Understanding Compression
Author: Debbie Grignani, 2BrightSparks Pte
Ltd.
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Many of us have experienced
difficulties in sending data files like text
documents and digital photos via email because
the file size is just too great. That is
where file compression comes in useful.
What is File Compression?
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File compression enables a computer
user to reduce the size of a data file
so that it is more convenient for storage
or sharing.
During file compression, excess information
is eliminated when some kind of pattern
in the data is recognized. There are
different algorithms used to search for
a pattern in order to reduce the size
of the file, and most compression software
programs use a variation of Lempel-Ziv
(1977) Compression, also known as LZ77.
LZ77 is an adaptive dictionary-based
compression algorithm that builds a dictionary
based on text that has been previously
encountered.
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A Simple Example of File Compression
To better understand this process we will
look at a simple example. Take the quote by
Aristotle:
“
One swallow does not make a summer, neither
does one fine day; similarly one day or brief
time of happiness does not make a person entirely
happy.”
Let us say that each character takes up 1
bit, therefore the size of the quote is 146
bytes. Ignoring capital letters, there are
words that are repeated which we can review
in the table that is shown below:
Index Number |
Repeated
Word |
Number of
repetitive times |
1 |
one |
3 |
2 |
does |
3 |
3 |
not |
2 |
4 |
make |
2 |
5 |
day |
2 |
By substituting the index number in the place
of the word, this is the result:
“1 swallow 2 3 4 a summer, neither 2
1 fine 5; similarly 1 5 or brief time of happiness
2 3 4 a person entirely happy.”
From a size of 146 bits to a 118 bit size,
which converts to only a 19% compression rate.
This example does not significantly compress
the data, but you get the idea. More complex
compression algorithms seek out patterns that
include eliminating punctuation and spaces
and therefore compressing files a lot more.
There are two forms of compression – Lossless and Lossy Compression.
Lossless Compression
What we have seen so far is known as lossless
compression. That is when a compressed file
is decompressed it looks identical to the original.
No information is lost in the process of lossless
compression. When compressing data and programs,
lossless compression must be used.
Lossy Compression
In order to achieve greater compression, lossy
compression is used. However, only certain
types of data can accept lossy compression.
They include graphics, audio and video files.
In this compression method, some degree of
data lost in inevitable since redundant and
unnecessary information are literally eliminated
and lost forever. The good side of this type
of compression is that it reduces the size
of the file tremendously.
Backup and Compression
What we have seen so far is known as lossless
compression. That is when a compressed file
is decompressed it looks identical to the original.
No information is lost in the process of lossless
compression. When compressing data and programs,
lossless compression must be used.
The two tasks of backup and compression go
hand in hand, since compression can help storage
sizes of backups to be kept to a minimum. You
can either manually compress your files using
programs like WinZip before each backup is
run, or simply make use of backup software
that has file compression as an option.
2BrightSparks Pte Ltd has two options of backup
and synchronization software that offer compression
capabilities. A freeware version called SyncBack and a shareware version called SyncBackSE.
SyncBackSE comes packed with more options and
its compression capabilities a significantly
more advanced as compared to SyncBack freeware.
Both programs provide ease of use and allow
you to manage your storage space better with
compressed backups.
For more information and guidance about backing
up read The
Backup Guide.
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