📚 Knowledge Library — Topic 1.5 — Data Representation

Data Compression

Understand why files are compressed, the difference between lossy and lossless compression, and how run-length encoding reduces repeated data.

Start LearningWorked Example
1. Invitation

Large files can slow everything down.

Images, audio and video files can contain a lot of data.

Compression reduces file size so files take less storage space and can be transmitted more quickly.

💡 Remember: compression makes a file smaller.
Figure 1.1
Why Compress Files?
Large file
↓
Compression
↓
Smaller file
Smaller files are quicker to store, send and download.
2. Big Idea

There are two main types of compression.

Lossy compression permanently removes some data to make the file much smaller.

Lossless compression reduces file size without removing data, so the original file can be perfectly rebuilt.

💡 Lossy loses data. Lossless keeps all data.
Figure 2.1
Lossy vs Lossless
Lossy
= data removed

Lossless
= data preserved
3. Bridge

Think of packing a suitcase.

Lossless compression is like folding clothes more neatly. Nothing is thrown away.

Lossy compression is like removing things you decide you do not really need.

💡 Lossless reorganises. Lossy removes.
Figure 3.1
Suitcase Model
Lossless
fold better

Lossy
leave items behind
Both reduce size, but only one keeps everything.
4. Worked Example

Calculating file size reduction.

To calculate how much a file has been reduced, subtract the compressed size from the original size.

File reduced from 12 MB to 4 MB

Original
12 MB
Compressed
4 MB
Reduction
?
12 MB − 4 MB = 8 MB reduction
Figure 4.1
Reduction Calculation
original size
− compressed size
↓
reduction
Always show the subtraction.
5. Run-Length Encoding

Repeated data can be stored more efficiently.

Run-length encoding, or RLE, is a lossless method that replaces repeated values with a count and the value.

It works best when there are long runs of repeated data, such as simple images with large areas of the same colour.

RLE example

AAAAA → 5A
Figure 5.1
RLE Pattern
AAABBBCC
↓
3A 3B 2C
RLE is lossless because the original can be rebuilt.
6. Exam Tip

Choose the right type for the file.

Lossy compression is often used for images, audio and video where a small loss of quality may be acceptable.

Lossless compression is needed when every bit of data must be preserved exactly.

🎯 Exam Tip: use lossy for files like JPEG and MP3; use lossless for files like PNG, FLAC and ZIP.
Figure 6.1
Common File Types
Lossy
JPEG • MP3 • MP4

Lossless
PNG • FLAC • ZIP
7. Common Mistake

Lossy is not suitable for every file.

Some files must keep every piece of data exactly the same.

⚠️ Common Mistake: using lossy compression for text files, legal documents, medical images or program files. If the data must be exact, use lossless compression.
Figure 7.1
When Accuracy Matters
Text files
Legal records
Medical images
Programs
↓
Lossless
8. Summary

Data compression in one screen.

Compression reduces file size.

Lossy compression removes data permanently. Lossless compression keeps all data and allows the original file to be rebuilt.

RLE is a lossless method that stores repeated values as a count and value.

💡 Key idea: smaller files need less storage space and transmit faster.
Figure 8.1
Compression Choices
Need exact data?
Use lossless

Need smallest file?
Use lossy