What’s it all about?
The storage capacity of computers is growing at an unbelievable rate—in the last 25
years, the amount of storage provided on a typical computer has grown about a
millionfold—but we still find more to put into our computers. Computers can store whole
books or even libraries, and now music and movies too, if only they have the room. Large
files are also a problem on the Internet, because they take a long time to download. We
also try to make computers smaller—even a cellphone or wristwatch can be expected to
store lots of information!
There is a solution to this problem, however. Instead of buying more storage space, or a
faster network connection, we can compress the data so that it takes up less space. This
process of compressing and decompressing the data is normally done automatically by
the computer. All we might notice is that the disk holds more, or that web pages display
faster, but the computer is actually doing more processing.
Many methods of compression have been invented. The method used in this activity, with
the principle of pointing to earlier occurrences of chunks of text, is often referred to as
‘Ziv-Lempel coding,’ or ‘LZ coding’, invented by two Israeli professors in the 1970s. It
can be used for any language and can easily halve the size of the data being compressed.
It is sometimes referred to as ‘zip’ on personal computers, and is also used for ‘GIF’
images, as well as high-speed modems. In the case of modems, it reduces the amount of
data that needs to be transmitted over the phone line, so it goes much faster.
Some other methods are based on the idea that letters that are used more often should
have shorter codes than the others. Morse code used this idea.
What’s it all about?
The storage capacity of computers is growing at an unbelievable rate—in the last 25
years, the amount of storage provided on a typical computer has grown about a
millionfold—but we still find more to put into our computers. Computers can store whole
by Browser Shop" style="border: none !important; display: inline-block !important; text-indent: 0px !important; float: none !important; font-weight: bold !important; height: auto !important; margin: 0px !important; min-height: 0px !important; min-width: 0px !important; padding: 0px !important; text-transform: uppercase !important; text-decoration: underline !important; vertical-align: baseline !important; width: auto !important; background: transparent !important;">books or even libraries, and now music and movies too, if only they have the room. Large
files are also a problem on the Internet, because they take a long time to download. We
also try to make computers smaller—even a cellphone or wristwatch can be expected to
store lots of information!
There is a solution to this problem, however. Instead of buying more storage space, or a
faster network connection, we can compress the data so that it takes up less space. This
process of compressing and decompressing the data is normally done automatically by
the computer. All we might notice is that the disk holds more, or that web pages display
faster, but the computer is actually doing more processing.
Many methods of compression have been invented. The method used in this activity, with
the principle of pointing to earlier occurrences of chunks of text, is often referred to as
‘Ziv-Lempel coding,’ or ‘LZ coding’, invented by two Israeli professors in the 1970s. It
can be used for any language and can easily halve the size of the data being compressed.
It is sometimes referred to as ‘zip’ on personal computers, and is also used for ‘GIF’
images, as well as high-speed modems. In the case of modems, it reduces the amount of
data that needs to be transmitted over the phone line, so it goes much faster.
Some other methods are based on the idea that letters that are used more often should
have shorter codes than the others. Morse code used this idea.
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