Specifications

Chapter 13: Impact 89
Side-Chain Processing
Compressors generally use the detected ampli-
tude of their input signal as a control source.
However, you can also use other signals, such as
a separate reference track or an external audio
signal as a control source. This is known as side-
chain processing.
Side-chain processing lets you control Impact
compression using an independent audio signal
(typically, another Pro Tools track). In this way
you can compress the audio of one track using
the dynamics of a different audio track.
The reference track or external audio source
used for triggering side-chain processing is re-
ferred to as the Key Input.
Using the Impact Side-Chain
Input
Impact provides side-chain processing capabili-
ties. Compressors typically use the detected am-
plitude of their input signal to cause gain reduc-
tion. This split-off signal is called the side-chain.
However, an external signal (referred to as the
Key Input) can be used to trigger compression.
A typical use for side-chain processing is to con-
trol the dynamics of one audio signal using the
dynamics of another signal (referred to as the
Key Input). For example, you could use a lead
vocal track to trigger compression of a back-
ground vocal track so that their dynamics
match.
To use a Key Input signal for side-chain
processing:
1 Click the Send button and select a bus path for
the audio track or Auxiliary Input you want to
use as the side-chain signal.
2 From Impact’s Key Input menu, select the in-
put or bus path carrying the audio you want to
use as the side-chain signal to trigger Impact
compression. The Key Input source must be
monophonic.
3 To activate external side-chain processing,
click
Ext.
4 Begin playback. Impact uses the input or bus
that you selected as a Key Input to trigger its ef-
fect.
5 If you want to hear the audio source you have
selected as the side-chain input, click
Listen. (To
stop listening to the side-chain input, click
Lis-
ten
again).
6 Adjust Impact’s Threshold parameter to fine-
tune Key Input triggering.
7 Adjust other parameters to achieve the desired
effect.
Selecting a Key Input
Remember to disable Listen to resume nor-
mal plug-in monitoring.