Surround : Multichannel Tracks and Signal Routing : Multichannel Audio Tracks : Track and Output Formats

Track and Output Formats
When you create new tracks, you specify mono, stereo, or a supported multichannel format for the new tracks.
In the Mix and Edit windows, the track format of a track’s output is always visible by the number of track meters contained in its fader strip (for example, a single meter for mono tracks, a pair of meters for stereo tracks, and six meters for 5.1 tracks).
Assigning track output determines the format of that output. For example, a mono track always has a single track meter, even when assigned to a stereo output path. If that same mono track is assigned to a 5.1 output path, it's output will be split among those six output channels, depending on the position of its panner.
Changing Format
Changing the output format for a given track has several effects:
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The panner that appears in the track will change to reflect the new output format.
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It may be necessary for one or more pan related automation playlists to be created or deleted.
Because changing the output format has these effects, a warning dialog appears whenever you change the output format of a track to a format of fewer channels. Specifically, this will occur whenever automation playlists will have to be deleted.
Multiple Output Assignments and Track Format
When a track is assigned to more than one path of differing formats, the main output for that track will match the format of the assigned path with the greatest number of channels.
Multiple Outputs and Automation Playlists
When a track has multiple output assignments, Pro Tools sorts panning data appropriately for each assigned path. For example, if you assign a mono track to a stereo path and a 5.1 path simultaneously, that track will have a 5.1 panner in the Mix or Edit window. When you pan the track, Pro Tools interprets the 5.1 panning moves into stereo panning moves.
This provides a type of parallel mixing. You can create a variety of mixes of differing formats all at the same time by routing your elements to multiple paths.
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Command-Control-click (Mac) or Control-Start-click (Windows) any control in an Output window to show its automation playlist in the Edit window and view any pan automation.