Optical drive's rotational mechanism differs considerably from hard disk drive's, in that the latter keep a constant angular velocity (CAV), in other words a constant number of revolutions per minute (RPM). With CAV, a higher throughput is generally achievable at an outer disc area, as compared to inner area.
Some drives further lower their maximum read speed to around 40x on the reasoning that blank discs will be clear of structural damage, but that discs inserted for reading may not be. Without higher rotational speeds, increased read performance may be attainable by simultaneously reading more than one point of a data groove , but drives with such mechanisms are more expensive, less compatible, and very uncommon.
