← ACARA Digital Technologies Years 9–10

Explaining what is lost and what is kept when a file is compressed

Linking word in an explanation: name the technique, say how it shrinks the file, and state what is lost or kept, e.g. MP3 is smaller because it drops sounds you can't hear.

Part of: ACARA Digital Technologies Years 9–10 — Data Compression — Module 40

Understanding Explaining what is lost and what is kept when a file is compressed in ACARA Digital Technologies Years 9–10

What it does: The sentence uses because to link the reason (no data thrown away) to what is kept (the exact original), which is how lossless compression works. A clear explanation names the technique, says how it shrinks the file, and says what is lost. The second sentence does not say what is removed. Good explainers use simple words and everyday comparisons that fit the audience, while still saying what is lost. Run-length encoding is lossless. It stores runs of repeated values, so the original image can be rebuilt exactly. A fix would be: JPEG makes photos smaller because it removes fine detail, so some detail is lost.

Common mistakes: MP3 gets smaller by dropping sounds, not adding them. The line should say 'because it drops quiet sounds.'. RLE is lossless because every pixel can be rebuilt from the counts. The line should say 'RLE is lossless compression', which also matches line 3.

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This module's key terms are also in a printable crossword (Modules 226–254) and word search (Modules 226–329).

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Learning
Duo
Which sentence is a clearer explanation of JPEG compression?
JPEG makes a photo smaller because it throws away tiny colour details your eyes barely notice.JPEG makes a photo smaller because it is a good file type.
Matches
JPEG makes files smaller. It is good for photos.
↓
It never says how the file shrinks or what is lost, and it has no because. A clear explanation links the method to the result and states what is lost or kept.
Applying
Meanings
Term
because
Meaning
Linking word in an explanation: name the technique, say how it shrinks the file, and state what is lost or kept, e.g. MP3 is smaller because it drops sounds you can't hear.
Practice
JPEG makes photos smaller because it removes tiny colour details.
Testing
Gaps
RLE is lossless no data is lost.
Check Answer
Spots
1MP3 is smaller than WAV
2because it adds extra sounds.

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Quick Questions

1. Which sentence is a clearer explanation of JPEG compression?
A. JPEG makes a photo smaller because it is a good file type.
B. JPEG makes a photo smaller because it throws away tiny colour details your eyes barely notice. ✓
A clear explanation names the technique, says how it shrinks the file, and says what is lost. The second sentence does not say what is removed.
2. Which explanation of run-length encoding (RLE) states what is kept?
A. RLE makes files smaller because it is a clever trick.
B. RLE stores 'W5' instead of 'WWWWW' because repeats can be counted, and no data is lost. ✓
RLE is lossless. A good explanation says how it shrinks the data (counting repeats) and that everything is kept.
3. Mia says: 'MP3 is smaller because it drops sounds most people can't hear.' What does her sentence do well?
A. It names the technique, says how the size shrinks, and says what is lost. ✓
B. It only names the file type and says nothing about what is lost.
The word 'because' links the technique (MP3) to how it saves space (dropping sounds) and shows what is lost.
4. Which sentence uses 'because' to link a cause to a result correctly?
A. A ZIP file can be restored exactly because lossless compression keeps all the original data. ✓
B. A ZIP file can be restored exactly because it has a smaller icon.
'Because' must link to a real reason. Lossless compression keeps all the data, so the original can be rebuilt.
5. You are explaining lossy compression to a younger student. Which is the better explanation?
A. It is like shrinking a drawing by leaving out tiny details, so the file is smaller but you can't get the details back. ✓
B. It applies a perceptual quantisation algorithm to the discrete cosine transform coefficients.
Good explainers use simple words and everyday comparisons that fit the audience, while still saying what is lost.

Match the Pairs

RLE shrinks data because it stores a count and a value instead of repeating the value. → A clear explanation of RLE: it names the technique, says how it shrinks the file, and states that nothing is lost.
JPEG is smaller because it throws away fine detail your eyes barely notice. → A clear explanation of JPEG: it names the technique, says why the file is smaller, and states that detail is lost.
Explaining to a younger student what is kept by a lossless method, using because → A ZIP file can be unzipped exactly because no data is thrown away, it is only packed more tightly.
A sentence starter that links a lossy method to what is lost → MP3 files are small because they remove sounds most people cannot hear, so some audio is lost for good.
JPEG makes files smaller. It is good for photos. → It never says how the file shrinks or what is lost, and it has no because. A clear explanation links the method to the result and states what is lost or kept.
RLE is lossy because it replaces repeats with counts, so the original cannot be restored. → The sentence wrongly says that data is lost. RLE is lossless, so the original can be rebuilt exactly from the counts and values.

Key Term

because
Linking word in an explanation: name the technique, say how it shrinks the file, and state what is lost or kept, e.g. MP3 is smaller because it drops sounds you can't hear.

Fill in the Blanks

1. RLE is lossless because no data is lost.
Hint: Type the linking word that introduces the reason.
2. MP3 is smaller because it drops sounds you cannot hear.
Hint: Type the name of the audio compression format being explained.
3. RLE is lossless because nothing is lost.
Hint: Type the kind of compression that keeps all of the original data.
4. JPEG is lossy because it removes detail.
Hint: Type the kind of compression that permanently removes some data.
5. A ZIP file is lossless because every bit of the original is kept.
Hint: Type the word that means saved, the opposite of lost.

Spot the Error

Question 1
A MP3 is smaller than WAV
B because it adds extra sounds.
C Those sounds are lost forever.
Line B — MP3 gets smaller by dropping sounds, not adding them. The line should say 'because it drops quiet sounds.'
Question 2
A RLE is lossy compression
B It replaces runs with counts
C so no pixels are lost.
Line A — RLE is lossless because every pixel can be rebuilt from the counts. The line should say 'RLE is lossless compression', which also matches line 3.
Question 3
A A ZIP file is lossless
B because some data is deleted.
C You get the exact original.
Line B — Lossless means no data is deleted, so the explanation must say 'because nothing is removed.' Deleting data would be lossy.
Question 4
A Explaining JPEG to a Year 5:
B A JPEG is a lossy photo file.
C It merges similar colours
D so the file is much bigger.
E Fine detail is lost forever.
Line D — Merging similar colours reduces the data, so the file becomes smaller, not bigger. The line should say 'so the file is smaller.'
Question 5
A RLE deletes every repeated run
B and a value, so AAAA is 4A.
C It works because data repeats.
Line A — RLE does not delete runs. It stores each run as a count and a value, which is what line 2 describes. The line should say 'RLE stores a run as a count'.

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