Apple TV/FM Radio System - Manual (Page 110)

Using compression
With compression you can store movies in a much smaller
space than they would normally require. To use
compression, follow these steps before you record a movie:
1 Choose Preferences from the Setup menu.
The Preferences dialog box appears.
2 Click one of the buttons under Movie Compression.
None means that no compression takes place. Normal, the
preset choice, uses anywhere from 12 to 50 percent of the
disk space that None uses; the less movement in each frame
of the movie, the greater the savings. Most uses slightly less
disk space than does Normal.
To save the most space you can, choose the smallest size
video window and the most compression.
Note: Compression affects picture quality: the more
compression you use, the grainier the picture.
103 Capturing TV and Video Images