Logic Pro 9 User Manual - Help Library - Apple

As an example: Imagine a simple song with a few bass, guitar, vocal, and drum tracks.
The bass track is routed through an audio channel that contains an effect that introduces
a latency of 10 ms. All guitar tracks are routed to an aux channel that contains several
inserted effects. The combined latency introduced by these effects is 30 milliseconds (ms).
The vocals are routed through another aux channel that has a set of effects that introduce
15 ms of latency. The drum tracks are routed straight to the main outputs, without being
routed through any effects. If latencies were not compensated for, the drum tracks would
play 30 ms ahead of the guitar tracks. The bass track would play 20 ms ahead of the guitar
track, but 10 ms behind the drums. The vocals would play 15 ms before the guitar track,
but 15 ms behind the drums and 5 ms behind the bass. Needless to say, this isn’t ideal.
With plug-in latency compensation set to All, Logic Pro shifts the bass track forward by
10 ms, thus synchronizing the bass and drum tracks. Logic Pro will then delay both streams
routed to the output channel by 30 ms, aligning them with the guitar tracks. The aux
channel that the vocals are streamed to is also delayed by 15 ms, aligning it with the
drum and guitar streams (in other words, the 15 ms delay is increased to 30 ms). The
precise calculations required for each stream are handled automatically.
Track
Bass (effect directly inserted in
audio channel)
Guitars (routed to aux 1)
Drums (direct to output)
Vocal (routed to aux 2)
Uncompensated
10 ms delay
30 ms delay
No delay
15 ms delay
Compensated
10 ms → (audio channel) then
← 30 ms (output channel)
Not changed
30 ms ← (output channel)
15 milliseconds ← (aux channel
2)
As you can see in the table, all output is effectively delayed by 30 milliseconds, to match
the largest amount of compensation required (by the effects in aux channel 1, which the
guitar tracks are routed to). This has the effect of perfectly aligning all tracks routed to
the output, and circumventing any delays introduced by plug-ins, regardless of where
they are used in the signal path.
Knowing the Limitations of Plug-in Latency Compensation
Plug-in latency compensation works seamlessly during playback and mixing. The delay
that is introduced—to compensate for latency-inducing plug-ins in output and auxiliary
channels—can be applied to non-delayed streams, before they are played back. Instrument
and audio tracks (that contain latency-inducing plug-ins) can also be shifted forward in
time, before playback starts.
1202 Chapter 41 Working with Plug-in Latencies