Character Sets (OCR A Level Computer Science): Revision Note

Character Sets

How are characters represented?

• Computers only understand binary and therefore we need to represent characters using binary codes

• For example, the letter 'A' might be represented as 01000001 in binary

Character sets

• A character set is a list of all of the characters and their associated binary code

• Character sets standardise the binary codes for each character

• Without a character set, one system might interpret 01000001 differently from another

• Two common character sets are:

  • American Standard Code for Information Interchange (ASCII)

  • UNICODE

ASCII

• ASCII uses 7-bits to encode each character, providing for 128 distinct characters

• For example, 'A' is represented as 65 in decimal, which is 1000001 in binary

• ASCII was created to provide a common standard for encoding characters, which was necessary for compatibility among various types of hardware and software

• An extended version of ASCII exists which encodes each character using 8-bits creating 256 characters

ASCII table

• The ASCII table shows the relationship between characters that humans recognise and the denary values that represent them in the system

• The denary values can then be converted to binary, representing the original character as binary

ASCII Table

Limitations of ASCII

1. It has a limited number of characters

ASCII is limited to 128 characters, which include English alphabets, numerals, and some special and control characters.

A, B, C, ..., Z
a, b, c, ..., z
0, 1, ..., 9
!, @, #, ...

2. It is not suitable for multilingual text

ASCII cannot represent characters from languages other than English, limiting its applicability globally.

No representation for: 'α', 'ö', 'ñ',

3. There is no provision for modern symbols

ASCII does not include modern symbols or emoji's common in today's digital communication.

Unicode

• UNICODE was created to be a solution to the limitations of ASCII

• UNICODE uses a much larger bit range, up to 32-bits (depending on the encoding method), allowing for a wide variety of characters from different languages and scripts

  • Example: The Greek character Lambda 'λ' is represented as U+03BB

  • U+03BB breaks down to:

    • U+, meaning this is a Unicode character

    • 03BB, meaning character 03BB in the UNICODE set

Impact on storage

• ASCII is more storage-efficient, with characters requiring only 7-bits

• UNICODE characters can require up to 32-bits, thus potentially using more storage space

Comparison