Character Sets and Bits Per Character (AQA GCSE Computer Science): Revision Notes

Character sets and bits per character

Understanding character encoding

When you type on a keyboard, your computer needs to understand what each key press means. This is where character encoding comes in - it's the system that converts the letters, numbers, and symbols you type into binary code that computers can process and store.

Think of character encoding like a translation dictionary. Each character you can type has a unique binary number assigned to it. When you press 'A', the computer looks up the binary code for 'A' and stores that code instead of the actual letter. This character code can then be used to display the character on screen or send it to a printer.

For computers to communicate properly, they need to agree on which binary codes represent which characters. This is why we have international standards for character encoding.

What is a character set?

A character set is like a complete list of all the characters that a computer system can understand and display. This includes everything from letters and numbers to punctuation marks and special symbols.

The size of a character set depends on how many bits are used to represent each character. More bits means more possible combinations, which means more characters can be included in the set. The first widely-used standard was designed for English text, but as computers became global, we needed much larger character sets to handle all the world's languages.

ASCII character encoding

In 1960, the American Standards Association created ASCII (American Standard Code for Information Interchange) to solve the problem of different computer systems using different codes for the same characters.

ASCII uses a 7-bit system, which means each character is represented by a 7-digit binary number. Since each bit can be either 0 or 1, this gives us $2^7 = 128$ possible characters.

The 128 ASCII characters are carefully organised into different categories:

• 52 letters (26 uppercase A-Z, 26 lowercase a-z)
• 10 digits (0-9)
• 33 punctuation marks and symbols (including the space character)
• 32 control characters (non-printable codes used for formatting)

ASCII code examples

Here are some common ASCII characters with their binary, hexadecimal, and decimal representations:

Important note about ASCII storage

While ASCII uses 7 bits to encode characters, computers actually store each ASCII character using 8 bits (1 byte). The extra bit was originally used for error-checking to ensure data wasn't corrupted during transmission. This means each ASCII character requires exactly 1 byte of storage space.

Unicode character encoding

As computers spread worldwide, ASCII's limitation became obvious - it could only handle English characters! Unicode was developed to solve this problem by supporting characters from all the world's languages.

Unicode uses either 16 bits or 32 bits per character, giving it the capacity to represent:

• 16-bit Unicode: $2^{16} = 65,536$ characters
• 32-bit Unicode: $2^{32} = 4,294,967,296$ (over 2 billion) characters

This massive capacity allows Unicode to include:

• All ASCII characters (for backwards compatibility)
• Characters from languages like Chinese, Arabic, Hindi, and Russian
• Mathematical symbols and special characters
• Even emojis and decorative symbols!

Relationship between ASCII and Unicode

Unicode was designed to be backwards compatible with ASCII. This means the first 128 characters in Unicode are identical to ASCII - so 'A' has the same code in both systems. This ensures that older ASCII text files can still be read by Unicode systems without any problems.

Comparing character sets

The key differences between ASCII and Unicode demonstrate the evolution of character encoding systems:

ASCII advantages:

• Simple and efficient for English text
• Uses less storage space (1 byte per character)
• Fast processing
• Universal support on all systems

Unicode advantages:

• Supports international languages
• Includes special symbols and emojis
• Future-proof as new characters are added
• Single standard for global communication

Storage consideration: Unicode characters require more storage space than ASCII. While ASCII needs just 1 byte per character, Unicode can need 2-4 bytes per character depending on the specific character being stored.