3.7 · File Formats

Goal: compare the common file formats listed in the C&A Guide and pick the right one for a use case.

Why "format" matters

A file format is a convention for arranging bytes so software can read them back. Same data, different formats → very different sizes, qualities and capabilities.

Formats listed in the C&A Guide

Category	Formats
Image	bmp, png, jpg
Audio	wav, mp3
Video	avi, mpeg4 (mp4)
Document / text	txt, docx, odt, pdf

You should be able to compare them on:

• compression (lossless / lossy / none)
• typical file size
• typical use case
• editability vs portability

Image formats

Format	Compression	Typical size	Best for	Notes
BMP	None (or RLE)	Largest	Legacy Windows, simple uncompressed storage	Wastes space on modern web
PNG	Lossless	Medium	Icons, screenshots, logos, transparency	Supports alpha channel
JPG (JPEG)	Lossy	Smallest	Photographs	Don't re-save repeatedly (quality degrades each time)
GIF (not on syllabus but commonly seen)	Lossless, 256-colour palette	Small	Simple animations	Largely replaced by MP4

Quick rule

• Photo of real life → JPG.
• Diagram, screenshot, icon, transparency → PNG.
• Archival / forensic → BMP or TIFF.

Audio formats

Format	Compression	Typical size (3 min)	Best for	Notes
WAV	None	~30 MB	Recording masters, short SFX	High quality, large
MP3	Lossy	~3 MB	Music distribution, podcasts	Universal compatibility
AAC (in MP4 container, not in syllabus list)	Lossy	~3 MB	Apple ecosystem, YouTube	Slightly better than MP3 at same bit rate
FLAC (not on syllabus)	Lossless	~15 MB	Audiophile downloads	Smaller than WAV, no quality loss

Quick rule

• Recording masters → WAV.
• Sharing / streaming → MP3.

Video formats

Format	Compression	Typical size (1 min, 1080p)	Best for	Notes
AVI	Container — can hold many codecs	Varies; often large	Legacy Windows video	Old, no longer recommended
MPEG-4 / MP4	Lossy (H.264, H.265, AV1)	~50–200 MB	Web streaming, mobile playback	The de-facto standard

Quick rule

• Today → MP4 (with H.264 or H.265 codec).

Document formats

Format	Editable?	Layout fidelity	Best for	Notes
TXT	Yes	No formatting	Plain text, code, logs	Smallest, universally readable
DOCX	Yes (Word)	Good	Microsoft Word documents	Proprietary but open spec
ODT	Yes (LibreOffice / OpenOffice)	Good	Open document standard	Open, royalty-free
PDF	Limited (without special tools)	Excellent	Sharing for viewing/printing	Looks identical on any device

Quick rule

• Sharing with formatting preserved → PDF.
• Editing collaboratively → DOCX or ODT.
• Code / config / logs → TXT.

Decision flowcharts

"What format should I save this image as?"

Is it a photograph?               → JPG
Does it need transparency?        → PNG
Is it a screenshot or icon?       → PNG
Is it a forensic archive?         → BMP or TIFF

"What format should I send this document as?"

Recipient must edit?              → DOCX (Microsoft) / ODT (Open)
Recipient must only view?         → PDF
Recipient is a programmer?        → TXT (or Markdown)

Container vs codec (good to know)

• A container is the file wrapper (e.g. .mp4, .avi, .mkv).
• A codec is the algorithm that compresses / decompresses the audio or video stream (H.264, H.265, AAC, MP3).
• An .mp4 file can contain H.264 video + AAC audio, OR H.265 video + Opus audio, etc.

The HKEAA mostly cares about containers (file extensions).

File-size comparison example

A typical short school promotional clip — 30 s, 720p, stereo audio:

Format / encoding	Approximate size
Uncompressed AVI	~2.5 GB
MP4 (H.264)	~30 MB
MP4 (H.265)	~15 MB
Old MPEG-1	~50 MB

The MP4 (H.265) is ~150× smaller than uncompressed — same content, smarter format.

Common student mistakes

• Calling MP4 lossless — it is lossy.
• Saving a photo as PNG and being surprised it is larger than JPG.
• Editing a JPG many times and not realising quality degrades each save.
• Calling DOCX an open format — it is proprietary but standardised; ODT is the open one.

Practice activity

Choose the best format for each:

1. A 200-page lecture notes you must publish for download — clear layout, no editing required.
2. A team logo with a transparent background for the school website.
3. A 6-second meme to share in a group chat (modern phones).
4. A recording master of the school choir for the music teacher's archive.
5. Source code for an SBA submission.

Suggested

1. PDF · 2. PNG · 3. MP4 (or animated GIF, though MP4 is smaller) · 4. WAV (or FLAC) · 5. TXT (or .py/.html etc, plain text)

Exam-style question

Q (5 marks): A school is preparing a digital prospectus to be downloaded from its website. The prospectus contains photographs of students, the school logo with a transparent background, and 30-page formatted text.

Recommend the most suitable file format for each, justifying your choice.

Sample answer:

• Photographs → JPG. Photographs have continuous tones; JPG's lossy compression dramatically reduces file size with imperceptible quality loss, important for fast download from a website.
• School logo with transparent background → PNG. PNG supports transparency (alpha channel) and uses lossless compression, preserving the crisp edges of the logo.
• Formatted text (30 pages) → PDF. PDF preserves layout, fonts and images across all devices and operating systems, making it ideal for distribution.

Key takeaways

• The C&A Guide names: bmp, png, jpg, wav, mp3, avi, mpeg4, txt, docx, odt, pdf.
• Match format to use case: photo → JPG, screenshot → PNG, video → MP4, viewable doc → PDF.
• Lossy formats trade quality for size; lossless formats preserve every bit.

➡️ Next chapter: 4 · Data Manipulation & Analysis