Compression

What is compression?

• Compression is reducing the the size of a file so that it takes up less space on 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

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

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

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”)

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]

• 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]