Key Principle: Remove What Humans Can't Perceive
For Images (JPEG):
- Human eye limitations: More sensitive to brightness than color
- Strategy: Keep brightness detail, reduce color detail
- Averaging: Group nearby similar pixels, store average
- Discard high frequencies: Remove fine texture details
- Result: 10× smaller file, looks nearly identical to most viewers
For Audio (MP3):
- Human hearing limitations: Can't hear very high/low frequencies
- Psychoacoustic masking: Loud sounds mask quiet nearby sounds
- Strategy: Remove frequencies humans can't hear
- Remove masked sounds: Quiet sounds near loud ones = discarded
- Result: 10× smaller, sounds nearly identical to CD
Quality Levels (User Control):
- Low quality: 90% compressed - visible/audible artifacts
- Medium quality: 85% compressed - good balance (default)
- High quality: 70% compressed - minimal loss, larger file
- Trade-off: More compression = smaller file but worse quality
This key facts covers How Lossy Compression Works within Compression for GCSE Computer Science. Revise Compression in 3.3 Data Representation for GCSE Computer Science with 16 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 7 of 12 in this topic. Use this key facts to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Compression
Which statement correctly describes lossy compression?
Explain how run-length encoding (RLE) works to compress data.