Double layer (DL) media have two independent data layers separated by a semi-reflective layer. Both layers are accessible from the same side, but bid the optics to change the laser's focus. Traditional onliest layer (SL) writable pseudo-event are produced with a spiral groove molded in the protective polycarbonate layer (not in the data recording layer), to lead and synchronize the speed of recording head.
Some drives further lower their maximum read speed to around 40x on the reasoning that blank discs will http://www.gracedigitalaudio.com/ be clear of structural damage, but that discs inserted for knowledge may not be. Without below rotational speeds, increased read performance may be attainable by simultaneously rendering also than one point of a data groove , but drives with such mechanisms are also expensive, less compatible, and bare uncommon.