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When working with video, you might want to place some sound effects, ambiances, or Foley where and when these events occur onscreen. These are timing-sensitive events, not bar-and-beat sensitive events. Changing the tempo of a song might not only change where a song starts in relation to the video, but it also shifts all these time-sensitive events with it as well. Changing the timebase of a track from musical to linear keeps events on this track from shifting in time when the tempo of the project changes. Furthermore, if you want to lock all events on this track from being edited or moved by mistake, you can lock the track.
Looking at Figure 14.13, you can see the same two audio events on the same two tracks. Even though all audio events are the same in both examples, the tempo setting is different in the upper portion than it is in the lower one. The track 2 (Narration 03) is set as a linear timebase track, whereas the track 1 (Audio 01) is set as a musical timebase. Track 2 contains a narration and track 1 a beat. In the top portion, you can see in the Transport panel that the tempo is set at 75 BPM and the display shows that the play line is at Bar 3, Beat 1, which at 75 BPM occurs 06 seconds and 11 frames after the beginning of the project.
In the lower portion, things have changed; the events were not moved, but the Tempo track has been deactivated, bringing the tempo up to 120 BPM from its previous 75 BPM. As you can see, the track 2 (Narration 03) did not move in time and neither did the play line, but both are now at Bar 4, Beat 1, past the fourth 16th-note. That's because in the time needed to play three bars at 75 BPM, you can play more bars at 120 BPM. Looking at the Transport panel, which displays both Bars & Beats (on the left) and Timecode (on the right), you can see that the time in frames remains the same, while the time in bars and beats changes. Also, the linear tracks remain unchanged, while musical tracks are adjusted according to fit the new tempo setting.
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