Sync and Video : Working with Video in Pro Tools : Video Engine Rate

Video Engine Rate
For more information, see the following topics:
The video engine rate represents the number of video frames output by Pro Tools per second, and is determined automatically by the frame rate of the first video file added to a session. Video files subsequently added to the session must have the same video frame rate as the video engine rate.
The video frame rate is displayed for each video file in the DigiBase browsers, and the current video engine rate is displayed in the I/O View of the main video track.
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See Video Engine Rate Indicator for information on the Video Engine Rate indicator in the I/O View.
The video engine rate is independent of the session's timecode rate, which determines the number of frames displayed per second in grids, rulers, and counters. You can set the session timecode rate manually with the Timecode Rate pop-up menu in the Session Setup window. In order for the grids and rulers to align with the actual frames of the video files in the Timeline, the session's Timecode Rate setting should be set to match the video engine rate.