Data Representation and Safety – SS1 Digital Technologies Lesson Note (First Term)

Data representation is the way a computer stores text, images and sound as numbers made of the digits 0 and 1.

Behavioural objectives

By the end of the lesson, students should be able to:

1. Define a bit and a byte, and state that 8 bits make 1 byte.
2. Arrange the units KB, MB, GB and TB by size and give an example of each.
3. Explain how ASCII and Unicode represent text, and state the main difference between them.
4. Explain how pixels, resolution and colour depth represent an image.
5. Explain how sampling and sample rate represent sound.
6. State common threats to data and the ways of keeping data safe.

What are bits and bytes?

A bit is the smallest unit of data in a computer, and a byte is a group of 8 bits.

• Switches: a computer is made of millions of tiny switches that are either ON or OFF.
• Binary: ON is written as 1 and OFF as 0, so all data is built from these two digits.
• Bit: one single 0 or 1. The word comes from Binary digIT.
• Byte: a group of 8 bits. One bit is too small to do much, so bits are grouped.

Why is a byte important?

One byte holds enough bits to store one character, such as the letter “A” or the number “7”.

• 1 bit: can make 2 different patterns (0 or 1).
• 2 bits: can make 4 patterns (00, 01, 10, 11).
• 8 bits: can make 256 different patterns, enough for letters, digits and symbols.
• Example: the capital letter “A” is stored as the number 65, which is 01000001 in binary.

What is the size scale for data?

Data is measured in bytes, and each larger unit is about 1,000 times the one before it.

• Order from smallest to largest: byte, KB, MB, GB, TB.
• Everyday use: phone storage and data bundles are measured in MB and GB.

How does a computer represent text?

A computer represents text by giving every character its own number code, which is then stored in binary.

• Code: when you press “B”, the computer does not see a letter. It stores the number code for “B”.
• Standard: all computers use the same code tables, so every computer agrees on which number means which character.

What is ASCII?

ASCII (pronounced ASK-ee) is an early code that gives each English letter, digit and common symbol a number.

• Standard ASCII: uses 7 bits, which gives 128 characters.
• Extended ASCII: uses 8 bits, which gives 256 characters.
• Strength: it works well for English letters, digits and basic symbols.
• Limit: it has no room for emojis or for languages such as Arabic and Chinese. It also cannot show every Yoruba letter with its marks.

What is Unicode?

Unicode is the modern standard that gives a number to characters from almost every writing system in the world.

• Size: it can use up to 32 bits per character, so it has room for over 140,000 characters.
• Coverage: it includes letters of many languages, symbols and emojis.
• Result: you can send an emoji or type in any major language and the computer understands it.

What is the difference between ASCII and Unicode?

How does a computer represent images?

A computer represents an image as a grid of tiny dots called pixels (picture elements), each stored as a binary code.

1. The image is divided into a grid of pixels.
2. Each pixel is given a colour.
3. The colour is stored as a binary number.
4. The screen reads the numbers and shows each pixel in its colour.

What is resolution?

Resolution is the number of pixels in an image.

• More pixels: a clearer, sharper image.
• Fewer pixels: a blurry, blocky (pixelated) image.

What is colour depth?

Colour depth is the number of bits used to store the colour of each pixel.

• 1 bit per pixel: only 2 colours, black or white.
• 8 bits per pixel: 256 colours.
• 24 bits per pixel: over 16 million colours.
• File size: more pixels and a greater colour depth make a larger file.

How does a computer represent sound?

A computer represents sound by measuring the sound wave many times a second and storing each measurement as a number.

• Problem: sound travels as a wave, and a computer can only store numbers.
• Sampling: taking a measurement, or “snapshot”, of the wave at a point in time.

1. A microphone picks up the sound wave.
2. The computer measures the height of the wave many times each second.
3. Each measurement is stored as a binary number.
4. To play the sound, the numbers are turned back into a wave.

What is sample rate?

Sample rate is the number of samples a computer takes in one second.

• Higher sample rate: the digital sound is closer to the real sound, but the file is larger.
• Lower sample rate: the sound is poorer, but the file is smaller.
• CD quality: 44,100 samples every second.

How do text, images and sound compare?

A computer changes every type of data into numbers before it can store or process it.

Why does data representation matter?

It explains why different files need different amounts of storage.

• Video: needs many pixels and many samples, so it is large.
• Text message: needs only a few bytes, so it is small.
• Phone storage: knowing file sizes helps you manage space.
• Sending files: large files take more data and time to send.

What is data safety?

Data safety is the protection of data from loss, theft, damage and misuse.

What are the common threats to data?

• Malware: harmful programs, such as viruses, that damage or steal files.
• Phishing: fake messages or websites that trick people into giving passwords or personal details.
• Hacking: unauthorised access to a computer, phone or account.
• Accidental deletion: a user removes an important file by mistake.
• Hardware failure: a damaged phone, flash drive or hard disk loses its files.
• Power failure: a sudden power cut can damage unsaved work.

How can data be kept safe?

Evaluation

1. How many bits are in 3 bytes?
2. What is the main difference between ASCII and Unicode?
3. If an image is “pixelated” (blurry and blocky), what does that tell you about its pixels?
4. Why does a 5-minute song take up more storage space than a 5-page Word document?
5. State two ways of protecting your files from loss or theft.

Assignment

1. Write short notes on how a computer represents text, images and sound, and name two ways of keeping your data safe.