2.1f Character Encoding, Images & Sound — Edexcel 1CP2

Character Encoding: ASCII

ASCII (American Standard Code for Information Interchange) assigns a unique 7-bit binary number to each character. It covers 128 characters (0–127), including uppercase/lowercase letters, digits, punctuation, and control characters.

Character	ASCII (Denary)	Binary
'A'	65	01000001
'a'	97	01100001
'0'	48	00110000
Space	32	00100000

Key relationships: uppercase 'A'–'Z' = 65–90; lowercase 'a'–'z' = 97–122. Adding 32 to an uppercase ASCII value gives the lowercase equivalent.

Unicode

Unicode is an extension of ASCII that can represent characters from all world languages, emojis, and special symbols. It uses more bytes per character:

UTF-8: 1–4 bytes per character (compatible with ASCII for the first 128 characters)
UTF-16: typically 2–4 bytes per character
Unicode supports over 1 million characters vs ASCII's 128

Bitmap Images

A bitmap stores an image as a grid of pixels. Each pixel is stored as a binary value representing its colour.

Pixel: the smallest element of a digital image
Resolution: number of pixels (width × height); higher resolution = more detail
Colour depth: number of bits used per pixel; more bits = more colours

Colour depth (bits)	Colours possible	Example
1	2¹ = 2	Black & white
8	2⁸ = 256	Early GIF
24	2²⁴ = 16,777,216	True colour

File size formula: width × height × colour depth ÷ 8 = bytes

Example: 200×150 pixels, 24-bit colour → 200 × 150 × 24 ÷ 8 = 900,000 bytes ≈ 879 KiB

Digital Sound

Sound is an analogue wave. To store it digitally, it must be sampled:

Sample rate: number of samples per second (Hz). CD quality = 44,100 Hz
Bit depth (sample resolution): bits used per sample. CD quality = 16-bit
Higher sample rate and bit depth = better quality but larger file size

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

For stereo, multiply by 2 (2 channels).

Example: 44,100 Hz × 16-bit × 10 s ÷ 8 = 882,000 bytes ≈ 861 KiB (mono)

Exam tip: Know the ASCII values for 'A' (65) and 'a' (97). For images: file size = width × height × colour depth ÷ 8 bytes. For sound: file size = sample rate × bit depth × seconds ÷ 8 bytes (× 2 for stereo). Always include units in your working.

⚠️ Common Mistakes

Forgetting to divide by 8 when converting bits to bytes in file size calculations
Not doubling for stereo sound files
Confusing ASCII (128 chars, 7-bit) with Unicode (millions of chars, variable bytes)
Confusing resolution (number of pixels) with colour depth (bits per pixel)

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Worksheet — 2.1f Character Encoding, Images & Sound

8 Edexcel-style questions · AI-marked

Q1State the ASCII code for 'A' and 'a' in denary, and explain the relationship between them.[3]

✅ Mark scheme

'A' = 65 [1]; 'a' = 97 [1]; lowercase = uppercase + 32 [1].

Q2State two advantages of Unicode over ASCII.[2]

✅ Mark scheme

Any two: supports more characters (over 1 million vs 128) [1]; supports all world languages and scripts [1]; includes emojis and special symbols [1].

Q3Define the term 'colour depth' and explain how it affects file size.[2]

✅ Mark scheme

Colour depth = number of bits used to represent each pixel's colour [1]; more bits per pixel means more possible colours and a larger file size [1].

Q4Calculate the file size of a 1024×768 pixel image with 24-bit colour depth. Give your answer in MiB.[3]

✅ Mark scheme

1024×768×24=18,874,368 bits [1]; ÷8=2,359,296 bytes [1]; ÷1024÷1024≈2.25 MiB [1].

Q5Explain what 'sample rate' means in digital audio.[2]

✅ Mark scheme

Sample rate is the number of times per second the sound wave is measured (sampled) [1]; higher sample rate = more accurate/better quality digital reproduction [1].

Q6Calculate the file size of a 30-second stereo audio recording at 44,100 Hz, 16-bit depth. Give your answer in MiB.[3]

✅ Mark scheme

44100×16×30×2=42,336,000 bits [1]; ÷8=5,292,000 bytes [1]; ÷1024÷1024≈5.05 MiB [1].

Q7A 1-bit image has 2 possible colours. How many colours can a 4-bit image represent?[1]

✅ Mark scheme

2⁴ = 16 colours [1].

Q8State one way to reduce an image file size without using compression.[1]

✅ Mark scheme

Any one: reduce resolution (width/height) [1] OR reduce colour depth (fewer bits per pixel) [1].

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Character Encoding: ASCII

Unicode

Bitmap Images

Digital Sound

Worksheet — 2.1f Character Encoding, Images & Sound

Mini Test — Encoding, Images & Sound

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