The synchronization protocol MTC (MIDI Time Code is the MIDI Version of SMPTE, which stands for "Society of Motion picture and Television Engineers") and word clock (Digital Sync) are fundamentally different, and fulfill different purposes. MTC (and SMPTE) is time-related, which means that SMPTE transmits information about absolute time positions. The SMPTE master tells the synchronized slave the time position of the tape so that the slave can lock to it at the same time position. Word clock is only a steady pulse defining a certain sample rate to which the slave can lock in to, but there is no absolute time position transmitted. If several devices have to be linked together, they have to be synchronized so that they run not only exactly with the same speed, but also locked to the same pulse phase. If not, pops, ticks and drop-outs can be heard more or less frequently. Wherever possible, it's best to combine both synchronization methods. Special synchronizers are needed to generate a word clock signal from a MTC signal (be it derived from a LTC or VITC video synchronization signal). If such a synchronizer is being used, you should switch Logic to external or free synchronization. Click the sync button in the Transport window, and select Synchronization > Audio to access the parameter. In any other situation, you should select trigger sync instead. Continuous Sync is not available with version 4.04 but will be implemented again in one of the next versions.