User Guide

AVI VIDEO COMPRESSION

If you thought audio files chewed up your hard-disk space quickly, wait until

you start using video! CD quality audio takes up about 10 MB of hard-drive

space per minute (60 seconds x 44,100 samples per second x 2 tracks for stereo

x 2 bytes per 16-bit sample). However, typical video for multimedia use can

easily take up about 200 MB per minute for a small window of video (60

seconds x 15 frames per second x 320 x 240 x 3 bytes for 24 bit pixels). Using

uncompressed video as a final distribution format is out of the question for

most practical purposes. Video compression is a necessary evil.

COMPRESSION ALGORITHMS

Many different compression algorithms exist and many more will continue to

be created in the next several years. With all algorithms, there will be trade-

offs between video quality, size reduction, and compression/decompression

processing time. Compressing by large amounts quickly will generally produce

visual artifacts such as jumpy or grainy video.

When saving an AVI file in Sound Forge XP, you can select from all of the

compressors installed in your system. Included with Windows are Cinepak by

Radius (cvid), Intel Indeo (IV32), and Microsoft Video 1 (CRAM). These can

be selected from the Video Compression dialog (see section above).

COMPRESSION SETTINGS

Many compressors use key frames during compression and decompression. A

key frame is a frame in the video stream that is not inter-frame compressed.

The following compressed frames are derived from the closest key frame.

Having fewer key frames will make the video size smaller. However, it will take

a lot more time to display the frames, especially when jumping around the end

video.

When editing AVI files in Sound Forge XP, the display speed will suffer

greatly if there is only one key frame in the entire file. In such cases,

Sound Forge XP needs to scan all preceding data when drawing every

single frame.

FILE FORMATS AND ATTRIBUTES

NOTE