Sync and Video : Working with Video in Pro Tools : Main Video Track

Main Video Track
The main video track is the video track that is currently online, and will be played back with any audio in the session.
When the main video track is offline, you can play back the audio in your session without playing back the video. When the video is offline, the Video Online button changes to gray so you can tell at a glance whether your video is offline or simply playing black fill.
If no video track is currently online, the main video track is the video track that was last online.
To select the main video track:
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Click the Video Online button of any track that is offline (gray) to put it online (blue), making that track the main video track.