Data Storage & Compression

Learn it by playing

Answer these questions to earn energy, then fish and explore. No account needed.

For teachers: ready-to-use lesson slides, revision notes, diagrams for Data Storage & Compression (Computer Science, CIE) — use them in your lesson, or run the topic as a live class game.

Notes

Units of Data Storage

A **bit** (b) is the smallest unit, representing a 1 or 0.
A **nibble** = 4 bits; a **byte** (B) = 8 bits.
Larger units: **Kibibyte (KiB)** = 1024 B, **Mebibyte (MiB)** = 1024 KiB, **Gibibyte (GiB)** = 1024 MiB, **Tebibyte (TiB)** = 1024 GiB, **Pebibyte (PiB)** = 1024 TiB, **Exbibyte (EiB)** = 1024 PiB.
Binary prefixes (KiB, MiB) use powers of 2 (1024); decimal prefixes (KB, MB) use powers of 10 (1000).
To convert to a larger unit, **divide by 1024**; to a smaller unit, **multiply by 1024**.
Between bits and bytes: multiply/divide by **8**.

Calculating File Sizes

Bitmap image size = **Resolution × Colour Depth** (or width × height × colour depth).
Resolution = width × height in pixels; colour depth = bits per pixel.
Sound file size = **Sample Rate × Duration (seconds) × Sample Resolution**.
Sample rate = samples per second; sample resolution = bits per sample.
Convert bits to bytes by dividing by 8; bytes to KiB by dividing by 1024.

Compression Overview

**Compression** reduces file size to save storage space, reduce bandwidth, and shorten transmission time.
Two types: **lossy** and **lossless**.

Lossy Compression

**Lossy compression** permanently removes data to reduce file size; it is **irreversible**.
Achieves large size reductions but at the cost of **quality loss**.
Suitable for images, video, and sound where some quality loss is acceptable.
Example: grouping similar colours in a photograph reduces colour count while keeping visual quality.

Lossless Compression

**Lossless compression** encodes data to reduce size without losing information; it is **reversible**.
Produces smaller files than original but not as small as lossy.
Suitable for text/documents where quality must be preserved.
Uses algorithms like **Run-Length Encoding (RLE)** to find patterns and repetition.

Run-Length Encoding (RLE)

**RLE** compresses consecutive identical elements into a count and the value.
Example: `AAAABBBCCDAA` becomes `4A3B2C1D2A`.
Used in bitmap images to compress runs of the same colour (e.g., `5R3B` for 5 red, 3 blue pixels).

Data storage units hierarchy: bit → nibble → byte → KiB → MiB → GiB → TiB → PiB → EiB, each step ×1024 (except bit/byte ×8).

Practice questions

Free preview — 8 of 40 questions. Sign up to see them all.

1.Which of the following is the smallest unit of data storage?
Easy
- ABit
- BByte
- CNibble
- DKilobyte
2.How many bits are there in one byte?
Easy
- A4
- B8
- C16
- D1024
3.What is the value of 1 KiB (kibibyte) in bytes?
Medium
- A1000
- B1024
- C8
- D1024²
4.Convert 2 MiB to KiB.
Medium
- A2048
- B2000
- C1024
- D4096
5.A sound file has a sample rate of 44,100 Hz, a duration of 10 seconds, and a sample resolution of 16 bits. What is the file size in bytes?
Hard
- A882,000
- B705,600
- C7,056,000
- D44,100
6.Which formula correctly calculates the size of a bitmap image?
Medium
- AResolution × colour depth
- BWidth × height × sample rate
- CSample rate × duration × colour depth
- DWidth × height × duration
7.What is compression?
Easy
- AReducing the size of a file
- BIncreasing the quality of a file
- CConverting a file to a different format
- DEncrypting a file for security
8.Which type of compression is irreversible?
Easy
- ALossy
- BLossless
- CRun length encoding
- DAll compression

Slides

Units of Data Storage

Data is measured in bits (b), bytes (B), and larger units like KiB, MiB, GiB, TiB, PiB, EiB.
1 nibble = 4 bits, 1 byte = 8 bits.
Each larger unit is 1024 times the previous (binary prefixes).

How many bits are in 3 bytes?

Converting Between Units

To convert to a larger unit, divide by 1024 (e.g., bytes → KiB).
To convert to a smaller unit, multiply by 1024 (e.g., MiB → KiB).
For bits and bytes, multiply or divide by 8.

Convert 4 KiB to bytes: 4 × ____ = ____ bytes.

Calculating Bitmap Image File Size

Formula: Resolution × Colour Depth (or width × height × colour depth).
Resolution = width × height (in pixels).
Colour depth is the number of bits per pixel (e.g., 24 bits = 3 bytes).
Result is in bits; convert to bytes (/8) then to KiB (/1024).

A bitmap image has a resolution of 100 × 100 and a colour depth of 8 bits. What is its file size in bytes?

10,000 bytes
100,000 bytes
80,000 bytes
8,000 bytes

Calculating Sound File Size

Formula: Sample rate × Duration (seconds) × Sample resolution.
Sample rate: number of samples per second (Hz).
Sample resolution: bits per sample.
Result in bits; convert to bytes (/8) then to KiB (/1024).

A sound file has sample rate 44,100 Hz, duration 10 s, sample resolution 16 bits. Size = ____ × ____ × ____ = ____ bits.

What is Compression?

Compression reduces file size to save storage space.
Benefits: less storage required, less bandwidth needed, shorter transmission time.
Two types: lossy and lossless.

Which of these are benefits of compression?

Reduces file size
Increases quality
Requires less bandwidth to transmit
Slows down transmission

Lossy Compression

Data is permanently lost to reduce file size (irreversible).
Greatly reduces size but reduces quality.
Suitable for images, video, and sound where quality loss is acceptable.
Example: grouping similar colours in a photo.

Which of the following is true about lossy compression?

It is reversible
It is suitable for text documents
It permanently removes data
It does not reduce file size

Lossless Compression

Data is encoded to reduce size without losing any information (reversible).
Reduces size less than lossy, but quality is preserved.
Suitable for documents, spreadsheets, and any data where loss is unacceptable.
Example: Run Length Encoding (RLE).

Run Length Encoding (RLE) compresses 'AAAABBBCCDAA' to '____'.

Run Length Encoding (RLE)

RLE is a lossless compression method that replaces repeated values with a count and the value.
Used in bitmap images to compress sequences of same-colour pixels.
Example: 5 red pixels followed by 3 blue → '5R3B'.

Using RLE, how would you compress 'WWWWBBWWW'?

4W2B3W
4W2B3W
W4B2W3
4W2B3W

Lossy vs Lossless Summary

Lossy: irreversible, greater size reduction, quality loss, for images/video/audio.
Lossless: reversible, smaller size reduction, no quality loss, for documents/text.
Choose based on whether data loss is acceptable.

Which statements about lossless compression are true?

It is reversible
It permanently removes data
It is suitable for text documents
It reduces file size more than lossy

Unlock all 40 questions, slides & more

Create a free account to see every question, the slides, flashcards and revision notes for this topic.

Past papers

Past-paper practice for this topic is coming soon.

🗂️ Coming soon