Sooner or later in your dealings with computers, (and many recent home entertainment technologies), you will come across the term compression. Broadly speaking it’s a way of reducing the size of files or compacting ‘streams’ of digital data, usually to make them more manageable, or to speed up transmission, when sending or transferring volumes of data from one place to another. Basically there are two types of data compression, ‘lossy’ and ‘lossless’.

Lossy compression is mostly used to process picture and sound information. Non-essential data is discarded, either because it is repeated unnecessarily (a static background in a moving picture that doesn’t vary from one frame to the next) or it relates to details or small changes that eye or ear cannot detect, e.g., subtle variations in hue or brightness in a picture, or sounds in a recording that cannot be heard because they are masked by louder sounds. Lossy compression schemes that you may be familiar with include JPEG (file extension *.jpg), used for sending image files over the Internet; MPEG is a whole family of video (and audio) compression systems used by DVD (digital versatile/video disc), digital television (ON digital and SKY digital) and MP3 Internet music files. Another system you may come across is ATRAC; this is used by the MiniDisc digital recording system.

However, the topic of this week’s Boot Camp is lossless compression, (sometimes referred to as archiving). This is a technique used to reduce the size of program and text files, where no loss of data or information can be tolerated. Most lossless schemes use algorithms to search out repetitive sequences of data. For example, a text file may contain the word ‘the’ many times, normally this short string of characters and spaces would be represented by up to 40 bits of data. A compression system typically assigns a three-digit code to each occurrence of  ‘the’, made up of just 9 bits, giving a space saving of more than 75%.

The best known lossless compression technique is ‘zipping’ it’s the one that you are most likely to encounter when downloading files or programs from the Internet. This type of compressed or archived file is instantly recognisable since it has the extension ‘*.zip’. There are more than fifty other types of file compression and coding in use with extensions that include *.arc *.arj *.gz *.lzh *.mim *.tar *.uue, etc., but you are unlikely to see more than a fraction of them unless you make a habit of visiting specialist sites or mess around with exotic programs and operating systems.

A lot of novice PC users are understandably wary about downloading strange sounding files or programs from the Internet but if you take a few simple precautions it should be safe and painless. The first thing you will need is a decompression program. The best known is WinZip and PKzip. Shareware versions of these programs are available from the distributors home pages  ( and, numerous sites on the Internet and they are regularly included on computer magazine cover-mount CD-ROMS. Both programs can also process the other types of compressed files mentioned earlier; WinZip can also decompress Windows ‘.cab’, (cabinet) files. Both programs are themselves compressed but contain their own decompression utilities and once loaded onto your PC automatically extract and install themselves on your hard disc drive. When that’s done open Windows Explorer and create a new folder (File > New > Folder) where you can store downloaded files, give the folder a name that you can easily remember and you are ready to begin.

When you click on a download button, or link to a download site on an Internet page Windows normally asks you if you want to open the file, or save it to disc, (the latter should be chosen by default), click OK and use the Browse button to direct the file to your specially created folder. When downloading is complete and you have logged off use Windows Explorer to open your download folder and click on the newly arrived *.zip file icon. This should automatically start your decompression program and the contents of the folder will be displayed. You can view files, before they are opened, so peruse any readme or text files, which may contain important set-up or installation information. 

When you are ready to proceed click on ‘Open’ or ‘Extract’, and you will be shown where the decompressed files will be sent. The extraction program may decide to open the compressed files into your download folder. This is a bad idea, as it will clutter up the folder, making it difficult to use for subsequent downloads. If that happens create a new folder by typing in a new path and name (i.e. C:\newprog) or accept the default if the zipped program asks permission to create a new folder. In all cases make a note of the name of the new folder. When you’re happy with the arrangements click the Extract button and the files will be decompressed and stored in the nominated folder.

Extraction usually only takes a second or two, after which it’s a good idea to give the decompressed files the once-over with your virus checker. If they get a clean bill of health you can open or install the program in accordance with the instructions in the associated Readme text file.

An often overlooked feature of both WinZip and Pkzip is their built-in compression or archiving utilities, which means you can create your own ‘zipped’ files. This is handy for sending large files over the Internet, exchanging programs or data with others on floppy disc or for backing up files on your PC. Both programs have a ‘spanning’ option that splits large compressed files into floppy disc sized chunks that can be spread across several discs.  Full instructions are contained in the program’s respective Help menus.

Next week – Elementary Dr Watson




Adaptive Transform Acoustic Coding – digital audio compression system used by the MiniDisc format


Joint Photographic Experts Group – division of the International Standards Organisation (ISO) responsible for developing digital compression systems for still images


Motion Picture Experts Group – ISO division developing digital compression systems for video and audio data


MPEG layer 3, digital audio compression system used to send high quality music files over the Internet



Windows Explorer often seems to have a mind of its own and you may find that it opens with different ‘Views’ (large icons, small icons, list etc.,) for no apparent reason. There is a way to force it to always open with your preferred layout. Open Windows Explorer and select your chosen View, next, go to the View menu and click Folder Options and choose the View Tab. In the Advanced Settings panel make sure the item ‘Remember each folder’s view settings’ is ticked, finally click on the button Like Current Folder’ and click Apply.

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