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 Pro anticipates
that your computer won’t be able to perform all the effects calculations in real time,
the render status bar for that segment becomes red, which means you have to render
before you can play back.
Before using the Print to Video or Edit to Tape commands to output to tape, you can
switch to Safe RT mode to see how many segments require rendering.
Even though the Safe RT option guarantees that no frames will be dropped
because of processor limitations, frames may still be dropped if your scratch disk
cannot maintain the necessary data rate during playback. This can be a problem if you
have multiple video streams playing from a relatively slow hard disk. In this case, the
Timeline may show a green render bar, indicating that the processor can handle the
effects load, but you may still drop frames because of slow disk performance. To solve
this issue, you can limit the acceptable real-time data rate of media playback in the
General tab of the User Preferences window. For more information, see Volume IV,
Chapter 23, “Choosing Settings and Preferences.”
Red render bars indicate
rendering is required.
Real Time and Rendering