File Compression (CIE IGCSE ICT)

• The size of a file determines:
• How much space it will require for storage 
• How long it will take to transmit from one device to another
• Often file sizes need to be reduced and will use a form of compression
• The two types of compression are:
• Lossy compression - Unnecessary data is removed and can significantly reduce file sizes. This is most often used for audio, video and image
• Lossless - Allows the original data to be perfectly reconstructed from the compressed version without any loss of information or quality. This is most often used for text and document and software compression 
• E.g. if 10 images were to be displayed on a website and their original size was 18MB each, with an internet connection of 32 megabits per second the webpage would take around 45 seconds to load and require 180 Mb of storage space online to host the images
• Using lossy compression it is possible to compress the files down to around 10% of their original size meaning that the same internet connection would require just 4 seconds to load the images and the storage requirement for the files would be 18MB

How are Files Compressed?

There are many different technical ways that file sizes can be reduced however some simple methods and techniques for multimedia files are below:

• Images can be compressed by the following:
• Reducing the number of colours in the image (known as the bit depth)
• Reducing the resolution of the image (the total amount of pixels that make up the image)

• Videos can be compressed by the following:
• Reducing the video resolution
• Reducing the number of video frames per second

• Sound can be compressed by the following:
• Removing frequencies that are outside of the human hearing range

• An uncompressed document containing both text and images
• A document can be compressed by converting it to a PDF file

• By converting the document to a pdf, the file size reduction is more than half

Compressing Multiple Images Together

• Hold Ctrl and select all the images you would like to compress, right click and choose ‘Compress to Zip file’

• Using the Zip file compression technique has reduced the file size considerably
  • Total for all files before compression: 8477KB
  • Total for all files after compression: 727KB

The same can be done using WinRar

• Hold Ctrl and select all the images you would like to compress, right click and choose ‘WinRAR followed by Add to Archive’

• Give the archive a title and click OK

• The compressed file is now in the directory

Note* Sometimes when using WinZip or WinRar the file sizes will not compress further as some file types are already compressed, such as JPEG images. Another reason may be that different compression algorithms are used by different compression software, and their effectiveness can vary depending on the file types being compressed