Bed tracks and object tracks in Logic Pro for Mac
A Dolby Atmos mix has two types of tracks, object tracks and bed tracks. They are fundamentally different in terms of how they handle audio signal routing and panning.
Bed tracks
A bed track in a Dolby Atmos mix behaves like a conventional channel-based surround track. This is the exception to the object-based concept of the Dolby Atmos format. The audio signal on a bed track is routed to a multichannel bus, and the surround panner control on each bed track pans the signal across each channel of that multichannel bus. Each channel corresponds to a dedicated speaker for that surround setup (L, R, C, Ls, Rs, LFE, and so on).
The implementation of bed tracks in Logic Pro for Mac is as follows:
Any newly created track in your project will be a bed track.
The channel mode of a bed track can be mono, stereo, or surround.
All bed tracks are routed to the same surround output, referred to as the surround bed.
A Dolby Atmos project in Logic Pro has only a single bed, which means all bed tracks in your project are routed to the same surround bed.
The Surround Format pop-up menu in the Logic Pro Audio project settings lets you set the channel mode of the surround bus to 7.1.2 (default), 7.1, or 5.1.
The signals of all bed tracks, which are routed to the surround bus, will be sent to the surround master channel strip and processed by any plug-in placed before the Dolby Atmos plug-in.
The maximum channel width of a surround bed is 7.1.2, a format that has only one pair of overhead speakers above the listener position (Ltm and Rtm). This means you can’t position a signal at the front or back of the ceiling.
Object tracks
Object tracks are different from bed tracks in several ways:
The audio signal of an object track is routed as a mono or stereo signal directly to the Dolby Atmos plug-in without going through any pan control.
The Dolby Atmos plug-in can have up to 118 objects. Think of it as a multitrack tape machine that has 118 tracks to record on.
Any track that is routed to an object becomes an object track. A stereo track uses two objects.
Mono tracks and stereo tracks can be routed to an object, but surround tracks cannot.
In Logic Pro, the pan control of an object track is always the 3D Object Panner.
The 3D Object Panner lets you position the audio signal in the three-dimensional space without the restriction of speaker positions. See Overview of the 3D Object Panner.
The audio signal of an object track is routed directly to the corresponding object input on the Dolby Atmos plug-in. The pan settings (static or automated) generated on its 3D Object Panner are sent as separate metadata to the same object input.
An object track can’t be routed directly to the LFE channel of a Dolby Atmos mix.
The audio signals of all the individual objects and their corresponding pan information are kept separate when exporting the Dolby Atmos mix to an ADM BWF master file (used for delivering a Dolby Atmos mix to Apple Music).