In SMPTE timecode synchronization, an option by which the slave machine is speed-controlled in such a way that the phase of its bi-modulated sync tonecode wave is held in phase with the sync tonecode wave on the master machine. This provides much closer alignment of the two than just frame lock. However, because the synchronizer must make continuous adjustments to the slave’s speed, phase sync can introduce noticeable flutter when the audio machine is slaved to video. In some synchronizers, only sub-frame information is used to achieve interlock, yielding a 1/100 frame accuracy between machines.