A movie is a series of images played rapidly to create movement. Remember webcasts are all about throwing away unneeded data. The Key Frame is a critical component in what the file size will be.
Video software takes the original movie and compresses it. The Key Frame compression process only keeps the changes from one frame to another. So if you have a PowerPoint slide on Frame 1 and the only difference in Frame 2 is the mouse moves, Frame 2 will only consist of the part that shows the mouse movement.
With compression there are two types of frames:
1. Key Frame: the entire frame
2. Delta Frame: only has the area that changed
Within video software is the option to change the Key Frame Rate. The fewer Key Frames you use the smaller the file size. Here are two examples:
1. For PowerPoint content, where little changes from frame-to-frame, use a high Key Frame Rate (10-80).
2. For live video, where the constant motion has lots of change from frame-to-frame, use a low Key Frame Rate (1-12).
Frames Per Second and Key Frame Rate
Within video software the Frames Per Second (fps) can be adjusted. We need to consider the fps when setting the Key Frame Rate. We need to really consider how much changes from frame-to-frame how often. Here are to examples:
1. If encoding at 30 fps and the Key Frame Rate is 80, a new Key Frame will occur very quick – about every 2.5 seconds.
2. If encoding at 5 fps and the Key Frame Rate is still set at 80, the Key Frames are nearly 40 seconds apart.
– Troy @ TLC