Binary Codes to Represent Characters Revision Notes for AQA GCSE Computer Science

Binary codes to represent characters

What is character encoding?

When you type on a keyboard, your computer needs a way to understand and store each letter, number, and symbol you press. This is where character encoding comes in. Character encoding is the process of converting characters (like letters and numbers) into binary codes that computers can understand and process.

Every time you press a key, the computer converts that character into a unique character code - a specific binary number that represents that character. This code can then be used to identify and display the character on screen or send it to a printer. Without these codes, computers wouldn't know the difference between an 'A' and a 'B'!

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Think of character encoding like a translation dictionary between human language and computer language. Each character you type has a corresponding "address" in the computer's memory system.

Why do we need agreed standards?

Imagine if every computer company created their own way of representing characters. A document created on one computer might display completely different text on another computer! This is why we need international standards - agreed ways of representing characters that all computer systems can understand.

These standards ensure that when you send a text message or email, the person receiving it sees exactly the same characters you intended to send. It's like having a universal language that all computers can speak.

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Without agreed standards, digital communication would be chaos! A simple email saying "Hello" might appear as random symbols or completely different words on the recipient's computer.

Character sets and bits per character

A character set is the complete collection of all characters that a computer system can recognise and work with. This includes everything from letters and numbers to punctuation marks and special symbols.

The number of characters in a character set depends on how many bits are used to represent each character. More bits mean more possible combinations, which means more characters can be represented. The early computer systems needed to balance having enough characters with keeping the system simple and efficient.

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The relationship between bits and possible combinations follows a simple mathematical rule: with n bits, you can create $2^n$ different combinations. So more bits = exponentially more possible characters!

ASCII - the first major standard

In 1960, the American Standards Association created ASCII (American Standard Code for Information Interchange - you can remember this as "Ask-ee"). This was designed to be a universal way of representing the main characters used in English text.

ASCII was created to provide codes for the essential characters needed for English text and computer operation. It was designed to be comprehensive yet efficient for the technology available at the time.

The ASCII character set is carefully organised into different types of characters:

52 alphabetic characters: These include all 26 uppercase letters (A-Z) and all 26 lowercase letters (a-z)
10 numeric symbols: The digits 0 through 9 that we use for numbers
33 punctuation and other symbols: This includes the space character, plus common punctuation like commas, periods, question marks, and mathematical symbols
32 non-printable control codes: These are special codes that control how text is displayed or processed (like codes for starting a new line or making a beep sound)

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ASCII Character Examples:

Letter 'A' = 1000001 in binary (65 in decimal)
Letter 'a' = 1100001 in binary (97 in decimal)
Number '5' = 0110101 in binary (53 in decimal)
Space character = 0100000 in binary (32 in decimal)

How ASCII uses binary numbers

ASCII uses 7-bit binary numbers to represent each character. This means each character code is made up of exactly 7 binary digits (0s and 1s). With 7 bits, you can create $2^7 = 128$ different combinations.

The ASCII codes range from 0000000 to 1111111 in binary (or 0 to 127 in decimal). This gives us 128 possible codes total. Originally, ASCII used 127 of these codes for actual characters, with the code 0000000 (decimal 0) meaning 'no character'.

Out of the 128 total ASCII codes, 95 represent printable characters - these are the characters you can actually see on screen, like letters, numbers, and punctuation marks. The remaining 33 codes are non-printable control characters that perform special functions.

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Binary to Decimal Conversion:

To convert 1000001 (the code for 'A') to decimal:

$1 \times 2^6 + 0 \times 2^5 + 0 \times 2^4 + 0 \times 2^3 + 0 \times 2^2 + 0 \times 2^1 + 1 \times 2^0$
$= 64 + 0 + 0 + 0 + 0 + 0 + 1$
$= 65$

So 'A' has the decimal ASCII code of 65.

Why 7 bits was chosen

The choice of 7 bits was practical for early computer systems. It provided enough combinations to cover all essential English characters while being efficient to store and process. Each ASCII character could fit comfortably within the 8-bit bytes that computers commonly used, leaving one bit available for other purposes like error checking.

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The "leftover" 8th bit in each byte was often used for parity checking - a method to detect errors in data transmission. This made ASCII both efficient and reliable for early computer communications.

Key Takeaways

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Essential Points to Remember:

Character encoding converts keyboard characters into binary codes that computers can understand and process
Character sets are collections of all characters a computer system can recognise and work with
ASCII uses 7-bit binary numbers to represent 128 different characters (including control codes)
ASCII includes 52 letters, 10 digits, 33 punctuation/symbols, and 32 control codes
Standards are essential so all computer systems can share and display text correctly

Binary Codes to Represent Characters (AQA GCSE Computer Science): Revision Notes