Analog source generally comes with certain established characteristics. For example, CD-quality audio is sampled at 44.1 KHz, 16-bit stereo sound, and video is usually 640 x 480 pixels in dimension and plays at 30 frames per second (fps). However, analog source digitized at full resolution would require enormous amounts of disk storage and is far too large to be used on a network. One way to prepare media for network delivery is to reduce the data by, for example, downsampling the audio material to 11.025 KHz, 8-bit mono sound. This reduces file size but also substantially reduces quality. Another way to reduce file size is to apply compression.

Compression first eliminates redundant data from a file and then removes less important data to shrink file size still further. This process is achieved using algorithms, or "codecs" (short for compressors-decompressors), that handle the media compression and the decompression when it is played. The codecs that are used for Web delivery use lossy compression: the process removes data from the original source material. You should never compress material multiple times, because each process will lower the video quality.

In preparing media for Web delivery, you should aim for files that can be managed by the average network connection and desktop machine of your target audience. The key measure is the data rate, normally measured in kilobytes per second (KBps), which is the amount of data that is used to represent one second of movie playback. For users to play your files in real time without hiccups or delays, you need to set a data transmission rate that is slightly lower than the throughput of your users' connections.

Illustration: Interaction of data rate and network capacity

Audio processing

Video processing