Lossy, Lossless

Types of compression: Lossy, Lossless

1.2.5 Compression

Compression — Note 2 of 2

What you need to know

There are two types of compression: lossy (which permanently removes some data to achieve smaller file sizes) and lossless (which compresses without losing any data). Each is suited to different applications.

What to know

Lossy is used for: images, audio, video — where slight quality loss is acceptable for much smaller files.
Lossless is used for: text files, program files, medical images — where NO data can be lost.
Lossy achieves GREATER file size reduction than lossless.

Key terms

Lossy Compression: Permanently removes some data to reduce file size. Quality is reduced and the original file CANNOT be fully restored.
Lossless Compression: Reduces file size without losing any data. The original file CAN be fully restored.

Exam technique

Tip:If asked which type is suitable, consider whether data loss is acceptable. For text/code = lossless (every character matters). For media = lossy (small quality loss is OK).
Mistake:Saying lossless 'doesn't compress as much'. It is still effective — it just does not reduce size as dramatically as lossy.
Tip:For text files, you MUST use lossless — lossy cannot be applied to text files because removing data would corrupt the file/make it unreadable.
Tip:When justifying lossy for images, say: 'lossy reduces the file size by the largest amount AND the changes are unlikely to be noticed by the user'.
Mistake:Just saying 'lossy compresses the file more'. That repeats the question. Explain what this MEANS — e.g. 'reduces file size significantly because it permanently removes unnoticeable data'.

Ready to test yourself?

Practise 1.2.5 questions

← Previous noteThe need for compression1.2.5 Compression Next topic →LAN (Local Area Network), WAN (Wide Area Network)1.3.1 Networks and topologies

Back to Compression

Revise another OCR GCSE topic: Data Storage: Images