Images, Sound & File Sizes

Watch first — Craig 'n' Dave

Craig 'n' Dave · Edexcel 1CP2Representing Images and Sound

Key facts

Images — key concepts

Pixel — smallest unit of an image. Each pixel has a colour value.
Colour depth — number of bits used per pixel. Higher depth = more colours = larger file.
Resolution — number of pixels (width × height). Higher resolution = more detail = larger file.
Image file size formula: width (pixels) × height (pixels) × colour depth (bits) ÷ 8 = bytes
Example: 1028 × 640 pixels, 15-bit colour: (1028 × 640 × 15) ÷ (8 × 1024 × 1024) megabytes
Jun 2022 Q3(d)(ii): full file size calculation — must show all working.

Sound — key concepts

Sample rate — number of samples taken per second (Hz). Higher = better quality = larger file.
Bit depth — number of bits per sample. Higher = more detail = larger file.
Sound file size formula: sample rate (Hz) × bit depth (bits) × duration (seconds) ÷ 8 = bytes
Jun 2024 Q2(b): "the time between samples" = sample interval (NOT sample rate definition).
Jun 2023 Q5(e)(ii): image file size with metadata: (100 × 600 × 32) ÷ 8 + 1024

Why hex is used for colour codes

Each colour channel (Red, Green, Blue) = 1 byte = 2 hex digits
#FF0000 = pure red (R=255, G=0, B=0)
Colour depth of 24-bit (true colour) = 3 bytes per pixel = 16.7 million colours
1-bit = 2 colours (black/white). 8-bit = 256 colours. 16-bit = 65,536 colours.

Exam questions — 3 questions · 9 marks · from real 1CP2 past papers

1 markColour depth effect1CP2 style

A bitmap image is stored with 1 bit per pixel. What colours can it represent?

A 256 colours

B Only black and white

C Red, green and blue

D Any colour but in a limited shade

3 marksImage file size calculation1CP2 Jun 2022 Q3(d)(ii) style

An image is 1028 × 640 pixels with a colour depth of 15 bits. Calculate the file size in megabytes. Show all working. (3 marks)

Hint: Formula: width × height × colour depth (bits) ÷ 8 ÷ 1024 ÷ 1024 = MB. Show each step for the marks.

+30 XP

3 marksSound file size1CP2 Jun 2023 Q5(e)(ii) style

A sound recording has a sample rate of 44,100 Hz, a bit depth of 16 bits, and lasts 3 minutes. Calculate the file size in megabytes. Show all working. (3 marks)

Hint: Formula: sample rate × bit depth × duration (seconds) ÷ 8 ÷ 1024 ÷ 1024 = MB. Convert minutes to seconds first.

+30 XP

Images — key concepts

Sound — key concepts

Why hex is used for colour codes

Module complete! 🎉