📁 Paper 2 · 3.3 Data Representation
3.3.8b Lossy Compression
AQA 8525 · GCSE Computer Science · ~10 min read
Notes
──
Video
──
Worksheet
──
Quiz

What Is Lossy Compression?

In lossy compression, some data is permanently removed from the file. The original data cannot be fully recovered — decompressing a lossy file does not give back exactly the original. In exchange for this loss, much greater file size reductions are possible compared to lossless compression.

Lossy compression works by removing data that humans are less likely to notice — exploiting limitations of human perception.

Lossy vs Lossless — Quick Comparison

✅ Lossless

  • Original data perfectly restored
  • Smaller compression ratio
  • Formats: PNG, FLAC, ZIP
  • Use for: programs, text, medical

⚠️ Lossy

  • Data permanently removed
  • Much smaller file sizes
  • Formats: JPEG, MP3, MP4
  • Use for: photos, music, video

Lossy Compression for Images — JPEG

JPEG (Joint Photographic Experts Group) is the most common lossy image format. It works by:

  • Dividing the image into 8×8 pixel blocks
  • Applying a mathematical transform (DCT — Discrete Cosine Transform) to each block
  • Discarding high-frequency details (fine edges, subtle textures) that the eye is less sensitive to
  • Storing a lower-precision version of each block

JPEG allows a quality setting (typically 0–100). Lower quality = more data removed = smaller file = more visible artefacts (blurring, blockiness). Higher quality = less data removed = larger file = better appearance.

JPEG qualityFile sizeVisual quality
High (80–100%)LargerExcellent — hard to tell from original
Medium (40–79%)MediumGood — some artefacts on close inspection
Low (0–39%)Very smallPoor — obvious blockiness and blurring

JPEG is ideal for photographs, which have many smooth colour gradients. It performs poorly on images with sharp edges (logos, text), where visible "ringing" artefacts appear.

Lossy Compression for Audio — MP3

MP3 removes audio frequencies that humans are least sensitive to, using a psychoacoustic model:

  • Sounds outside the range of human hearing (above 20 kHz) are removed
  • Very quiet sounds masked by loud simultaneous sounds are removed
  • The remaining audio is compressed and stored at a lower bit rate

Bit rate determines quality: 128 kbps = acceptable quality; 320 kbps = near CD quality. Lower bit rate = smaller file = more noticeable quality loss.

Lossy Compression for Video — MPEG/MP4

Video compression (MPEG, H.264, H.265) combines lossy image compression (for each frame) with temporal compression: storing only the differences between frames rather than full images every time, since most of a video frame is similar to the frame before it.

When to Use Lossy Compression

Suitable for lossyNot suitable for lossy
Website photosMedical X-ray images
Music streamingSource code files
Video streamingExecutable programs
Social media imagesArchive backups
Exam tip: The key trade-off is: lossy gives smaller files but permanent data loss; lossless preserves data but compresses less. A common question asks when lossy is unsuitable — answer with programs, text, and medical images where any data loss could cause serious problems.
⚠️ Common Mistakes
  • Saying lossy compression "degrades the file a little" — the original data is permanently gone; you cannot fully recover it
  • Thinking JPEG is good for logos — it isn't; PNG (lossless) is better for images with sharp edges
  • Confusing bit rate (audio quality) with sample rate — bit rate is the data rate of the compressed audio
▶
Video coming soon

Key points

  • Lossy compression permanently removes data; original cannot be recovered
  • Much smaller files than lossless — achieved by removing imperceptible data
  • JPEG: removes fine image detail using DCT; controlled by quality setting
  • MP3: removes frequencies humans can't hear (psychoacoustic model)
  • Unsuitable for executables, text, source code, medical images
Click slide or press arrow keys to navigate
✍️

Worksheet — 3.3.8b Lossy Compression

8 questions · 19 marks

Q1What is lossy compression and how does it differ from lossless compression?[2]
✅ Mark scheme
Mark scheme
Lossy compression reduces file size by permanently removing some data [1]; unlike lossless, the original data cannot be perfectly reconstructed after decompression [1].
Q2Give two examples of lossy compression file formats and the media type each is used for.[2]
✅ Mark scheme
Mark scheme
Any two: JPEG — images/photos [1]; MP3 — audio/music [1]; MP4/MPEG — video [1].
Q3Explain how MP3 compression removes audio data while keeping perceived quality high.[2]
✅ Mark scheme
Mark scheme
MP3 uses a psychoacoustic model [1]; it removes frequencies outside human hearing range and/or sounds masked by louder simultaneous sounds — data that humans would not notice is missing [1].
Q4A graphic designer reduces the JPEG quality setting of a logo from 100 to 20. Describe the effect on the image.[2]
✅ Mark scheme
Mark scheme
File size decreases [1]; visible artefacts appear — blockiness, blurring, loss of sharp edges — so image quality degrades [1].
Q5State two situations where lossy compression is NOT appropriate, and explain why.[4]
✅ Mark scheme
Mark scheme
Executable programs [1] — any data loss could corrupt the program/cause crashes [1]; medical images, e.g. X-rays [1] — loss of detail could lead to missed diagnoses [1]. (Also: text files, source code — any supported with explanation.)
Q6Why is JPEG a poor choice for storing a logo, but a good choice for a photograph?[2]
✅ Mark scheme
Mark scheme
JPEG removes high-frequency detail — logos have sharp edges and flat colours that suffer visible artefacts (ringing/blocking) [1]; photos have smooth gradients where the same data removal is barely noticeable [1].
Q7What is meant by bit rate in the context of MP3 audio?[2]
✅ Mark scheme
Mark scheme
Bit rate is the number of bits of audio data transmitted or stored per second [1]; higher bit rate = better quality audio but larger file size [1].
Q8A music streaming service needs to reduce mobile data usage. They decide to switch streams from 320 kbps to 96 kbps. Evaluate this decision.[3]
✅ Mark scheme
Mark scheme
Advantage: file/stream size is roughly 3× smaller, reducing mobile data usage and buffering time [1]; Disadvantage: audio quality is noticeably lower — more compression artefacts [1]; trade-off: acceptable for casual listening or weak network conditions, but audiophiles or premium subscribers would be dissatisfied [1].
✓
Check your answers above.
Topic Quiz
Q 1 of 10
You scored
out of 10
Card 1 of 5
Click to flip
🎉
All done!
TermDefinition
🎯

Mini Test — 3.3.8b Lossy Compression

Timed exam conditions.

  • 8 questions · 10 minutes
  • 5 MCQ + 3 short answer
← 3.3.8a Lossless Compression
32 of 57 · AQA 8525
3.3.9 Encryption →