Pixel Perfect
How computers store text, images and sound as bits: ASCII character codes, bitmaps built from pixels, and sound captured by sampling. Master the file-size calculations and the quality-versus-size trade-off.
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Everything is bits 🖼️
A computer stores **everything** as bits, 1s and 0s: text, images and sound alike. This module covers how each is represented: **ASCII** codes for characters, **bitmaps** for images, and **sampling** for sound. You will learn the **file-size** calculations and the **trade-off** between quality and file size.
The words you need 🔑
Five ideas run through data representation:
Capitals and lowercase 🔤
In 7-bit ASCII the code for 'A' is 65 and the code for 'a' is 97. What is the relationship between a capital letter's code and the same lowercase letter's code?
- The lowercase code is 32 more than the capital code
- They always have exactly the same code
- There is no fixed relationship; the codes are random
- The lowercase code is always double the capital code
Encode a character 🔢
An interactive activity.
Sizing a bitmap 🟦
A **bitmap** image is a grid of coloured **pixels**. Two numbers decide its size: **Resolution** = width x height, the number of pixels. **Colour depth** = the bits per pixel; n bits gives 2 to the power n possible colours. **File size (bits) = width x height x colour depth.** Divide by 8 for bytes, then by 1024 for kilobytes.
Match each term to its meaning 🧩
- Pixel
- Resolution
- Colour depth
- Sample rate
- A single point in a bitmap image
- The number of pixels across and down
- The number of bits used per pixel
- The number of sound samples taken per second
Size the bitmap 📐
An interactive activity.
Fill in the representation 🧱
A bitmap image is made of tiny squares called _____. The number of pixels across and down is the _____. The number of bits used for each pixel is the _____. For sound, the number of samples taken each second is the _____, and the number of bits stored per sample is the _____.
Images versus sound ⚖️
Both are stored as bits, but they are measured in different ways.
Size the sound file 🔊
An interactive activity.
How sound is sampled 🪜
An interactive activity.
What grows the file? ✅
Select the TWO changes that would increase the file size of a digital sound recording.
- Increasing the sample rate
- Increasing the bit depth
- Making the recording shorter
- Lowering the sample rate
Explain the trade-off ✍️
An interactive activity.
The grade-9 habit 🏆
Top marks show the **full calculation** and **convert units** at the end. Work in **bits**, then divide by **8** for bytes and by **1024** for kilobytes, and give the answer in the **unit asked for**. And never just say 'bigger file'. Explain the **mechanism**: a higher sample rate means **more samples per second**, so **more binary numbers to store**, so a larger file, but a closer match to the original wave. Mechanism, then trade-off, then the number in the right unit - top band.