← Years 9–10 Data & Information

Lossy compression

Shrinks a file by permanently throwing away detail people are unlikely to notice, so the original can't be rebuilt exactly. JPEG and MP3 use it.

Part of: 9-10 Digital Technologies: Data compression — Module 9, IGCSE Computer Science 0478 — 1.3 Data Storage and Compression — Module 33, ACARA Digital Technologies Years 9–10 — Data Compression — Module 26, QCE Digital Solutions Unit 4 — Digital Methods for Exchanging Data — Module 55

Understanding Lossy compression in Years 9–10 Data & Information

What it does: A low quality setting throws away more detail, so the file gets smaller but looks or sounds worse. Lossy compression removes detail for good, so the original can't be rebuilt exactly. Lossless compression is the type that can restore the original. JPEG is a common lossy image format. It removes small colour details most people will not notice. The mistake is thinking lossy compression can be reversed perfectly. The thrown-away data is permanently lost.

Common mistakes: JPEG uses lossy compression, which permanently throws away detail. It should be compression ← "lossy". An MP3 uses lossy compression, so the removed sound detail is gone for good. The original cannot be rebuilt exactly, so it should be FALSE.

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This module's key terms are also in a printable crossword (Modules 6–16) and word search (Modules 6–132).

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Learning
Duo
Which statement describes lossy compression?
It permanently throws away some detail to make the file smallerIt shrinks the file and always lets you rebuild the exact original
Matches
Unzip the JPEG to get back the original photo exactly
↓
JPEG is lossy, so the discarded detail is gone for good. Unzipping or reopening cannot rebuild the exact original.
Applying
Meanings
Term
lossy compression
Meaning
Shrinks a file by permanently throwing away detail people are unlikely to notice, so the original can't be rebuilt exactly. JPEG and MP3 use it.
Practice
photo <- LOAD("beach.bmp")
Testing
Gaps
JPEG photos use compression
Check Answer
Spots
1format ← "JPEG"
2compression ← "lossless"

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Quick Questions

1. Which statement describes lossy compression?
A. It permanently throws away some detail to make the file smaller ✓
B. It shrinks the file and always lets you rebuild the exact original
Lossy compression removes detail for good, so the original can't be rebuilt exactly. Lossless compression is the type that can restore the original.
2. Which file format uses lossy compression?
A. ZIP archive of a document
B. JPEG photo ✓
JPEG discards fine detail in photos that our eyes rarely notice. A ZIP archive uses lossless compression so every bit comes back.
3. An MP3 file is smaller than the original song recording. How is this done?
A. Sounds that most people can't hear well are removed ✓
B. Repeated notes are swapped for a count, so nothing is lost
MP3 uses lossy compression. It removes sounds that are very quiet or hard for human ears to notice, and that detail can't be recovered.
4. What is the trade-off when you use stronger lossy compression?
A. A smaller file and better quality
B. A smaller file but lower quality ✓
The more detail you throw away, the smaller the file gets, but the quality drops. You balance size against quality.
5. You save a JPEG, open it, edit it, and save it as a JPEG again, over and over. What happens?
A. The quality gets worse each time it is saved ✓
B. The quality stays exactly the same every time
Each save as a lossy file throws away a bit more detail. Over many saves, the image can look blocky or blurry.

Match the Pairs

photo.jpg → A JPEG image that uses lossy compression, so some fine detail has been permanently discarded to make the file smaller
song.mp3 → An audio file that uses lossy compression, so sounds that are hard to hear are permanently removed to save space
Lossy compression lets you trade quality for size: a lower quality setting gives a smaller file → quality = 30 -> small file, more detail lost
Saving a lossy file again and again keeps removing detail, so quality gets worse each time → save.jpg -> save.jpg -> save.jpg (quality drops)
Unzip the JPEG to get back the original photo exactly → JPEG is lossy, so the discarded detail is gone for good. Unzipping or reopening cannot rebuild the exact original.
Lossy compression keeps every bit, so nothing is ever lost → Lossy compression deliberately discards detail that people are unlikely to notice. Keeping every bit is lossless compression.

Key Term

lossy compression
Shrinks a file by permanently throwing away detail people are unlikely to notice, so the original can't be rebuilt exactly. JPEG and MP3 use it.

Fill in the Blanks

1. JPEG photos use lossy compression
Hint: Type the word for compression that permanently throws away detail.
2. MP3 is a lossy format for music files
Hint: Type the name of the lossy audio format.
3. Lower quality setting = smaller file size
Hint: Type the word describing what happens to the file size when quality is lowered.
4. REPEAT 5 TIMES SAVE image as JPEG quality gets lower each time
Hint: Type the keyword that makes the saving step run again and again.
5. Lossy: original rebuilt exactly = FALSE
Hint: Type the Boolean value that shows the original cannot be restored exactly.

Spot the Error

Question 1
A format ← "JPEG"
B compression ← "lossless"
C detail_thrown_away ← TRUE
Line B — JPEG uses lossy compression, which permanently throws away detail. It should be compression ← "lossy".
Question 2
A song ← "track.mp3"
B removed ← "quiet sounds"
C can_rebuild_original ← TRUE
Line C — An MP3 uses lossy compression, so the removed sound detail is gone for good. The original cannot be rebuilt exactly, so it should be FALSE.
Question 3
A quality ← 90
B size_KB ← 800
C quality ← 30
D size_KB ← 2000
Line D — Lowering the quality throws away more detail, so the file gets smaller, not bigger. 2000 KB is larger than 800 KB; it should be a smaller number such as 200.
Question 4
A keeps_all_detail ← TRUE
B file_size ← SMALLER
C original_exact ← FALSE
Line A — Lossy compression does not keep all the detail. It permanently discards some, so this should be FALSE.
Question 5
A quality_start ← "good"
B REPEAT 5: re-save as JPEG
C quality_end ← "better"
Line C — Each time a JPEG is saved again, a little more detail is thrown away. Repeated saving lowers quality, so the end quality should be "worse".

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