That Blue Square Thing

Syllabus content:

Unit 3 compression content – just compression

Note: this syllabus content is a slightly amended version of the one published freely on the web by AQA. I have made very minor adjustments to remove some content less suitable for students to use and it is presented here simply to allow the children I teach to download a usable copy of the syllabus content. It is copyright AQA and reproduced here simply to make access easier for students. No attempt to claim copyright is being made, although I could have copied the text into my own interpretation...

AQA Computer Science GCSE

Data Representation – Compression

Compression involves the reduction in file size of data. Basically finding clever ways to make it smaller.

This is dead handy and makes using the modern internet much easier – iPlayer, Spotify, online multiplayer games, Skype – all of it uses compression to make it possible.

There are two specific methods of compression you need to know about:

• Run Length Encoding
• Huffman Coding

Compression – intro slides

Run Length Encoding

Run Length Encoding is fairly straightforward. In an exam you might see it applied to text but it's more likely that it will be applied to a bitmap image of some kind - quite possibly a black and white bitmap.

Compression and Run Length Encoding – everything you need to know

Run Length Encoding examples – slides from class

RLE summary – single slide

Huffman Coding

Huffman Coding is more complex - to the extent that it might well appear to be just weird. It uses an idea called a Binary Tree – which is actually used quite widely in all sorts of ways in computing.

The exam board will use Huffman Coding to encode short pieces of text – the chances are they will use a word with no more than 4 or 5 letters in it.

Huffman Coding slides – the slides I use in class

Huffman Coding basics

Example Huffman Tree – a Huffman Tree to work through, preferably with a teacher

Huffman Tree questions – the easiest way to learn Huffman Trees is probably to work through some questions