The red line at the top of the timeline indicates that the clip must be rendered before it will play back.
This could result from many things, such as having many intensive filters applied to the clip and being in “Safe” RT Mode.
But for novices, nine times out of ten, the reason the clip “redlines” is because the sequence settings do not match the footage type you are dropping into it.
Check the clip format by highlighting it and pressing Cmd 9.
Check the sequence format by clicking in an empty area of the timeline, then pressing Cmd 0 (zero).
The two should match, meaning frame size, frame rate, video and audio format should be identical.
If they don’t match you have two choices:
1. Transcode your incoming media to the format of your timeline, e.g., if your timeline is HDV 1080i 29.97 fps, with audio at 48 KHz, 16-bit uncompressed, then transcode all the video you want to edit on that timeline to that format. We always use DV Kitchen for batch transcoding, it’s fast and high quality. This is the best option.
2. If the video clip you want to use in your movie is already in an editing codec, like Pro Res, HDV, DV, DVCPRO, JPEG etc. then you can just import it in its native format and hope it plays in real time or close to it.