All topics › Edexcel GCSE Computer Science › Lossless and lossy compression (incl. RLE)2.3.2 Lossless and lossy compression (incl. RLE)Edexcel GCSE Computer Science (1CP2) · Data › Data storage and compression
Practise Lossless and lossy compression (incl. RLE). 11 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.
Build a paper on this topic ▶ Watch videos on Lossless and lossy compression (incl. RLE) (Craig 'n' Dave Edexcel on YouTube) · Practise all of Data storage and compression
Quick recall Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Give two reasons why files are compressed. Any two from: less storage space; faster transfer; less bandwidth; streaming; size limits.
Give one type of file that is often compressed using lossy compression. A photo, a music track or a video. Sample questions Written for this site in the style of Edexcel exam questions. They are not taken from real past papers.
Question 1 Easy 3 marks
(a) Give two reasons why files are compressed.[2]
(b) Identify the file that must be compressed using lossless compression.[1]
A photo for a social media post A song for a music-streaming service A program's executable file A video for a video call Show the answer and mark scheme (a) Answer: Any two from: less storage space; faster transfer; less bandwidth; streaming; size limits.
to use less storage space so files can be transferred / downloaded / uploaded faster to use less bandwidth / data allowance so that media can be streamed to fit within file-size limits, e.g. for email attachments (b) Answer: A program's executable file
Question 2 Medium 4 marks
(a) A novelist emails the draft of a book to a publisher.[2]
(b) A video-streaming service uses lossy compression.[2]
Show the answer and mark scheme (a) Answer: Lossless keeps every bit, so the text is restored exactly; lost data would corrupt the words.
lossless compression allows the original file to be restored exactly / no data is lost every character of the text must stay the same; losing data would change or corrupt the words (b) Answer: Much smaller files stream smoothly over limited bandwidth, and the lost detail is barely noticeable.
lossy compression makes files much smaller than lossless compression so video can be streamed quickly / without buffering using less bandwidth, and viewers hardly notice the loss of some detail Question 3 Hard 4 marks
(a) A photographer edits a photo and saves it in a lossy format several times.[2]
(b) Explain why lossless compression reduces the size of a cartoon drawing much more than the size of a photograph of a forest.[2]
Show the answer and mark scheme (a) Answer: Each save compresses the image again, throwing away more data, so the damage accumulates.
each time the file is saved, lossy compression is applied again so more data is permanently removed each time and the errors / artefacts build up (b) Answer: The cartoon has big areas of identical colour to compress; the photo's pixels change constantly, leaving few repeated patterns.
the cartoon has large areas of exactly the same colour / many repeated patterns (long runs) the photograph has lots of small changes in colour between neighbouring pixels, so there are few repeated patterns for lossless compression to use Related subtopics
Stuck? Get 1-to-1 help. Chhetri Academy tutors GCSE and A level Maths and Science online, with a free 30-minute trial lesson.
Book a free trial