Why we compress data

In this lesson

Explain what data compression is and why data is compressed, and - on the two boards that use the words - compare lossless and lossy compression, giving an advantage, a disadvantage and a suitable use for each, with a justification that says why rather than how.

What it covers

• What compression IS: making a file take up fewer bits than it did, in a way that can be undone or lived with (AQA 3.3.8a: explain what data compression is)
• WHY data is compressed: less storage used, faster transmission and download, less bandwidth, fitting inside a limit such as an email attachment size, smoother streaming
• THE PAIR, on OCR and Edexcel: LOSSLESS keeps every bit, so the original file can be restored exactly; LOSSY permanently discards data and the original cannot be restored
• Advantages and disadvantages of each, and the effect on the file - lossy gives a smaller file and a higher compression ratio but the loss is permanent; lossless is bigger but nothing is gone
• Choosing which is appropriate for a given scenario AND JUSTIFYING IT - why this type suits this file, not how the compression works
• Typical uses: lossless where every bit matters, such as text, documents and program files; lossy where a human sense is the judge, such as audio, photographs and video
• Compressed is not damaged: a compressed file still opens and is still readable
• Compression is not encryption - one makes data smaller, the other hides its meaning
• AQA'S SHAPE IS DIFFERENT and must be said as such: AQA requires what compression is, why it is done, and that there are different ways of doing it - and the two ways it names are Huffman coding and run length encoding, which are CS03-06 and CS03-07

Key words

On the specification