Compression for streaming and websites

Streaming means sending audio or video that is played while it is still arriving. For streaming to work, the data must arrive at least as fast as it is played, so the connection has to keep supplying data quickly enough for playback to run without stopping. If the data arrives too slowly, playback pauses while more is downloaded.

Why streamed video is compressed heavily

Uncompressed video is far too large to send over a normal internet connection quickly enough. A few minutes of video can be several gigabytes, which would take far too long to download before anyone could watch it. Video is therefore compressed heavily before it is streamed: the file is made much smaller, so the bits needed for each second of video arrive in time to be played without stopping.

For example, a school streams its winter concert live to families at home. The video is compressed heavily so that each moment of the concert reaches viewers just in time to be shown, letting them watch continuously instead of waiting for large chunks of video.

Why images are compressed for websites

A web page is made from several files, including images. Before the page can appear, your browser must download these files. Images are often the largest files on a page, so compressing images reduces how many bits have to be downloaded. With smaller image files, the page can be displayed sooner.

For example, a pupil on a slow rural connection opens an animal rescue centre's web page to check its opening times. Because the photos on that page are compressed, the page and its pictures appear quickly; the full-size originals would have left the pupil staring at empty boxes for much longer.