Compression (Cambridge (CIE) IGCSE ICT)
Revision Note
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
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]
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