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

11 File Management

Objective

Know and understand why file sizes need to be reduced for storage or transmission, and be able to apply the appropriate techniques required by the Cambridge IGCSE ICT (0417) syllabus.

Why Reduce File Sizes?

• Limited storage on USB sticks, memory cards, smartphones and cloud accounts.
• Faster transmission over networks, especially where bandwidth is restricted.
• Lower data‑transfer costs (mobile data, ISP caps).
• Improved performance of applications that open or process large files.

Key Concepts

1. File size – measured in bytes (B), kilobytes (KB), megabytes (MB), gigabytes (GB).
2. Compression – reducing the amount of data required to represent a file.
3. Lossless compression – no data is lost; the original file can be restored exactly.
4. Lossy compression – some data is permanently discarded to achieve a greater size reduction; the original quality cannot be fully recovered.
5. File‑extension conventions – extensions are normally lower‑case (e.g. .docx), indicate the file type, and may be hidden by default in some operating systems.
6. Generic vs. specific file formats
   • Specific formats are tied to a particular application (e.g. .docx, .xlsx, .pptx).
   • Generic formats can be opened by many applications and are often used for sharing (e.g. .pdf, .txt, .csv, .rtf).

Mandatory File Formats (Cambridge IGCSE ICT 0417)

Methods of Reducing File Size

1. Changing the File Format

• Different formats use different compression algorithms.
• Examples:
  • Save a document as .pdf instead of .docx when only a read‑only version is needed.
  • Save photographs as .jpg rather than .png when a small size is more important than lossless quality.
  • Group several files into a .zip archive (preferred for the exam – universally supported).

2. Adjusting Image Resolution

Resolution = width × height (pixels). An estimate of the uncompressed size is:

Size (bytes) = Width × Height × (Colour‑depth ÷ 8)

• DPI (dots per inch) affects print size, not the raw file size; for web use 72 dpi is typical.

Reducing width and/or height lowers the pixel count and therefore the file size. A common web‑friendly size is 800 × 600 px.

3. Reducing Colour Depth

• Colour depth = bits per pixel.
• Typical values:
  • 24‑bit (true colour) – ~16.7 million colours.
  • 8‑bit – 256 colours.
  • 4‑bit – 16 colours.
• Changing from 24‑bit to 8‑bit can cut the file size to roughly one‑third, especially for simple graphics such as logos.

4. Using Compression Tools

Common utilities: 7‑Zip, WinZip, WinRAR, or the built‑in OS “Compress” function.

Exam‑friendly steps (Maximum/Best compression):

1. Select the file(s) or folder.
2. Right‑click → Compress or Add to archive.
3. Choose the Maximum (or Best) compression level.
4. Name the archive clearly, e.g. Project1.zip.
5. Write your centre, candidate number and name on the archive (or on the accompanying cover sheet) – a requirement for the practical papers.

Why .zip rather than .rar? The exam marks the technique, not the brand; .zip works on all operating systems.

5. Trimming Unnecessary Data

• Delete unused worksheets, hidden rows/columns, or empty pages.
• Remove embedded objects (large pictures, videos) that are not required.
• Use “Save As” to create a fresh copy after cleaning up the file.

6. Removing Metadata

Metadata (author, revision history, EXIF data, etc.) can add several kilobytes.

How to remove it (common applications):

• Word / Excel / PowerPoint: File → Info → Check for Issues → Remove Properties & Personal Information.
• Images (Windows): Right‑click → Properties → Details → Remove Properties and Personal Information.
• Images (macOS): Open in Preview → Tools → Show Inspector → More Info → Remove Metadata.

Impact on Storage and Transmission

Example: a 5 MB video sent over a 1 Mbps connection.

Time without compression:

Time (seconds) = (File size × 8) ÷ Bandwidth

$$\frac{5 \times 8 \times 10^{6}}{1 \times 10^{6}} = 40\text{ s}$$

If compressed to 0.5 MB (90 % reduction):

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

Note: real‑world transfers include protocol overhead, so actual times are slightly longer.

Choosing Between Lossless and Lossy Compression

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

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.
5. Create a .zip archive of the three images, using the “Maximum” compression level, and name it Centre‑C123‑Name‑Images.zip (replace with your details).

Suggested Diagram

Summary Checklist

• Identify when storage space or bandwidth is limited.
• Choose an appropriate file format (specific or generic) that offers built‑in compression.
• Adjust image resolution and colour depth where visual quality can be compromised.
• Use a compression tool with the Maximum/Best setting; prefer .zip for exam tasks.
• Remove unnecessary data and metadata to shave off extra kilobytes.
• Select lossless compression for data that must remain unchanged; use lossy compression only when a small quality loss is acceptable.
• When creating an archive for a practical exam, label it with your centre, candidate number and name.