The name of the original Dolby Laboratories recording/monitoring unit used by re-recording stages during a Dolby Stereo mix. Prior to the 2-track print master, the unit is used for 4:2:4 monitoring purposes, encoding a 4-channel composite mix into two tracks and then decoding it back into four channels. Later versions of these units include theSEU4 and SDU4 units which offer, respectively, the ability to encode and decode print masters, although without either the container of the optical track simulation featured in the DS4. The DS10 contains a magneto-optical recorder for theatrical Dolby Digital mixes and also records the Lt-Rt SR-encoded print master. None of the above units can be purchased; their use is free for films that have paid for the appropriate license fee and/or trademark agreement.