Apple Logic Express 7 - Manual (Page 452)

452 Chapter 18 Synchronization
MTC Interpretation
As the MIDI standard only supports four of the possible six time code formats (the
30 fps and 29.97 fps formats cannot be differentiated), Logic needs to decide which
format is “intended”, when it encounters incoming time code:
Incoming MTC format is interpreted as:
• 24 fps–24 fps
• 25 fps–25 fps
• 30 drop fps–29.97 drop fps
• 30 fps–30 fps
In other words, the much more commonly used 29.97 fps and 30 drop fps formats are
used in place of the uncommon 30 fps, and the virtually-unheard-of 29.97 drop fps
formats.
You can, however, manually set the format to whatever you like in the Song Settings >
General > Frame Rate pull-down menu: to convert material to 30 fps for black and
white TV transmission in the NTSC format, for example.
Receiving MIDI Clock/SPP
Synchronization via MIDI Clock/SPP is the most accurate method to use—if Logic is
being synchronized to a bar-referenced master.
MIDI Clock has a resolution of 24 PPQN (pulses per quarter note), while Logic has an
internal resolution of 960 PPQN (some 40 times more accurate). For this reason, Logic
needs to interpolate the 39 steps between two incoming clock pulses.
If you experience small variations in the timing between master and slave, you can
tighten the sync (when in external sync mode) by entering the expected tempo
changes from the master into Logic’s internal tempo list as well.
Even if you don’t take this step, synchronization should still be fairly tight, as long as
you avoid large deviations, such as an internal setting of 200 bpm, with an external
tempo of 40 bpm.
Continue Event
When a MIDI Continue Event is received, Logic doesn’t leap to the last valid position
received via MIDI Clock. Rather, playback continues from the current song position. This
allows you to manually change the song position while the sequencer is stopped, and
then restart from the new position, with a MIDI Continue command.