Back to Hub

Media Representation

AQA 8525 §3.3.3 & §3.3.4
Formula: Width × Height × Depth
🎨 How Computers Store Images (Plain English)

Zoom close into any screen and you'll see a grid of coloured squares called pixels. In RAM, every single pixel is just a binary number (1s and 0s). The more bits per pixel (colour depth), the more colours you get, but the larger the file!

1-bit: 2 colours ($2^1$). Pure Black & White.
2-bit: 4 colours ($2^2$). Classic Game Boy green.
4-bit: 16 colours ($2^4$). Retro arcade / pixel art.
8-bit: 256 colours ($2^8$). VGA computing.
Canvas

Pixel Grid Editor

Click & Drag to Paint
Grid Size:
Colour Depth:
Tool:
✨ 1-Click Starter Sprites: (Click any sprite to load instantly into the grid & binary stream)
Active Palette: Selected: #0000aa (0001)
Coordinates: Hover over a pixel -
Inspector

File Size & Metadata

AQA 8525 §3.3
Dimensions 16 × 16 256 pixels
Colour Depth 4 bits 16 colours (2⁴)
Raw Bits (W×H×D) 1024 256 px × 4 bpp
Bytes (Bits ÷ 8) 128 B 1,024 ÷ 8
Kilobytes (kB) vs Kibibytes (KiB) Both Accepted
Decimal (SI) • ÷ 1,000
0.128 kB
1 kB = 1,000 bytes
Binary (IEC) • ÷ 1,024
0.125 KiB
1 KiB = 1,024 bytes
⚠️ Exam tip: Show your working clearly. Either divide by 1,000 or by 1,024, but explicitly write down which one you used so the examiner can give you the marks!
Exam Working:
1. Pixels = 16 × 16 = 256 pixels
2. Raw Bits = 256 pixels × 4 bits/pixel = 1,024 bits
3. Bytes = 1,024 bits ÷ 8 = 128 Bytes
4. Size in kB = 128 ÷ 1,000 = 0.128 kB  |  in KiB = 128 ÷ 1,024 = 0.125 KiB
Add Image Metadata (Shipping Label)
Data about data: tells the computer the image width, height, and colour depth.
Image Header Properties:
Width: 16 px Height: 16 px Colour Depth: 4 bpp Resolution: 72 dpi File Type: BMP/PNG
📌 Think of metadata as a shipping label: Without it, the computer just sees a continuous stream of 1s and 0s and wouldn't know when a row ends or which colour belongs where.
Total Stored File Size
128 Bytes (1,024 bits)
RAW SIZE
Raw Stream

Binary Stream (Row by Row)

Stored as bytes (8 bits each). Hover over a byte to see which pixels it represents:

Byte Count: 128 Bytes Encoding: Left-to-right
Compression

Run-Length Encoding (RLE)

-

How Run-Length Encoding works: Instead of repeating individual pixels (e.g. Blue, Blue, Blue, Blue), RLE stores the count and the pixel value (e.g. 4, Blue). It's lossless compression—no detail is thrown away!

✨ Hover over any RLE block below (or hover over canvas pixels) to highlight the exact run!
Space Saved: 0%
Raw: 1024 bits Compressed: 0 bits
⚠️ Exam tip: RLE only saves space when there are long runs of the same colour. If colours alternate frequently (like a checkerboard), RLE actually increases file size.
AQA 8525 §3.3.3 (Bitmaps)
🌈 Why Real Photos Need Millions of Colours (Colour Banding)

Human eyes can perceive around 10 million distinct colours. In 24-bit True Colour, computers use 8 bits for Red, 8 for Green, and 8 for Blue (256 × 256 × 256 = 16,777,216 colours), allowing buttery-smooth gradients in skies and skin tones.

⚡ Look for "Colour Banding" (Posterization): When you drop to 8-bit, 4-bit, or 2-bit, the computer runs out of intermediate color codes. Smooth sky gradients split into harsh, visible blocks of solid stripes!

