Abstract: Disclosed is a tracking bit synchronizer for use in digital data apparatus such as high density digital magnetic tape recorders. The disclosed bit synchronizer effects synchronization of data if the data rate is known within one octave. Moreover, data rates can change during operation over an octave range without bit errors or other loss of data. The bit synchronizer includes a phase locked loop which produces a clocking signal in synchronism with incoming encoded data. A data error detector uses information from a data decoder to determine if phase lock at the proper data rate has occurred. Control circuitry uses this information, along with the output voltage of the phase locked loop filter, to operate the phase locked loop in either a seek mode or a tracking mode. A squelch circuit prevents phase corrections during data signal dropouts.