Lossy
Compression

Data Is Permanently Removed · JPEG · MP3 · Trade-offs

CSZoneAQA GCSE Computer Science 8525

What is Lossy Compression?

Smaller Files — Permanent Data Loss

Lossy compression achieves much smaller file sizes by permanently removing data that humans are unlikely to notice. The original file cannot be perfectly restored — some quality is permanently lost.

Used for: JPEG images, MP3/AAC audio, MP4/H.264 video

Can achieve much higher compression ratios than lossless

Acceptable when minor quality loss is not critical — e.g. web images, streaming music

Lossy vs Lossless Comparison

Choosing the Right Type

Feature	Lossless	Lossy
Data after decompression	Identical to original	Some data permanently lost
Compression ratio	Moderate	High (much smaller files)
Quality after decompression	Perfect	Reduced (artefacts possible)
File formats	PNG, ZIP, FLAC, GIF	JPEG, MP3, AAC, MP4
Best used for	Text, code, medical	Photos, music, video

How Lossy Compression Works

Removing Imperceptible Data

JPEG (Images)

Groups of similar-coloured pixels are replaced with a single average colour. Fine details and subtle colour differences are removed.

MP3 (Audio)

Sounds outside the range of normal human hearing are removed. Very quiet sounds masked by louder ones are also discarded.

Key principle:Remove data the human senses are unlikely to notice — exploit limitations of human perception.

Exam Practice

Have a go at this question

AQA-style question

A web developer wants to compress a photograph for display on a website. Explain why lossy compression is more suitable than lossless compression in this situation.

3 marks

Lossy compression creates much smaller files [1] which load faster on a website [1]. Minor quality loss in a web photograph is unlikely to be noticed by viewers [1] so the reduced file size is worth the trade-off.

Key Takeaways

What to Remember

Lossy compression permanently removes data — file cannot be fully restored

JPEG (images), MP3 (audio), MP4 (video) — all use lossy compression

Much higher compression than lossless — small files, fast to send/load

Best when small file size matters more than perfect quality — streaming, web, social media