Compression software

File compression means that you take a file, some files or a folder and you give it to some compression software that 'squashes it'. This makes it much smaller than the original size. This is sometimes known as zipping up a file or folder. What file compression software actually does is to look at each file you want to compress and see if it can construct rules to reduce the size of the file.

For example, suppose you had a word processed document with the word 'elephants' 50 times in different places.

You would need 50 * 9  = 450 bytes to store all of the characters in elephant fifty times.
The file compression software creates a dictionary and puts elephant in it.
It also gives elephant a code, let us say E to keep it simple.
Now it replaces all the instances of elephant in your document with E.

zipThat's it! All you need now are 50 * 1 = 50 bytes to store your document (and a little bit for the dictionary). Instead of 450 bytes, you only need 50 bytes to store the word 'elephant'! If you did that for all the words in your document, you would soon have a much smaller document. You've now created a zip or compressed document. If you wanted to unzip it, or decompress it, you just get some compression software to put the words from the dictionary back into the document. This is the basic idea but there is a really good description of the whole process here.

    • There are a number of reasons why you might want to compress documents. Compressed files and folders take less time to transmit across the Internet than the original files, because they are smaller. If you had, for example, video files or music files to send, which can be very large, this might be important.
    • If you were the BBC and wanted to 'stream' video (where people watch the video at the same time the BBC is sending it) then you need your large files to be as small as possible or the video will slow or stop from time to time.
    • You can store more files, programs and data on any storage device such as your pen drive if they are compressed.
    • If you want to send ten files by email, an efficient way to send them is to first put them in a zip folder. The zip folder can be attached to an email in the same way that a file can be attached to an email. (Note, however, that you can't attach a normal folder to an email usually.)

 There are lots of free data compression programs available and some that you have to pay for. Windows comes with a free utility. in the File Manager, just right-click and then select 'New' folder by 'Compressed Folder'. 7-Zip is open source and very good, as is Hamster Free Zip application. You can download and try out an evaluation copy of WinZip. 

Q1. What is meant by a zip file?
Q2. Are compressed files bigger or smaller than the original contents?
Q3. What unit is used to measure file size?
Q4. Describe with an example how file compression is achieved.
Q5. You want to save lots of photos and videos on your pen drive but they won't fit. What could you do about this problem?
Q6. What is meant by 'video streaming'?
Q7. Why are videos compressed during the streaming process?
Q8. What is meant by open source software?
Q9. State one open source application for compressing files.
Q10. Visit here and read how files are compressed.