System Configuration : System Setup : Configuring Pro Tools System Settings : Current Engine

Current Engine
In the Playback Engine dialog, Pro Tools lets you select the audio engine for use with your audio interfaces. The available options are determined by which audio interfaces are connected and have compatible drivers installed.
Changing the Current Engine setting can be useful if you have multiple audio interfaces connected to your computer with different routing configurations in your studio, or if you want to prepare a session for use with a specific interface on a different system (for example, you might want to prepare a session created on your Pro Tools|HD system for use with the built-in audio on your Mac laptop).
To select which audio engine for Pro Tools uses:
1 Choose Setup > Playback Engine.
2 From the Current Engine selector, select your audio interface:
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For Avid HDX, Pro Tools|HD, and HD Native systems, use the default setting of HDX, HD TDM, or HD Native.
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For supported Pro Tools and M-Audio audio interfaces (such as the Mbox Pro or ProFire 2626), selecting the name of the corresponding interface uses the Direct I/O engine for Pro Tools.
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For third-party audio interfaces with compatible CoreAudio (Mac) or ASIO (Windows), select the name of the corresponding Core Audio (Mac) or ASIO (Windows) compatible audio interface.
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On Mac systems, if you are using built-in audio, select any of the available built-in options for playback or select Pro Tools Aggregate I/O for simultaneous input and output with the built-in I/O hardware options.
3 Click OK.
When changing engines with a Pro Tools session open, Pro Tools must close and relaunch the session to initialize the new engine.
On Avid HDX and Pro Tools|HD systems, changing engines requires that you quit and relaunch Pro Tools for the new setting to take effect.
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When changing engines on any Pro Tools system, you may need to reset the default I/O settings to match the selected audio interface.
Pro Tools Aggregate I/O
(Mac Only)
On Mac systems using Core Audio, you can use Pro Tools with the built-in audio inputs and outputs on your Mac by selecting any of the available built-in inputs and outputs, or by selecting the Pro Tools Aggregate I/O option for using a combination of built-in inputs and outputs simultaneously (for recording and monitoring).
 
Selecting Pro Tools Aggregate I/O in the Playback Engine (Mac only)
You can configure the Input and Output options for Pro Tools Aggregate I/O in the Mac Audio Setup, which can be accessed from the Pro Tools Hardware Setup.
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If you need simultaneous input (recording) and output (playback and monitoring) with Pro Tools using the built-in audio options on Mac, use the Pro Tools Aggregate I/O option. If you only need to play back audio for editing and mixing, select the appropriate built-in audio output option.