Data Storage and Compression

Data Representation · 4 question types

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 4% of your exam marks.

stable

Rare

Stable4%

File size calculations and lossless vs lossy compression are regular 3 to 4 mark questions.

reduces file size by permanently removing data that the user is unlikely to notice. The original cannot be perfectly reconstructed.

Key features:

Data is permanently lost. The original file can never be exactly recovered.
The reduction in size is much greater than with lossless (often 10:1 or higher).
It is the right choice when small size matters more than perfect fidelity: streaming media, web previews, email attachments, social media images.

Lossy methods take advantage of the limits of human perception:

For sound:

Reduce the sample rate so high frequencies above human hearing are no longer captured.
Reduce the sample resolution so each sample uses fewer bits.
Use that removes sounds the human ear cannot hear (very quiet sounds drowned out by louder ones nearby).

For images:

Reduce the colour depth so similar shades are merged into one code.
Reduce the resolution so several adjacent pixels are merged.
Group visually similar colours together so the image stores fewer distinct colour values.

For video:

Apply image lossy methods to each frame.
Store only differences between successive frames rather than each frame separately.