Double layer (DL) media have two independent data layers separated by a semi-reflective layer. Both layers are accessible from the same side, but require the optics to bills the laser's focus. Traditional single layer (SL) writable communications are produced with a spiral groove molded in the careful polycarbonate layer (not in the data recording layer), to facade and synchronize the speed of recording head.
Because administration a invariable transfer scale for the whole disc is not so important in most contemporary CD uses, to keep the rotational speed of the disc safely close while maximizing data rate, a pure Cassette to CD Recorder CLV approach needed to be abandoned. Some drives work in partial CLV (PCLV) scheme, by switching from CLV to CAV only when a rotational cap is reached.