Track (optical disc) Track (optical disc) On an optical disc, a track (CD) or title (DVD) is a subdivision of its content. Specifically, it is a consecutive set of "sectors" (called "timecode frames" on audio tracks) on the disc containing a block of data. One "session" may contain one or more tracks of the same or different types. There are several kinds of tracks, and there is also a sub-track index for finding points within a track. Blu-ray discs can also be written in a sequential, session based mode modelled on CD and DVD. A Blu-ray "track" refer to the entire physical storage
Optical medium Optical medium An optical medium is material through which electromagnetic waves propagate. It is a form of transmission medium. The permittivity and permeability of the medium define how electromagnetic waves propagate in it. The medium has an "intrinsic impedance", given by where formula_2 and formula_3 are the electric field and magnetic field, respectively. In a region with no electrical conductivity, the expression simplifies to: For example, in free space the intrinsic impedance is called the characteristic impedance of vacuum, denoted "Z", and Waves propagate through a medium with velocity formula_6, where formula_7 is the frequency and formula_8 is the wavelength