or Selective Synchronous recording is the process of selectively using audio tape record heads as play back heads so that new signals can be recorded on other tracks in perfect sync with the existing tracks. Sel-sync recording dramatically changed the recording process allowing overdubbing of individual recorded tracks. D-2 (video) was the first digital recording video tape format to offer Sel-Sync "read before write" (an Ampex term) also known as "preread" on Sony magnetic tape recorders. Read before write allowed simultaneous playback and recording on the same VCR.