### Adding and Editing Sidechains

Summary: This section describes how to add sidechains to the document. There are two different types of sidechains that can be added: input sidechains which route an additional input to an Audio Unit with an unused element, and output sidechains which add a new sidechain input track group to the document. To add, or edit an input or output sidechain for an effect in a track, select Edit Inputs… from the effect popup menu of that Audio Unit.

A sidechain is an additional input or output routing path for an audio unit. Apple provides two special sidechain units that exist specifically for signal routing- AUMerger and AUSplitter. Additionally, any audio unit that provides more than one input or output element is capable of supporting a sidechain.

Input sidechains
An audio unit with a variable number of input elements or an audio unit with more than one fixed input element (such as the AUMerger) can support an input sidechain. To add, remove, or edit an input sidechain of an effect in a track, select Edit Inputs… from the effect popup menu. If that menu item is not visible, the selected effect does not support input sidechains.

The sidechain editor for a fixed input audio unit

The input sidechain editor displays the current configuration of the Audio Unit. The dialog contains a visual representation of the state of the Audio Unit including its name at the top of the window. The first input element is locked and cannot be modified since it receives input from the previous effect or the track source. Additional input elements may be attached to an audio input.

The sidechain editor for a variable input audio unit

Adding an Input Sidechain

To add a sidechain to an audio unit with a fixed number of inputs, open the sidechain editor by selecting Edit Inputs… from the effect popup menu of a track insert effect. Then select the input from the Selected Input popup menu and click the Audio Input button from the Input Source radio button group. For Audio Units with a variable number of inputs, simply click on the Add Input button to add a new sidechain.

The track will display a chain icon () to indicate that it has an active sidechain. You can then specify the starting channel for the sidechain on the document's audio device. If the audio unit supports it, you may also change the channel configuration of the input sidechain by choosing the number of channels from the Input Channels popup menu.

Click the OK button to add the sidechain.

Removing an Input Sidechain

To remove an existing sidechain, open the sidechain editor and select the sidechain by choosing the input from the Selected Input popup menu or clicking on the input directly in the view. For Audio Units with a fixed number of inputs, remove the sidechain by clicking the None button in the Input Source radio button group. For Audio Units with a variable number of inputs, click the Remove Input button to remove the sidechain.

Output Sidechains
An Audio Unit with a variable number of output elements or an Audio Unit with more than one fixed output element (such as AUSplitter or AULooper*) can support an output sidechain. When an output sidechain is created, it adds a new sidechain track to the document window. The source of the track is the output of the specified element of the Audio Unit.

Adding an Output Sidechain

To add an output sidechain to an effect in a track, open the sidechain editor by selecting Edit Outputs… from the effect popup menu.

For Audio Units with fixed outputs, select the output from the Selected Output popup menu and click the As Track button from the Output Destination radio button group.

The sidechain editor for a fixed output audio unit

For Audio Units with a variable number of outputs, click the Add Track button.

The sidechain editor for a variable output audio unit

Once a output sidechain is designated, the track name may be specified by typing the name in the Track Name edit text field. The default track name is the name of the Audio Unit element. Clicking the OK button dismisses the dialog and adds the sidechain track(s).

Removing an Output Sidechain

To remove an output sidechain, open the sidechain editor by choosing Edit Outputs… from the effect popup menu, or by double clicking on the sidechain group in the document window. Select the output that you want to remove by selecting it from the Selected Output popup menu or clicking directly on the output in the view. For fixed output Audio Units, remove the sidechain by clicking the None button in the Output Destination radio button group. For Audio Units with a variable number of outputs, click the Remove Track button.

You can also remove all sidechains associated with a given audio unit by selecting the sidechain group in the document window and removing it by choosing Delete Selected Group from the Edit menu or using the key equivalent (command-delete).

*AULooper ships with the CoreAudio SDK and is not installed as part of Tiger