timebase

Timebase information is used to synchronize playback of the current item with the master clock. You can use this property to access the timebase information, but you cannot use it to set the time or rate of playback.

If you need to respond to changes in the effective playback rate, the best way to do so is to respond to notifications posted the player item’s timebase (see kCMTimebaseNotification_EffectiveRateChanged in <CoreMedia/CMSync.h>). These notifications reliably announce when the effective rate of playback changes, including any compensation needed for drifting behaviors of audio output hardware.