IMPORTANT VARIABLES

Data rate

The speed material is delivered to the viewer - closely tied to the connection speed of the end user. If you create material that run at a higher data rate than your mode of delivery you will grind to a standstill. Nothing will be delivered.

Here are some upper limits for data rate settings for various delivery modes. Remember this includes video and audio.

• 56K modem - 4K/second
• ISDN - 12K/second
• T1 or cable modem - 20K/second
• CD-ROM - 100K/second
• Hard drive - 250K/second

A good way to calculate the approximate data rate you should be aiming to produce for a given mode of delivery is to use this simple calculation:

Frame rate of media

At least half the rate of original media, ie 12.5 as opposed to 25 frames per second.

Less for streaming to a 56K modem connection - 1/3 original rate

(DVD is the only format which will deliver full frame rate and full screen video).

Frame size of media

• Original media - 768 x 576
• DVD - maintains original
• CD-ROM - 320 x 240
• Internet - 240 x 180 or 160 x 120

Keyframe frequency

Rule of thumb is one per second. You can set this to natural and the chosen codec will set its own key frames.

Audio rate and size

Again there are trade-offs regarding the rate and resulting file size when using audio compressors.

• 11Khz, 8 bits — low quality — 662 K per minute
• 11Khz, 16 bits — much better — 1.3 mb per minute
• 22Khz, 16 bits — very acceptable — 2.6mb per minute
• 44.1Khz, 16 bits — CD quality — but 5.3mb per minute mono and 10.6mb per minute in stereo

All of these variables work together to determine the file size and deliverability of your completed project. Compression is as much an art as it is a science and your own experiments will carry you further down the path to understanding compression.