Introduction

(See also our article on Choosing appropriate settings for optimising audio and video files.)

New media developers are familiar with the constraints of data size and data rate when authoring for fixed capacity storage media (such as CD-ROM) and network bandwidth. Even small images must be optimised to their smallest possible size. Because video and audio files contain a great deal of data, extra care needs to be taken to ensure fast delivery and smooth playback on desktop computer systems.

Playback of multiple audio tracks is no longer problematic for desktop systems (although speedy transfer over networks is). For a variety of reasons, playing back full frame PAL or NTCS video (with its associated audio tracks) is still a major challenge. Transferring files over networks is even more problematic. Therefore video files usually need optimising or compressing before being published for delivery.

You may need to ...

Have video running from a CD-ROM or DVD-ROM
Have video running from an internal HD
Have video running off a floppy disc
Have video running off a Zip disc
Have video files download from a web site
Have video streaming over the internet
Have video running over an intranet

You may therefore need to consider ...

End-user system
Network performance (bandwidth, traffic etc)
Frame size
Frame rates
Compression settings (audio and video)
File format
Storage devices available on an end-user system
Capacity of delivery media (CD-ROMs etc)

What is Data rate (or Bit rate)?

"Data rate"(or bit rate) is used to measure ...

The average amount of data per second that a file (video, audio etc) contains.
The maximum speed of a connection technology such as USB, Firewire or an internet connection.
The speed with which data can be transferred from a storage device to RAM (normally referred to as "sustained data transfer rate")

Clearly data will arrive in RAM faster from an internal hard disc drive than a 56Kbps internet connection.

The info dialogue box showing the compression codecs, settings, data rate/size etc for a Hollywood film QuickTime video clip trailer.

What is Data size?

"Data size" is the total size of a given video or audio file (data rate multiplied by the video or audio file's duration in seconds).

You will be concerned with data size for the following reasons ...

Because all storage devices have finite capacity you will need to ensure that your data will fit on your chosen storage media.
If an end-user must download a complete file before viewing and time is an issue. How long will your end-user be willing to wait?
If your data files are so large that they place unreasonable demands on the free application RAM of your end-user's system.