Apple Logic Pro 7 - Manual (Page 611)

Chapter 19 Synchronization 611
MTC Trigger
Audio regions are started in sync, but are then played at a constant sample rate,
regardless of any variations in the time code master signal. Logic always uses the set
nominal sample rate (44.1, 48, 88.2, 96, 192 kHz).
This mode is suitable when it is vital to retain the absolute pitch of a recording. If the
speed of the time code master deviates from the nominal value, you may need to split
long Regions into shorter sections.
External or Free
Logic has no influence on the sample rate. The audio hardware is responsible for
ensuring that the position and sample rate of audio Regions match. This mode is only
advisable if you are sure that the word clock and time code master are running in
sync—by using an external SMPTE/wordclock synchronizer, for example.
Digital
Similar to External or Free, but the hardware is also set up to synchronize to the sample
rate of the incoming digital signal.
SSD/VSD Type
Exactly the same as Digital, but a SMPTE Slave Driver (SSD) or Video Slave Driver (VSD)
is used in addition—to synchronize with the sample and frame rates used in Logic. The
SMPTE slave driver is a device for synchronizing ProTools hardware to time code or
word clock.
MIDI
Transmit MIDI Clock
The checkboxes activate transmission of MIDI Clock. The pull-down menus determine
the MIDI output ports for the MIDI Clock signal.
The two Destinations allow you to specify discrete MIDI ports.
Every time you start, “Song Position Pointer” (SPP) is also sent.
As not all devices can process SPP, the MIDI system realtime “Continue” message is also
sent. The exception to this is when you start at position 1 1 1 1. In this situation, the
realtime “Start” message is sent.
MIDI Clock can easily be sent with other normal MIDI events (notes, controllers). When
using multiport MIDI interfaces (such as the Unitor8), better timing is achieved by
sending MIDI Clock to All ports, rather than to several individual ports.
If MIDI Clock is transmitted to all ports, the events are only sent once from the
computer to the interface. If you address individual ports, one event needs to be sent
for each port, placing a higher strain on the bandwidth of all ports.