Curriculum Corporation
Video Research Project

Dehydrated files ... just add water
Compression is not that mysterious. If you've ever downloaded a file with the .zip suffix to your Windows PC or .sit on the Macintosh, then you’ve downloaded a file that has been subject to file Compression in order to make the file more compact and minimise the download time. Software such as WinZip for Windows or Stuffit Expander for the Macintosh uncompress or reconstitute these compressed files back into identical copies of the original. Remember that you won’t be able to open these files without these applications or other applications that support the compressed archives.

The Need for Compression

Useful background whitepapers

Further reading

Key concepts

  • Multimedia files are large and consume lots of hard disk space. The files size makes it time-consuming to move them from place to place over school networks or to distribute over the Internet.
  • Compression shrinks files, making them smaller and more practical to store and share. {glossary:windowsmedia}Windows Media{~glossary} and {glossary:quicktime}QuickTime{~glossary} are popular examples of Compression formats for video clips.
  • Compression works by removing repetitious or redundant information, effectively summarising the contents of a file in a way that preserves as much of the original meaning as possible.
  • Some Compression formats may require a relatively fast computer to de-compress or play back the footage, and can therefore behave poorly on a slower system.

Introduction

Have you ever waited impatiently for a file to download from the web? This file may well have been a video clip of some sort. Video files contain a great deal of information and can be very large. Consequently, they are slow to download.

Consider this example:

Your students are participating in a project with another school and have shot a 5-minute video about the status of the project on a {glossary:dv}DV{~glossary} {glossary:camcorder}Camcorder{~glossary}. When transferred to a computer for editing, this footage will consume around one GB (one {glossary:gigabyte} Gigabyte{~glossary} or 1000 {glossary:megabyte}megabytes{~glossary}) of hard drive space. How long would it take to send this clip using the fastest {glossary:dialup}dialup{~glossary} {glossary:modem}Modem{~glossary} available today? Five minutes? An hour? A day? In fact around two and a half days!

In contrast text files can be sent quickly because they are relatively small and don’t take too long to transfer. In comparison, multimedia files can be massive.

If only these files weren’t so big then they wouldn’t be such a headache. In fact files can be made more compact. This squeezing process is called, appropriately, {glossary:compression}Compression{~glossary}. A compressed file requires a smaller amount of storage space and moves more quickly over a computer network, whether this is the school local area network (LAN) or the Internet.

Figure 1 shows the dramatic reduction in file size due to {glossary:compression}Compression{~glossary}. On the left-hand side of the graph, it can be seen that a single minute of full-screen {glossary:dvd}DVD{~glossary} video weighs in at 37MB. {glossary:dvd}DVD{~glossary} uses the {glossary:mpeg2}MPEG2{~glossary} digital video {glossary:compression}Compression{~glossary} format that was introduced in 1995. Compare this with the same full-screen video clip encoded with the latest {glossary:windowsmedia}Windows Media{~glossary} technology from Microsoft - it is about one-third of the size - revealing how far {glossary:compression}Compression{~glossary} technology has advanced.

Figure 1: Reduction in file size due to compression
Figure 1 Reduction in file size due to compression

On the right, we can see that a 3-minute audio track, when compressed using the popular {glossary:mp3}mp3{~glossary} format, is only 3MB, less that one-tenth of the size of the original size.

The fact that a digital multimedia file such as a video clip, sound file or image can be shrunk might make you wonder what was ‘wrong’ with the original file in the first place. Was it simply padded out with a lot of unnecessary information that can be thrown away?

In fact, {glossary:compression}Compression{~glossary} does work by removing ‘unnecessary’ information (we’re not talking here about cutting those wobbly shots of the grass when you forgot to turn the camera off - that’s called editing).

Text documents can be compressed and yet retain their original essence; similarly, multimedia files are compressed by eliminating information whose absence will (ideally) go unnoticed. Typically, this is repetitive information or falls outside the range of human perception - in other words it is redundant.

More information about {glossary:compression}Compression{~glossary} techniques, as they apply to still images, is contained in the Image Compression Whitepaper. In the Video Compression Whitepaper, this discussion is extended to moving images.

Key terms

{glossary:bandwidth}Bandwidth{~glossary}
{glossary:camcorder}Camcorder{~glossary}
{glossary:compression}Compression{~glossary}
{glossary:dialup}Dialup{~glossary}
{glossary:dv}DV{~glossary}
{glossary:dvd}DVD{~glossary}
{glossary:gigabyte}GB{~glossary}
{glossary:gigabyte}Gigabyte{~glossary}
LAN
{glossary:megabyte}MB{~glossary}
{glossary:megabyte}Megabyte{~glossary}
{glossary:modem}Modem{~glossary}
{glossary:mp3}mp3{~glossary}
{glossary:mpeg2}MPEG2{~glossary}
{glossary:quicktime}QuickTime{~glossary}
{glossary:redundancy}Redundancy{~glossary}
{glossary:windowsmedia}Windows Media{~glossary}