Arranging : Time, Tempo, Meter, Key, and Chords : Tick- and Sample-Based Timebases : Sample Rounding and Edit Operations

Sample Rounding and Edit Operations
When audio material in Pro Tools is sample-based, some amount of sample-rounding may occur with some edits when the Main Time Scale is set to Bars|Beats. This is most evident when you need audio clips to fall cleanly on the beat (as when looping) and notice that the material is sometimes a tick or two off. With a few simple precautions, this can be avoided.
To ensure that selections are precisely aligned to the grid for looping, copying, duplicating, or repeating, consider the following:
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If you use the Selector tool, enable Grid mode and select the appropriate grid value (such as sixteenth notes).
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For the greatest accuracy, type the selection Start and End points in the Event Edit area.
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Do not select the material with any of the Grabber tools (or by double-clicking with the Selector tool).