Plugin Delay
When used in real time, a plugin takes a certain fraction of time to carry out its task. This time is called plugin delay, and its effect is that the signal passing through that plugin is delayed relative to the signal that doesn't.
This can cause several tracks with different plugins to fall out of sync. To avoid this problem, DAWs often have a buffer that provides a certain amount of time as a "cushion."
However, some plugins can slightly exceed this buffer and introduce small delays of, for example, 16 samples. At these amounts, the delay isn't a big problem, since we are talking about delays of less than a millisecond.
Specifically, if you are working at a sample rate of 48 kHz, 16 samples amount to a delay of 16/48000 = 0.3 ms, a time that is insignificant most of the time. Even delays of two or three milliseconds would probably go unnoticed between, say, a guitar and a bass.
However, this delay can be significant in two different scenarios:
When does it matter?
On the one hand, when you are working with tracks that must maintain absolute phase coherence with each other, a small 1 ms delay can mean a major change in the sound, because of the comb filtering it can create.
A typical example of this is when you have a multi-mic take of an electric guitar, an acoustic drum kit, etc., or when you are using parallel compression.
In these cases, keeping the phase aligned between all the mics, or between the original signal and the parallel-compressed one, is essential to avoid cancellation problems in the signal. The second case in which the delay introduced by a plugin matters is, naturally, when that time is very long—so long that the resulting signal is clearly out of time with the rest.
Some plugins need a large number of samples to do their job, as is the case with look-ahead plugins. This category includes some compressors, most brickwall limiters, Auto-Tune-style pitch correction, noise reducers, drum replacers...
When these plugins are inserted on a channel, the signal coming out of it can be heard clearly out of time with the rest of the instruments. If our DAW doesn't have a way to compensate for this problem automatically, we need to look for alternatives to avoid it or at least minimize its effect.
How to fight it manually
Naturally, one way to avoid this problem is to apply the plugin that introduces the delay offline. To do so, you adjust the plugin's parameters and, once you are happy with the result, apply the effect to the audio. That way, the plugin isn't used in real time, and the sync problem disappears.
Taking Pro Tools LE (a widely used DAW) as an example, the solution would be to open the AudioSuite version of the plugin in question and apply it to the audio regions.
This method is useful when, for example, you are using pitch correctors or noise reducers, since once the plugin has been applied, there is no need to change it again.
However, this is often not possible, since the plugin's parameters change throughout the mix depending on other decisions, and you can't "print" the effect every time.
In these cases, the solution is to deliberately delay the rest of the tracks so that during playback they line up correctly with the tracks affected by a plugin's delay.
To make this correction, most DAWs have a system through which plugins report how much delay they introduce, so the user can adjust the rest of the tracks manually.
This adjustment can be made directly by moving the region to shift it in time, or by using special plugins whose sole purpose is to introduce an adjustable delay on the track.
Although each method has its pros and cons, it is generally more convenient to insert a correction plugin on each track and adjust it as the mix evolves, since when working on projects with many tracks, keeping track of how far each region has been dragged can become very complicated.
Sticking with Pro Tools LE, the procedure is to insert a Time Adjuster on each channel before starting the mix, and to delay each one until they all have the same delay as the channel with the most delay.
To see the delay on each track, Command+click (or Control+click on Windows) on the numeric volume display (vol); it then switches to showing the peak level (pk), and if you click again, the plugin delay display (dly) appears.
Figure 1 shows a detail of a session in which each track has a different delay. In this example, the longest delay is on the last track, with 1048 samples.
Therefore, to adjust the first one, you need to enter a value of 1044 samples in its Time Adjuster (1048 – 4 = 1044). To adjust the second, you need to delay it by 1048 samples (1048 – 0 = 1048), the third by 536 samples and the fourth by 792.
This way, once the appropriate delay values have been entered in each Time Adjuster, all the tracks have the same overall delay and therefore play in sync (Fig. 2).
Mellow Muse
A few months ago this company (www.mellowmuse.com) released Auto Latency Compensation (ATA), a product designed to automatically combat plugin delay in Pro Tools LE and M-Powered.
Much as with the Time Adjuster, an instance of ATA is inserted on each channel, whether audio, aux or master. Each of these instances communicates internally with the others to report each channel's delay and automatically synchronize all the channels.
These delay times are calculated by sending short audio impulses and measuring the actual delay that occurs. This avoids a problem that sometimes comes up, in which some plugins report a plugin delay different from the one they actually introduce.
So every time a new plugin is inserted on a track, you have to press the Ping button. The Auto Time Adjuster automatically calculates the existing delay relationships and applies the appropriate delay to each track accordingly.
ATA even lets you adjust the delay on up to five effect send buses, an adjustment that is usually more complex to do manually because of the interaction between the dry signal and the effect signal.