Interactive Studio

Photo Quantization Playground

Sample Photo:
Target Depth:
8-bit • 256 Colours Aspect: 16:9 Landscape
800 × 560 px (448,000 pixels) Colour Banding: Noticeable in skies & skin
File Size: 448.0 kB (66.7% saved vs 24-bit)
📊 AQA File Size Comparison Table (Formula: Width × Height × Colour Depth) Current Photo: 400 × 280
Colour Depth Max Colours Bytes per Pixel Total File Size (Bytes / kB) Space Saved vs 24-bit Visual Effect
Sample Image:
Option A

Lower Quality

Smaller File
Resolution:
Colour Depth:
Pixels 256
Max Colours 2 2¹
File Size 32 B 256 bits
Option B

Higher Quality

Larger File
Resolution:
Colour Depth:
Pixels 1,024
Max Colours 16.7M 2²⁴
File Size 3,072 B 3 KiB (24,576 bits)
Transmission

Download Time Calculator

Select a connection speed to see estimated download times (Time = File Size in Bits ÷ Speed in bps):

Option A Download Time
0.005 s
Option B Download Time
0.439 s
AQA 8525 §3.3.4 (Sound)
🌊 How Sound is Digitized (Plain English)

Real sound is a smooth, continuous wave of vibrating air molecules. A computer can't store infinity, so an Analogue-to-Digital Converter (ADC) measures the wave's height thousands of times per second (Sample Rate) and rounds each measurement to the nearest binary number (Quantization).

Sample Rate (Hz): Frequency of measurements per second. CD = 44,100 Hz (44.1 kHz).
Resolution (Bit Depth): Bits per measurement (2depth height levels). CD = 16-bit.
Quantization Error: The difference between the real wave height and the rounded digital step.
Interactive Canvas

Waveform Sampling Visualizer (Melody Theremin)

Vertical Axis = Musical Pitch (130Hz - 520Hz) • Horizontal Axis = Time (2.0s)

The height of the curve directly controls musical pitch! The continuous blue wave produces a smooth pitch glide. The red staircase shows how digital sampling quantizes that glide into discrete stair-step notes based on your Bit Depth and Sample Rate.

Sample Rate:
Bit Depth (Quantization Levels):
Melody Preset:
✏️ Interactive Waveform: Click & drag anywhere across the canvas to draw your own custom melody! IDLE • CLICK PLAY TO HEAR
Continuous Analogue Curve (Smooth Pitch Glide)
Quantized Staircase (Stepped Pitch Snapped to Bit Depth)
Sample Points (Triggered at Sample Rate)
Quantization Pitch Levels
Real-World Audio Lab

🎙️ Voice & Microphone Audio Degradation Lab

Record your voice, then hear sample rate & bit depth reduction live!

Test real sound degradation! Record a quick 2-second voice clip (or use a built-in sample). Dial down the Sample Rate to hear high frequencies get lost (like speaking on an old landline telephone), and dial down the Bit Depth to hear the gritty crunch of retro video games (quantization noise)!

or choose sample:
Sample Rate (Frequencies Captured):
Bit Depth (Amplitude Precision):
Waveform Display (Processed Audio Buffer) READY • PRESS PLAY
Uncompressed File Size (2.0s Audio)
176.4 kB (176,400 Bytes)
Baseline CD Quality (16-bit at 44.1 kHz, Mono)
💡 What your ears are hearing (GCSE Exam Rule):
Full Studio Quality: Crystal clear reproduction with full treble, rich harmonics, and zero audible background noise.
Formula

Sound File Size Calculator (AQA §3.3.4)

Formula: Rate (Hz) × Resolution (bits) × Duration (s) × Channels
10 seconds
Sample Rate 8 Hz
Bit Depth 3 bits
Total Raw Bits 240 bits
Bytes (÷ 8) 30 B
Dual Unit Breakdown:
Decimal (kB): 0.03 kB
Bytes ÷ 1,000
Binary (KiB): 0.029 KiB
Bytes ÷ 1,024
⚠️ AQA Mark Scheme Rule: Remember to multiply by 2 if the question specifies stereo sound! Show whether you divided by 1,000 or 1,024.
AQA 8525 §3.3 Specification Focus

