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1.2.5Compression

OCR GCSE Computer Science (J277) · Computer systems (Paper 1) › Memory and storage

Practise Compression. 12 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.

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Sample questions

Written for this site in the style of OCR exam questions. They are not taken from real past papers.

Question 1Easy3 marks
(a) State one reason why files are compressed.[1]
(b) Which type of compression permanently removes some of the data?
Tick (✓) one box.[1]
  • Lossless
  • Both lossy and lossless
  • Neither
  • Lossy
(c) Give one example of a file format that uses lossy compression.[1]
Show the answer and mark scheme
(a)
  • to reduce the file size, so it takes up less storage space / is quicker to send or download
(b) Answer: Lossy
(c)
  • JPEG / MP3 / MP4 / AAC
Question 2Medium4 marks
Describe the differences between lossy and lossless compression.[4]
Show the answer and mark scheme
  • lossy compression permanently removes some of the data
  • the original file cannot be restored exactly from a lossy file
  • lossless compression does not lose any data
  • the original file can be reconstructed exactly from a lossless file
  • lossy compression usually reduces the file size by more than lossless
  • lossy compression removes detail that people are least likely to notice
Question 3Hard4 marks
A company's website uses lossless compression for its logo but lossy compression for the photographs on its product pages.
Explain why the company uses different types of compression for these images.[4]
Show the answer and mark scheme
  • the logo has sharp edges / text / a few flat colours
  • lossy compression could blur the edges / create visible marks (artefacts) / change the exact brand colours
  • lossless keeps the logo exactly as designed, and its file is small anyway
  • photographs have large file sizes with many colours
  • lossy compression reduces them much more, so the pages load faster
  • the small loss of detail in a photograph is hard to notice

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