Apple Final Cut Express HD - Manual (Page 867)

Chapter 59 Using RT Extreme 867
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Playback video quality and frame rate can be controlled independently. For example, if
you need to see every frame during playback, you can set your sequence playback
frame rate to Full and video quality to Dynamic. In this case, your sequence video
quality may be reduced during playback, but every frame will play. On the other hand,
if you are doing detailed compositing work that depends on high quality video display,
you can set your sequence playback video quality to High and your sequence frame
rate to Dynamic.
If neither frame rate nor video quality is critical, you can choose Dynamic for both
playback video quality and frame rate.
About Safe Real-Time Playback
The Safe RT mode guarantees that effects are played back at the quality and frame rate
you specify, and that no frames are dropped during playback. If Final Cut Express HD
anticipates that your computer won’t be able to perform all the effects calculations in
real time, the render status of that portion of the Timeline becomes red, which means
you have to render before you can play back.
Red video render bars indicate
rendering is required.