0984
Text, Sound and Images
Data Representation · 3 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 5% of your exam marks.
stable
Rare
Stable5%
ASCII/Unicode, sound sampling and pixel/colour depth appear regularly as 2 to 4 mark questions.
(American Standard Code for Information Interchange) was the first widely-used character set, dating from the 1960s.
| Feature | Standard ASCII | Extended ASCII |
|---|---|---|
| Bits per character | 7 bits | 8 bits |
| Number of characters | 2⁷ = 128 | 2⁸ = 256 |
| Covers | English letters (A-Z and a-z), digits 0-9, basic punctuation, control codes | Standard ASCII plus extra symbols like ©, ±, accented letters |
Limitations of ASCII
Even extended ASCII has serious limits:
- Cannot represent characters from non-Latin alphabets (no Chinese, Arabic, Hindi, Cyrillic, Greek, etc.)
- Cannot represent modern symbols such as emoji
- Different "extended" versions disagreed about which extra symbols belonged where, causing compatibility problems
Part of the ASCII table:
| Character | Denary code |
|---|---|
| P | 80 |
| Q | 81 |
| R | 82 |
| S | 83 |
If you are given P = 80 and the question asks for the code for W, count forward four letters: Q = 81, R = 82, S = 83, T = 84, U = 85, V = 86, W = 87. The same trick works for finding a character from its code: ASCII 78 must be 80 − 2 = N.