Knowledge Organiser: Compression
This topic summary covers Knowledge Organiser: Compression within Compression for GCSE Computer Science. Revise Compression in Memory & Storage for GCSE Computer Science with 15 exam-style questions and 16 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 12 of 12 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 12 of 12
Practice
15 questions
Recall
16 flashcards
Knowledge Organiser: Compression
Key Terms
- Compression: Reducing file size by encoding data more efficiently
- Lossless compression: File size reduced with no data permanently lost — exact original recoverable
- Lossy compression: File size reduced by permanently removing data — original cannot be recovered
- RLE (Run-Length Encoding): Lossless algorithm that replaces repeated values with a count and value
- Compression ratio: How much smaller the compressed file is compared to the original
Must-Know Facts
- Lossless: 30–70% file size reduction; reversible; exact original restored
- Lossy: 80–95% file size reduction; irreversible; some data permanently lost
- Lossless formats: PNG, ZIP, FLAC, GIF
- Lossy formats: JPEG, MP3, MP4, AAC
- Use lossless for: text, programs, medical images, legal documents
- Use lossy for: photos, music, video (human perception is the limit)
- RLE example: AAAAAABBB → 6A3B (count then value)
- RLE can make files larger if there are no long runs of repeated data
Key Concepts
- RLE method: Scan for runs of identical values; store as (count, value) pairs
- Why lossy works for media: Removes data humans cannot perceive (inaudible frequencies, invisible detail)
- Lossy is irreversible: Once data is removed it cannot be recovered — never compress originals with lossy
- Trade-off: More compression = smaller file but lower quality; less compression = better quality but larger file
Common Mistakes
- Mixing up lossy and lossless: Lossy permanently removes data (used for audio/video/images where quality loss is acceptable); lossless keeps all data and is reversible (used for text, software, ZIP files)
- Saying lossless compression is always better: Lossless achieves less size reduction — lossy compression is preferred when file size matters more than perfect quality (e.g. streaming music)
- Thinking RLE always reduces file size: Run-length encoding only helps when there are long runs of repeated values — images with many different colours (e.g. photographs) can actually get larger with RLE
- Saying compressed files take longer to transmit: Compression reduces file size, which reduces transmission time — this is one of the main reasons compression is used
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Practice Questions for Compression
Which statement correctly describes lossy compression?
Explain how run-length encoding (RLE) works to compress data.
Quick Recall Flashcards
15 questions on Compression — practise free
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