Encoding Text,
Images & Sound

ASCII · Unicode · Pixels · Colour Depth · Sample Rate · Bit Depth

CSZoneEdexcel GCSE Computer Science 1CP2

Character Encoding

ASCII and Unicode

ASCII (American Standard Code for Information Interchange): 7-bit encoding = 128 characters. Covers English letters, digits, punctuation. 'A' = 65, 'a' = 97, '0' = 48.

Unicode: up to 32-bit encoding. Supports over 1 million characters including all world scripts, emoji. UTF-8 is backwards compatible with ASCII.

Unicode is essential for international software; ASCII is only sufficient for English

Representing Images

Pixels, Resolution, and Colour Depth

Images are made of pixels — each pixel stores a colour value in binary

Colour depth — number of bits per pixel. 1-bit = black/white; 8-bit = 256 colours; 24-bit = 16.7 million colours

Resolution — number of pixels (width × height). Higher resolution = more detail = larger file

File size (bytes) = (width × height × colour depth) ÷ 8

Representing Sound

Sampling Analogue Audio

Sound is an analogue signal. To store it digitally, it is sampled at regular intervals. Each sample captures the amplitude as a binary value.

Sample rate — how many samples per second (Hz). 44,100 Hz = CD quality. Higher = better quality, larger file.

Bit depth — bits per sample. 16-bit = 65,536 amplitude levels. Higher = more accurate, larger file.

File size = sample rate × bit depth × duration (seconds) ÷ 8 (for bytes)

Exam Practice

Have a go at this question

Edexcel-style question

Calculate the uncompressed file size in bytes of a 300×400 pixel image with a colour depth of 24 bits per pixel.

3 marks

Number of pixels = 300 × 400 = 120,000 [1]
Total bits = 120,000 × 24 = 2,880,000 bits [1]
File size = 2,880,000 ÷ 8 = 360,000 bytes [1]

Key Takeaways

What to Remember

ASCII: 7-bit, 128 chars; Unicode: up to 32-bit, global characters

Images: size = width × height × colour depth ÷ 8 bytes

Higher colour depth/resolution → better quality, bigger file

Sound: higher sample rate + bit depth → better quality, bigger file