Compression (Cambridge International IGCSE ICT)

The Need For Compression

What is compression?

• Compression is reducing the size of a file so that it can be transmitted or stored in secondary storage

• There are scenarios where compression may be needed, such as:

  • Maximise the amount of data you can store on a digital device such as a mobile phone or tablet
  • Minimise the transfer time of data being uploaded, downloaded or streamed across a network such as the Internet

• Reducing the file size of an image can be achieved by reducing the colour depth or resolution of the image

• Compression can be achieved using two methods, lossy and lossless

Lossy Compression

What is lossy compression?

• Lossy compression is when **data is lost **in order to reduce the size on secondary storage

• Lossy compression is irreversible

• Lossy can** greatly reduce the size** of a file but at the expense of losing quality

• Lossy is only suitable for data where reducing quality is acceptable, for example images, video and sound

• In photographs, lossy compression will try to group similar colours together, reducing the amount of colours in the image without compromising the overall quality of the image

• Example file types that use lossy compression are:

  • JPEG Images
  • MP3 Audio
  • MPEG-4 Video

• In the images above, lossy compression is applied to a photograph and dramatically reduces the file size
• Data has been removed and the overall quality has been reduced, however it is acceptable as it is difficult to visually see a difference
• Lossy compressed photographs take up less storage space which means you can store more and they are quicker to share across a network

Lossless Compression

What is lossless compression?

• Lossless compression is when data is encoded in order to reduce the size on secondary storage
• Lossless compression is reversible, the file can be returned to its original state
• Lossless can reduce the size of a file but not as dramatically as lossy
• Lossless can be used on all data but is more suitable for data where a loss in quality is unacceptable, for example documents
• In a document, lossless compression uses algorithms to analyse the contents looking for patterns and repetition. For example, repeating characters are replaced with a single character and the number of occurrences in the document (“EEEEE” becomes “E5”)

• In the image above, lossless compression is **automatically **applied to document formats such as DOCX and PDF with a different rate of success

• When you open a lossless compressed document the decompression process reverses the algorithms and returns the data** back to its original state**

• Lossless compressed documents take up less storage space which means you can store more and they are quicker to share across a network

• Example file types that use lossless compression are:

  • .PNG Images
  • .Zip files
  • .Rar files

Worked Example Sarah uses her computer to record an audio file of herself narrating a video for work.

She emails her recording to friend for proofing. She uses lossy compression to produce the sound file.

Explain two reasons why using lossy compression is beneficial. [4]

How to answer this question

• What are the differences between lossy and lossless?
• Can you state two differences? [2 marks]
• Can you say why each point is a benefit? [2 marks]

Answer

• Lossy will decrease the file size [1]
• ... so it can sent via email quicker [1]
• Lossy means data is lost [1]
• ... the difference is unlikely to be noticed by humans [1]