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1.4.2. Busses, Master Bus, and Sub-Master Bus

An audio bus sends audio signals from one place to another. Many different signals can be inputted to a bus simultaneously, and many different devices or applications can read from a bus simultaneously. Signals inputted to a bus are mixed together, and cannot be separated after entering a bus. All devices or applications reading from a bus receive the same signal.
How audio busses work
An example audio bus combining three signals. The combined output is sent to two separate devices or applications.
Figure 1.2. How audio busses work

All audio routed out of a program passes through the master bus. The master bus combines all audio tracks, allowing for final level adjustments and simpler mastering. The primary purpose of the master bus is to mix all of the tracks into two channels.
A sub-master bus combines audio signals before they reach the master bus. Using a sub-master bus is optional. They allow you to adjust more than one track in the same way, without affecting all the tracks.
The relationship between the master bus and sub-master busses
Two example sub-master busses, each combining two different signals and sending them to a master bus. The master bus combines the two signals into one signal that contains all four original signals.
Figure 1.3. The relationship between the master bus and sub-master busses

Audio busses are also used to send audio into effects processors.