1.2.5 Compression

What is compression?

• to make a file smaller in size

Why is compression used?

• files take up less storage space (so more files can be stored on the strage medium)

• files can be transferred quicker (because they are smaller) - less time to transmit/upload/download

• files can be read from or written to quicker

• less bandwidth to transmit

• would otherwise exceed email storage

What is lossy compression?

• some data which cannot be seen or heard by humans is removed by an algorithm from a file to reduce its size

• further lossy compression removes data that humans can see or hear, negatively affecting the quality

• data is completely lost once removed - can never get back original file

What is lossy compression commonly used for?

• images, audio and video, for example to send over the internet

• the reduction in file size is significant while the reduction in quality is barely noticeable

What is lossless compression?

• reduces the size of a file - Run-length encoding (RLE) - looks at a file for consecutive runs of the same data and stores them as one item of data instead of many

• no data is removed permanently - file can be restored to its original form when decompressed

• no quality is lost but doesn’t reduce file size as much

What is lossless compression commonly used for?

• files that would be useless if data was removed permanently - text files / executable