The big idea: Compression stores the same thing in fewer bits.
Smaller files take up less room and travel faster over a network — which is why every photo, song and video you meet has been compressed.
Why it is possible at all: Because real files repeat themselves. A photo of a sky has thousands of almost identical blue pixels in a row.
Anything that repeats can be written down more briefly than spelling it out.
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Run-length encoding: Instead of storing every pixel, store how many in a row and what colour.
Six white pixels become "6 white". Expand it again and you get all six back, exactly.
One row of an image: 20 values become 5 count-and-colour pairs, and every original pixel can be recovered.
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What lossless means: The original can be rebuilt perfectly. Nothing has been discarded — only written more briefly.
Essential for text, code and spreadsheets, where one wrong character matters.
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Throw away what nobody notices: Lossy compression permanently removes detail people are unlikely to miss — slight colour differences, sounds too quiet to hear behind louder ones.
It saves far more room, but the original is gone for good.
Lossless
- Original rebuilt exactly
- Saves less space
- Text, code, spreadsheets, ZIP, PNG
- Run-length encoding is one method
Lossy
- Original cannot be recovered
- Saves much more space
- Photos, music, video — JPEG, MP3
- Transform coding is the usual method
Transform coding, briefly: The method behind JPEG and MP3. It converts the data into a form that separates detail people notice from detail they do not, then keeps the first and discards the second.
How this is tested — you must say WHAT each method does to the data and match it to a file type. It comes up two ways:
Paper 1 Section A
- Describe compression, or contrast lossy with lossless, 2-4 marks
- Explain run-length encoding with an example
- Choose a method for a stated kind of file
Paper 1 Section B — case study
- Recommend compression for the case study's files
- Justify it from what those files are used for
The classic trap: Saying lossy "makes files smaller" and lossless "keeps quality". Both make files smaller. The difference is whether the original can be rebuilt exactly — say that.
A company stores scanned contracts and promotional photographs. Explain which type of compression suits each, justifying your choices.
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