Data compression

• Benefits of reducing file size when transferring files:

  • The file will be sent/transferred faster if it has a smaller file size. ​
  • Some applications, e.g., Gmail, have a file size limit - if you exceed the file size limit, you will not be able to send your file. Less bandwidth is used as transfer limits may apply. ​
  • Buffering on audio and video streams is less likely to occur. ​
  • Less storage is required. 
• A compression technique is an algorithm, i.e. a sequence of steps, that is followed to reduce file size.
• Once a file is compressed, it can be saved and moved around in this form. But it must be decompressed so that it can be accessed in a meaningful way (e.g. viewed or listened to).
• Whether the file is the same as the original depends on whether the compression technique is lossy or lossless.
• Lossy compression is a term used to describe compression techniques that always result in a loss of data, i.e. data is permanently removed from the file. When the file is decompressed, the data that was removed (in the compression process) is re-created from the data that remains in the file. The re-created data will never be an exact copy of the original. Instead, it will be an approximation of the original data.
• Lossy compression is commonly used for image and sound files as some of the data can be lost without a noticeable effect on quality. This is because the human eye and ear cannot typically detect the subtle changes that the compression algorithm makes to the original file.
• Some types of data are not suitable for lossy compression. For example, files that contain words or numbers could not go through a lossy compression process, as their contents need to be exactly the same (when decompressed); the data cannot be approximated.
• It is important to remember that lossy compression is a destructive process, which means that it causes the original data to be lost. Performing repeated rounds of lossy compression can cause such severe loss of data that the file output becomes unrecognisable from the original.
• Lossless compression refers to techniques where no data is lost in the compression process. When the file is decompressed, the contents are identical to the original file (before it was compressed).
• Lossless compression refers to techniques where no data is lost in the compression process. When the file is decompressed, the contents are identical to the original file (before it was compressed).
• Most users do not actively choose a compression method. When a file is saved, compression is applied automatically to reduce the file size. When the file is opened, the data is re-created so that it can be viewed and worked with. When you save a piece of work, you rely on the software to store the data in the most appropriate form.