Ex 2.3.1 - File Compression

• Portability
• For use on the Internet

Portability
In the early days of computing files sizes were very small and graphics did not in fact feature.  These days documents can contain text, graphics, sound and even video.  The file size requirement therefore has increased dramatically.  One thing that has not changed however is the size of a floppy disc which has remained the same at 1.44MB.  Hence, how do you squeeze a file of 10MB onto a disc of size 1.44MB?  The answer is that you compress the file using "Zip" software.  Zip software utilises special mathematical techniques that eliminate repeatable unnecessary code within a file.  This effectively reduces the file size.  The degree of compression can vary from anything between 0% and 95% depending on the nature of the original file.  Compressions of between 70% and 90% are typical though.  Once the file has been zipped it can then be copied onto a floppy disc and transported to another machine where it is then un-zipped.  Un-zipping returns the document to its exact original format and there is no data loss or change in the overall process.  Zipped files can also be sent over the Internet as attachments thus saving time.  The software required to zip a file is available from a number of sources and in some instances it is available as freeware.

For use on the Internet
Because the speed of data transfer over the Internet is typically quite slow it is vital that all elements that make up a web page are as small as possible.  Graphics in particular need to be kept small and manageable (this is changing with Broadband access) otherwise they can take so long to download that the user does not wait and clicks to go on to another page.  To this end a number of graphic file formats have been developed specifically for use over the Internet.  *.gif graphics were developed for this reason and typical images are of the order of 1-20KB for a small picture.  The disc graphic shown here is 3KB file size.  To achieve this small file size the image quality is quite poor.  If the image were scanned from a magazine and saved in a format that is photo quality the file size could be 300KB.  In other words changing the format to a *.gif file has reduced/compressed the file by a factor of 100.  Clearly some loss of quality has to be accepted.  The other commonly used file format for graphics on the Internet is *.jpg or *.jpeg.  Here the file sizes are marginally higher but the image quality is much better.  It should be noted than once a file has been compressed to a *.gif or *.jpg file it can NOT be expanded back to its former size.

In addition to graphics text files can also be compressed for use over the Internet by using Adobe Acrobat format.  This is used in this course for the Practical Exercises.  Adobe Acrobat Distiller takes word processed documents and typically reduces the file size of them by 80%-90% changing them into *.pdf (portable document format) files.  This means that when the file is sent over the Internet it will be much quicker.  Adobe Acrobat Reader is then required to read the *.pdf file.  Adobe Acrobat Reader can also be used to print the *.pdf file.  At this point the file size is expanded to include all the information required for printing the document as opposed to just viewing it.

www.pkware.com - Compression Software

www.adobe.com - PDF File Creator Software