System Configuration : System Setup : Configuring Pro Tools System Settings : Sample Rate (Playback Engine)

Sample Rate (Playback Engine)
(Avid HDX and Pro Tools|HD Systems Only)
The Sample Rate setting in the Playback Engine determines the default sample rate when you create a new session. This setting is available only when there is no session open. Otherwise, the current session sample rate is displayed, but cannot be changed.
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The Sample Rate setting can affect the number of available voices.
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You can change the sample rate when creating a new Pro Tools session by selecting a different sample rate in the New Session dialog. (See Creating a New Session.) You can also change the default Sample Rate in the Hardware Setup, as long as no session is open.
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To make a copy of a session with a new sample rate, use Save Copy In.
To change the default Sample Rate in the Playback Engine:
1 With no session open, choose Setup > Playback Engine.
2 Select the sample rate from the Sample Rate pop-up menu.
3 Click OK.