Apple Final Cut Pro 6 - Manual (Page 1734)

Chapter 18 Exporting Using QuickTime Conversion 251
III
 Key Frames: Key frames are available if your selected codec uses temporal
compression. Most video frames in close proximity have a high percentage of visual
redundancy. Compression key frames reduce data rate by only storing complete
images at regular intervals or when abrupt visual shifts occur. The remaining frames
only store information about the change, or delta, between themselves and the key
frame. Increasing the number of frames between key frames increases the amount
of compression and makes the final file size smaller.
Depending on the codec you use, the movie file’s quality, especially for clips and
sequences with a lot of motion, may decrease if there are too few key frames
specified. If there’s a lot of motion in your media file, you need key frames more often
than you do with more static footage, such as an interview sequence. A setting of
one key frame every ten frames is a good starting point.
Some video codecs allow you to insert a key frame every certain number of frames,
while other video codecs use natural key frames by scanning the whole file to
detect where the major similarities and differences occur and inserting the key
frames accordingly.
 Automatic: QuickTime adds temporal compression key frames when necessary.
 Every N frames: Temporal compression key frames are created every N frames.
Because key frames require more data to store than the in-between frames, a
higher value here results in a movie with a lower data rate.
 All: A key frame is added on every frame. This increases the data rate of the
movie significantly.
Key frame
Delta frames
Key frame