TABLE OF CONTENTS Data is compressed by reducing its redundancy, but this also makes the data less reliable, more prone to errors. Increasing the integrity of data, on the other hand, is done by adding check bits and parity bits, a process that increases the size of the data, thereby increasing redundancy. Data compression and data reliability are therefore opposites, and it is interesting to note that the latter is a relatively recent field, whereas the former existed even before the advent of computers. The sympathetic telegraph, discussed in the Preface, the Braille code of 1820 (Section 1.1.1), and the Morse code of 1838 (Table 2.1) use simple, intuitive forms of compression. It therefore seems that reducing redundancy comes naturally to anyone who works on codes, but increasing it is something that "goes against the grain" in humans. This section discusses simple, intuitive compression methods that have been used in the past. Today these methods are mostly of historical interest, since they are generally inefficient and cannot compete with the modern compression methods developed during the last several decades.
This well-known code, which enables the blind to read, was developed by Louis Braille in the 1820s and is still in common use today, after having been modified several times. Many books in Braille are available from the National Braille Press. The Braille code consists of groups (or cells) of 3 2 dots each, embossed on thick paper. Each of the 6 dots in a group may be...
