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DAVJ277 1.5

J277 GCSE CS 1.2.5 Compression

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Why do we use compression? To reduce the number of bits in a file therefore the size of a file is reduced. Making data transfer of the file quicker. Making the storage capacity of the file lower. EXAM TIP—relate to the scenario
What is lossy compression? Can greatly reduce file size. Some of the data is permanently lost—cannot be restored to original state. Reducing the quality of the sound/image/video Cannot be used on text or software files.
What is lossless compression? Reduction in file size by using an algorithm. None of the data is lost Can be decompressed (restored) into original format. The only type of compression suitable for text files and computer programs(as no data is lost) Can also be used on media files but less compression than lossy.
Why don’t we always use lossy compression if that can achieve higher rates of compression than with lossless? Lossy isn’t suitable for all files. Not suitable for text files or programs as they would no longer be useful if data has been removed.
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