Playback and Recording : Playing Back Track Material : Scrolling Options : Center Playhead Scrolling

Center Playhead Scrolling
(Pro Tools HD Only)
When this scrolling option is selected, the Edit window’s contents scroll continuously past the Playhead, which is a blue line in the center of the window (or a red line when recording).
The Playhead indicates where playback begins when clicking Play in the Transport window.
 
To move the Playhead to a particular location for playback, you can scroll there in a ruler (see Scrolling in a Timebase Ruler), use the Edit window’s horizontal scroll bar, or type the location into one of the Edit Selection indicators or one of the Counters.
Moving the Playhead with these methods does not update the Timeline selection. However, updating the Timeline selection automatically moves the Playhead to the Timeline insertion point.
With the Playhead enabled, you can jump to and play an Edit or Timeline selection. For details, see Playing Timeline and Edit Selections with the Playhead.
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Center Playhead Scrolling and Dynamic Transport mode cannot be used at the same time. Selecting Center Playhead Scrolling disables Dynamic Transport mode (if it is enabled). Likewise, enabling Dynamic Transport mode when Center Playhead Scrolling is selected automatically changes the Scrolling option to No Scrolling.