WJEC GCSE Digital Technology Data: representation, units, compression and storage

The Data content of WJEC GCSE Digital Technology Unit 1 explains how digital systems store everything as binary, how that data is measured and compressed, and where it is kept. This guide maps the whole topic and links to a focused answer page for each examinable point. Everything here is assessed in Unit 1, The Digital World, the on-screen examination.

How data is represented

At the lowest level, every file is a pattern of binary digits.

Binary

Computers use binary because their components have two states, on or off, mapping onto the two digits 0 and 1. A bit is one binary digit, a nibble is 4 bits, and a byte is 8 bits. With $n$ bits you can represent $2^n$ different values.

Images

A bitmap image is a grid of pixels, each storing one colour as a binary number. Resolution is the number of pixels (width times height); colour depth is the bits per pixel, giving $2^b$ colours. More of either improves quality but enlarges the file.

Sound

Real sound is analogue and must be sampled: its amplitude is measured at regular intervals and stored as numbers. Sample rate is samples per second; bit depth is the bits per sample. Higher values give a more faithful but larger recording.

Measuring and calculating

File-size calculations are the most reliable source of marks in this topic.

• Units. bit, byte, KB, MB, GB, TB, each roughly 1000 times the last (or 1024 if specified). Convert down by multiplying, up by dividing.
• Image size $=$ width $\times$ height $\times$ colour depth (bits), then $\div 8$ for bytes.
• Sound size $=$ sample rate $\times$ bit depth $\times$ seconds (bits), then $\div 8$ for bytes.
• Capacity questions convert both values to the same unit, then divide the capacity by the file size to find how many fit.

Compression

Compression makes files smaller so they use less storage and transfer faster.

Lossy compression permanently discards the least noticeable data: files shrink a lot but quality drops and the original cannot be recovered. It suits photos, music and video.

Lossless compression keeps every bit, so the original is rebuilt exactly, but the saving is smaller. It suits text, spreadsheets, databases and programs.

Storage media

You must compare four families of storage and recommend one for a scenario.

Choose by weighing capacity, speed, portability, durability and cost against what the scenario actually needs.

How to study the Data topic

1. Learn the units in order and the 1000-versus-1024 rule; conversions underlie every calculation.
2. Drill the two size formulae until automatic: image (width times height times depth) and sound (rate times depth times seconds), both then divided by 8.
3. Always show working and units. Method marks survive an arithmetic slip, and the unit is part of the answer.
4. Practise recommend-and-justify storage questions by matching qualities to needs.
5. Be precise on compression: lossy loses data permanently; lossless does not.

The Data dot points

Each examinable point has its own answer page with worked exam questions and cross-links:

• Representing data in binary
• Representing images
• Representing sound
• Measuring and storing data
• Data compression
• Storage devices and media

For the official specification

WJEC publishes the full Digital Technology specification, past papers and mark schemes at wjec.co.uk. Always revise from the current specification and WJEC's own past papers, because question style is board-specific.