System Configuration : System Setup : Configuring Pro Tools Hardware Settings : Configuring Clock Source

Configuring Clock Source
The Pro Tools Hardware Setup lets you select the Clock Source for the system.
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Changes made to Clock Source in the Session Setup window will be reflected in the Hardware Setup window.
Internal
If you are recording an analog signal directly into Pro Tools, you will usually use the Pro Tools Internal clock source.
External
If you are transferring material into Pro Tools from an external digital device, or if you utilize a common house clock signal, you will synchronize Pro Tools to that digital device or common signal.
Depending on your audio interface, external options can include AES/EBU [Encl], S/PDIF, Optical [Encl], AES/EBU 1–8, TDIF, ADAT, and Word Clock. For details, see the guide for your audio interface.
To select the Clock Source:
1 Choose Setup > Hardware.
2 Choose the clock source from the Clock Source pop-up menu.
3 Click OK.
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Your digital input device must be connected and powered on for Pro Tools to synchronize to it. If your input device is not powered on, leave the Clock Source set to Internal.