 On Blu-ray Disc, DD+ can only be used to extend a primary Dolby Digital (DD) 5.1 audiotrack.In this method 640 kbit/s is allocated to the primary DD 5.1 audiotrack (which is independently playable on players that do not support DD+), and up to 1 Mbit/s is allocated for the DD+ extension.The DD+ extension is used to replace the rear channels of the DD track with higher fidelity versions, along with adding additional channels for 6.1/7.1 audiotracks.