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Sample lesson plan: data compression

This is an example of one approach to teaching data compression. It uses the specification content to set the learning objectives for the lesson. It assumes that the lesson duration is 1 hour.

Topic

3.5.6.7 (AS), 4.5.6.9 (A-level) Data compression

This example of one approach to introducing the topic is designed for a lesson lasting one hour.

Learning objectives

Know why images and sound files are often compressed and that other files, such as text files, can also be compressed.

Understand the difference between lossless and lossy compression and explain the advantages and disadvantages of each.

Explain the principles of run length encoding (RLE) as a technique for lossless compression.

Next lesson: dictionary-based methods.

Prior knowledge needed

Ability to design, code and test simple programs.

Experience of using image viewing and/or sound files and software.

Lesson preparation

Appropriate computer facilities to explore solutions, develop and run code.

Datafile 1: simulated or real data from a weather station or other sensor taking frequent observations over 24 hours. There are likely to be extended runs of unchanging values, making this a good example for compression.

Datafile 2: RAW image file of highest available resolution, ideally showing a fine-detailed subject such as a map.

Image processing software capable of importing RAW image and exporting in both lossy (eg JPEG) and lossless (eg GIF) formats. (eg Irfanview, with plugins installed.)

CD recording of a timeless track (personal taste dictates the choice, but The Grateful Dead have a place in computer science – through the WELL, which has links to the Electronic Frontier Foundation); and an MP3 or compressed YouTube version of the same.

Activity

Duration Activity Group size Assessment objective
5 mins Discuss purpose of compression, such as reduced bandwidth and storage.

Suggested resource: Run Length Encoding Visualization video (1:37)

Whole class Understand the point of compressing data.
5 mins

Consider Datafile 1 or part of it.

Estimate a compression ratio.

Individual Understand the concept of RLE.
20 mins

Write and test code to implement a lossless compression algorithm for Datafile1. Compare the output with the estimated compression ratio.

Suggested resource for teachers: http://rosettacode.org/wiki/Run-length_encoding

Note: if students’ skills and experience make completion of this activity in this time unrealistic, it could be set as homework.

Individual or pairs

Practical experience with RLE.

Enhance experience of algorithms, coding and testing.

3 mins Compare the performance of a standard compression utility (eg Zip) on the same file. Individual or pairs Experience of widely used utility
15 mins Load Datafile 2 and experiment with different compressed versions, with varying settings for lossy compression (eg JPEG) and exploring small areas at high magnification. Individual or pairs

Observe compression artefacts – perhaps with difficulty

Compare file sizes

5 mins Compare audio quality of CD and compressed versions, using best speakers available. Whole class

Observe compression artefacts – or not

Discuss uses of different formats

7 mins

Plenary discussion:

  • can we tell the difference, with images and video? Limits of human perception are variable, especially high-frequency audio with age
  • usefulness of small file sizes in portable devices
  • impact of lossy compression on text files such as Datafile1.
Whole class

Know why images and sound files are often compressed and that other files, such as text files, can also be compressed.

Understand the difference between lossless and lossy compression and explain the advantages and disadvantages of each.

Further work and reading

Extension

Consider the effect of open or restricted licensing on the use of compression algorithms (eg GIF vs PNG).

Explore the working of the Lempel-Ziv-Welch lossless compression algorithm. Where is LZW used? (eg GIF, Pkzip). For example, Elegant Compression in Text (The LZ 77 Method) – Computerphile video.

Preparation for next lesson

Research Huffman compression, eg Text Compression with Huffman Coding

Homework

Compile revision notes or podcasts, perhaps using a class Wiki, on the topic.

Answer specimen/past exam questions.

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