That Blue Square Thing

Computer Science GCSE

Note: this page deals with the old Edexcel GCSE (grades A-G) which no longer exists. I'm keeping it as an archive and because a number of the resources will still apply to newer (grades 1-9) courses. The AQA GCSE CompSci pages deal with a current course.

Data Compression

As computers have become more powerful we have started to save more and more data. Digital photographs, for example, used to be restricted by the secondary storage devices (such as floppy discs and memory cards) which were available. They had to be small in order to fit onto a floppy disc with 1.44MB capacity.

Storage has changed and storage memory and secondary storage devices have much greater capacities nowadays and are much cheaper. Network speeds are quicker as well, so we can transfer files quicker over networks such as the internet.

Even with cheap storage memory it is still useful to reduce the size of a file where possible. This allows, for example, a computer to process the data quicker and for web users to be able to download image or sound files much more quickly, particularly over slower mobile networks.

Reducing the size of a file is known as compressing it.

How compression works

The key things to make sure that you understand here are:

This all links to an understanding of how files are stored and how file size and file transfer rates work.

PDF iconData Compression - key notes, including on Run Length Encoding (RLE)

PDF iconCompression question - exam style question

The BBC Bitesize site has sections about data compression for image fileswiki link and for sound fileswiki link. The video is the same on both pages - and talks about the key differences between lossy and lossless compression.

Run Length Encoding is dealt with on Bitesizewiki link as well, although in relation to video files. There is a little more detail, linked to image files, on the Scottish Higher Bitesize pagewiki link (note: this is sixth for material!)

The Impacts of Data Compression

There are many obvious impacts of data compression - smaller files to transfer across networks, for example, or to store on mp3 devices or video devices.

There are other, wider, impacts as well though. This crosses over into the impacts of computing on society section of the syallabus.

PDF iconThe Impacts of MP3 Compression - a long, exam style question

PDF iconNotes about the impacts of MP3 - to help with the question

PDF iconBBC news article, July 2015 - it's illegal to rip copies of your own CDs or DVDs again...