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Data Compression Explained – Lossy vs Lossless Compression (O Level Computer Science 2210 / IGCSE 0478)

What Is Data Compression?

Data compression is the process of reducing the size of a file so that it uses less storage space and less bandwidth when transmitted.

Compression is widely used when storing or sending:

  • Images

  • Audio files

  • Videos

  • Documents

The main goal of compression is to make files smaller without significantly affecting their usefulness.

According to the Cambridge Computer Science syllabus, compression is used to reduce file size, reduce transmission time and save storage space.

computer ol

Why Data Compression Is Important

Data compression helps to:

✅ Save storage space
✅ Reduce internet bandwidth usage
✅ Speed up file transmission
✅ Improve efficiency in digital systems

For example:

  • Sending a compressed photo through messaging apps

  • Streaming music efficiently

  • Storing thousands of images on a smartphone

Types of Data Compression

There are two main types of compression used in computer science:

  1. Lossless Compression

  2. Lossy Compression

Lossless Compression

Lossless compression reduces file size without losing any original data.

This means the file can be perfectly reconstructed after decompression.

Example formats:

  • ZIP files

  • PNG images

  • Text files

A common method used in lossless compression is Run Length Encoding (RLE).

Example

Original data:

AAAAAAABBBBCCCC

Compressed form:

7A4B4C

This significantly reduces the amount of data stored.

Lossy Compression

Lossy compression reduces file size by permanently removing some data.

The removed data usually has minimal effect on perceived quality.

Example formats:

  • JPEG images

  • MP3 audio

  • MP4 video

For example, when compressing an image:

  • Some colours may be removed

  • Some details may be simplified

This results in much smaller file sizes.

Lossy vs Lossless Compression

FeatureLossless CompressionLossy CompressionData lossNoYesFile qualityPerfectly preservedSlightly reducedFile size reductionModerateVery highExamplesZIP, PNGJPEG, MP3

When Each Type Is Used

Lossless compression is used when:

  • Data must remain exactly the same

  • Storing documents or databases

  • Archiving files

Lossy compression is used when:

  • Very small file size is required

  • Storing images, music or videos

  • Streaming media online

Exam Tip (2210 / 0478)

Students are commonly asked to:

  • Define data compression

  • Explain lossy and lossless compression

  • Give examples of when each type is used

A common exam question is:

Explain the difference between lossy and lossless compression.

Always mention:

  • Lossless = no data lost

  • Lossy = some data permanently removed

Practice Question

Explain one advantage of data compression.

Answer

Data compression reduces the file size, which decreases storage space required and transmission time when sending files over a network.

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