Know and understand the need to reduce file sizes for storage or transmission

11 File Management

Objective

Know and understand the need to reduce file sizes for storage or transmission.

Why Reduce File Sizes?

• Limited storage capacity on devices (e.g., USB sticks, memory cards, cloud quotas).
• Faster transmission over networks – especially where bandwidth is restricted.
• Reduced costs for data transfer (e.g., mobile data, ISP data caps).
• Improved performance of applications that need to open or process files.

Key Concepts

1. File size – measured in bytes (B), kilobytes (KB), megabytes (MB), gigabytes (GB).
2. Compression – process of reducing the amount of data needed to represent a file.
3. Lossless compression – no data is lost; original file can be perfectly restored.
4. Lossy compression – some data is discarded to achieve greater size reduction; original quality cannot be fully restored.

Methods of Reducing File Size

1. Changing File Format

Different formats use different compression algorithms. Choosing an appropriate format can dramatically affect size.

2. Adjusting Image Resolution

Resolution is the number of pixels in an image, expressed as width × height.

File size (uncompressed) can be estimated by:

\$\text{Size (bytes)} = \text{Width} \times \text{Height} \times \text{Colour depth (bits)} \div 8\$

Reducing width and height reduces the total number of pixels and therefore the file size.

3. Reducing Colour Depth

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

• 24‑bit (true colour) – 16.7 million colours.
• 8‑bit – 256 colours.
• 4‑bit – 16 colours.

Lowering colour depth can halve or quarter the file size, especially for simple graphics.

4. Using Compression Tools

Software such as WinZip, 7‑Zip, WinRAR, or built‑in OS utilities can compress files or folders.

Typical steps:

1. Select the file(s) or folder.
2. Choose “Compress” or “Add to archive”.
3. Select compression level (e.g., “Maximum” for smallest size).
4. Save the new archive (usually .zip or .rar).

5. Trimming Unnecessary Data

• Delete unused worksheets, hidden rows/columns in spreadsheets.
• Remove embedded objects (e.g., large pictures in documents).
• Clear metadata and revision history where not required.

Impact on Storage and Transmission

Consider a 5 MB video file to be sent over a 1 Mbps (megabit per second) connection.

First calculate the time required without compression:

\$\text{Time (seconds)} = \frac{\text{File size (bits)}}{\text{Bandwidth (bits/s)}} = \frac{5 \times 8 \times 10^6}{1 \times 10^6} = 40\text{ s}\$

If the file is compressed to 0.5 MB (90 % reduction), the transmission time becomes:

\$\frac{0.5 \times 8 \times 10^6}{1 \times 10^6} = 4\text{ s}\$

This illustrates how reducing file size speeds up transmission and saves bandwidth.

Choosing Between Lossless and Lossy Compression

• Lossless is essential when data integrity must be preserved (e.g., text documents, spreadsheets, source code, archival images).
• Lossy is acceptable when a small loss of quality does not affect the purpose (e.g., web photos, streaming video, audio podcasts).

Practical Activities

1. Take a high‑resolution photograph (e.g., 4000 × 3000 px, 24‑bit). Record its original size.
2. Save the image as JPG with quality settings 90 %, 70 %, and 50 %. Record each new size.
3. Calculate the percentage reduction for each setting.
4. Discuss which quality level is acceptable for a school website versus a printed poster.

Summary Checklist

• Identify when storage space or bandwidth is limited.
• Select an appropriate file format that offers built‑in compression.
• Adjust resolution and colour depth where visual quality can be compromised.
• Use compression tools to create archives for multiple files.
• Choose lossless compression for data that must remain unchanged; use lossy compression for media where small quality loss is acceptable.