📌 AQA Core Revision Topics & Mark Scheme Rules

AQA RULE 1: FILE SIZE & PREFIXES

Dual Prefix Conventions (kB vs KiB)

The formula Width × Height × Colour Depth yields raw BITS. Divide by 8 to obtain Bytes (B). AQA tests both decimal and binary prefix conventions:

Decimal (SI Units):
• 1 kB = 1,000 bytes
• Bytes ÷ 1,000 = kB
Binary (IEC Units):
• 1 KiB = 1,024 bytes
• Bytes ÷ 1,024 = KiB
⚠️ AQA Mark Scheme Rule: In arithmetic questions, marks are awarded for dividing by either 1,000 or 1,024, provided your working clearly states which convention you used. Always state your units explicitly!
AQA RULE 2: IMAGE PARAMETERS

Resolution vs Colour Depth (Impact & Trade-offs)

Be prepared to explain the distinct impacts of changing image resolution versus changing colour depth:

Increasing Resolution:
• Packs more pixels into the image (W × H or DPI).
• Results in sharper, crisper detail.
• File size increases proportionally to pixel count.
Increasing Colour Depth:
• Allocates more bits per pixel ($2^n$ colours).
• Allows smoother gradients and realistic shades.
• File size increases proportionally to bit depth.
💡 Key Trade-off: Higher quality requires more memory, greater storage capacity, and longer network transmission / download times.
AQA RULE 3: SPECIFICATION §3.3

Role & Examples of Image Metadata

Never write only "metadata is data about data"—that is worth at most 1 mark. For full marks, specify the exact attributes and explain why the computer needs them.

AQA Required Header Attributes:
• Width and Height (in pixels)
• Colour Depth (bits per pixel, bpp)
• Resolution (dots/pixels per inch, dpi)
• File Format / Encoding (e.g. BMP, PNG)
✓ Why Metadata is Essential: Raw image data is simply a continuous sequence of 1s and 0s. Without metadata, rendering software cannot determine where each row of pixels breaks or how many bits belong to each pixel colour, resulting in corrupt rendering or display failure.

📝 AQA Past Paper Style Questions with Mark Schemes

Calculation • 4 Marks

Q1: An image has dimensions of 200 pixels by 150 pixels with a colour depth of 8 bits per pixel.
(a) Calculate the file size of the image in bytes. Show your working.
(b) State the size of the image in kilobytes (kB) using the decimal convention, or in kibibytes (KiB) using the binary convention.

[1 mark] Calculating total pixels: 200 × 150 = 30,000 pixels.
[1 mark] Calculating total bits: 30,000 × 8 bits = 240,000 bits.
[1 mark] Converting bits to bytes: 240,000 ÷ 8 = 30,000 Bytes. (Part a answer).
[1 mark] Conversion to kB or KiB (Part b answer - either accepted if unit stated):
• Decimal convention: 30,000 ÷ 1,000 = 30 kB
• Binary convention: 30,000 ÷ 1,024 ≈ 29.3 KiB (or 29.30 KiB)
Theory • 3 Marks

Q2: State two items of metadata stored in an image file, and explain why metadata is essential for displaying a bitmap image.

[1 mark] Any two correct metadata properties (Width, Height, Colour Depth, Resolution in DPI, File type).
[1 mark] Definition / role: Metadata describes the structure and format of the raw binary data.
[1 mark] Explanation of necessity: Without metadata, software cannot determine where lines break (width/height) or how many bits represent each pixel colour, making it impossible to reconstruct and display the image correctly.
Evaluation • 2 Marks

Q3: A digital photograph's colour depth is increased from 8 bits per pixel to 24 bits per pixel.
Explain one advantage and one disadvantage of this change.

[1 mark] Advantage: The image will have higher visual fidelity / smoother colour gradients, representing up to 16.7 million colours ($2^{24}$) compared to only 256 colours ($2^8$).
[1 mark] Disadvantage: The file size will triple (3× larger, 24 bits vs 8 bits per pixel), requiring more storage space and taking longer to transmit or download.