A method of writing red book audio CD’s. Disc-at-once, as opposed to track-at-once and other methods, writes the entire CD in one pass. That is the laser never turns off once the writing has begun. This is significant because it is the only way to produce an audio disc suitable for mass duplication. If the laser turns off for any reason while burning a disc it creates a small inconsistency in the reflective material that is interpreted by CD mastering machines as a fatal error. The CD is still entirely playable on most CD players, but the machines at the mastering house will kick it out. If you are making audio CD’s for mass duplication you need to make sure your hardware and/or software supports and is set to use disc-at-once mode. Nowadays most systems default to this when writing audio CD’s.