What are the units of data, and how do you calculate the size of an image or sound file?
The units of data (bit, nibble, byte, kilobyte, megabyte, gigabyte, terabyte), how images are stored as pixels (resolution and colour depth), how sound is sampled (sample rate and bit depth), and calculating file sizes.
An Eduqas GCSE Computer Science answer on the units of data, how images (resolution and colour depth) and sound (sample rate and bit depth) are represented in binary, and full worked calculations of image and sound file sizes.
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What this dot point is asking
Eduqas wants you to know the units of data and to calculate the size of an image (from its resolution and colour depth) and a sound file (from its sample rate, bit depth and duration). These are the main calculation questions on Component 1, and they are heavily method-marked, so the working matters as much as the answer.
The units of data
Representing and sizing images
Representing and sizing sound
Try this
Q1. State how many bits are in a byte. [1 mark]
- Cue. bits.
Q2. An image is by pixels with a colour depth of bits. Calculate the file size in bytes. [3 marks]
- Cue. bits, divided by bytes.
Q3. A -second clip is sampled at Hz with a bit depth of bits. Calculate the file size in bytes. [3 marks]
- Cue. bits, divided by bytes.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas Component 1, 20224 marksA bitmap image is 200 pixels wide by 150 pixels high and uses a colour depth of 4 bits per pixel. Calculate the file size in kilobytes. Show your working.Show worked answer →
Total pixels: 200 times 150 = 30000 pixels.
Total bits: 30000 times 4 = 120000 bits.
Convert to bytes: 120000 divided by 8 = 15000 bytes.
Convert to kilobytes: 15000 divided by 1000 = 15 KB (using 1 KB = 1000 bytes, as Eduqas accepts at GCSE).
Markers award one mark each for pixels, bits, bytes and the final KB. Carrying the wrong unit (leaving the answer in bits) loses the final mark.
Eduqas Component 1, 20234 marksA sound clip is recorded for 10 seconds at a sample rate of 8000 Hz with a bit depth of 8 bits. Calculate the file size in kilobytes, showing your working.Show worked answer →
Samples per second times bit depth gives bits per second: 8000 times 8 = 64000 bits per second.
Multiply by duration: 64000 times 10 = 640000 bits.
Convert to bytes: 640000 divided by 8 = 80000 bytes.
Convert to kilobytes: 80000 divided by 1000 = 80 KB.
One mark each for bits per second, total bits, bytes, and KB. The formula is sample rate times bit depth times seconds, all in bits, then divide to the unit asked for.
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Sources & how we know this
- WJEC Eduqas GCSE Computer Science specification (from 2016) — Eduqas (2020)