A company runs a website advertising holiday cottages. Each page shows photographs of a cottage and has a short audio guide that visitors can play. The company uses lossy compression for the photographs and the audio.
Discuss the use of lossy compression on the company's website.
Your answer should include:
- how lossy compression makes the files smaller
- the effect this has on the photographs and audio
- the benefits to the company and its visitors
Mark scheme
Indicative content
How lossy compression makes the files smaller
- Some of the data in the file is permanently removed, so the file that is stored is smaller.
- The data that is removed is the data that a person is least likely to notice, such as sounds outside the range of human hearing or small differences in colour in a photograph.
- Removing data means a much higher compression ratio is possible than with lossless compression.
The effect on the photographs and audio
- The original file cannot be reproduced exactly when it is decompressed.
- Photographs may show artefacts, such as blocks or noise where there are contrasting colours, so they are lower quality.
- Some of the sound is lost, so parts of the audio guide may be less clear to listen to.
- If the company later wants to use the photographs at a larger size, it cannot recover the lost detail.
The benefits to the company and its visitors
- The smaller images load faster, so each page of the website appears more quickly for a visitor.
- The audio can be streamed, so a visitor can play the guide without waiting for the whole file to download.
- Less bandwidth is used by the web server, so it can cope with more visitors at the same time.
- Less storage space is needed on the server for the photographs and audio files.
Levels
- Level 0 (0 marks): No rewardable content.
- Level 1 (1–2 marks): Basic, independent points are made, showing elements of understanding of key concepts/principles of computer science. (AO1) The discussion will contain basic information with little linkage between points made or application to the context. (AO2)
- Level 2 (3–4 marks): Demonstrates adequate understanding of key concepts/principles of computer science. (AO1) The discussion shows some linkages and lines of reasoning with some structure and application to the context. (AO2)
- Level 3 (5–6 marks): Demonstrates comprehensive understanding of key concepts/principles of computer science to support the discussion being presented. (AO1) The discussion is well developed, with sustained lines of reasoning that are coherent and logically structured, and which clearly apply to the context. (AO2)
Level 3 responses must cover both the effect on the files and the benefits.
Do not accept the idea that lossy compression keeps all of the original data.
Example answers and how they were marked
Lossy compression permanently removes some of the data, for example the parts of the sound that the human ear cannot hear, or small differences in colour in a photograph, so the file that is stored is much smaller than it would be with lossless compression. This means the original cannot be recreated exactly: parts of a photograph with contrasting colours may show blocks or noise, and parts of the audio guide may be less clear. However, the smaller images mean each page loads faster for a visitor, and the audio can be streamed so a visitor can play the guide without waiting for the whole file. Less bandwidth is used too, so the server can cope with more visitors at once.
Why: Explains how data is removed and its effect on the photographs and audio, then links the benefits to visitors and to the server, so Level 3.
Lossy compression makes the files smaller by removing some of the data. This means the photographs might not look as good as the original. The website pages will load faster for visitors because the files are smaller.
Why: Correct basic points about the method, the effect and a benefit (AO1/AO2), but the points are not developed and the audio is not considered, so Level 2.
Lossy compression does not lose any data, so the photographs can be decompressed back to the original picture. The web pages will load faster because the photographs are smaller.
Why: One basic point about faster loading; the description of lossy compression is the wrong way round because data is permanently removed, so Level 1.