How are images and sound represented digitally?
Understand how bitmap images are represented using pixels, colour depth and resolution, how analogue sound is sampled, and the effect of sample rate, resolution and metadata on quality and file size.
A focused answer to AQA A-Level Computer Science 4.5.10 and 4.5.11, covering bitmap images with pixels, colour depth and resolution, the sampling of analogue sound, and the effect of sample rate, resolution and metadata on quality and file size.
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What this dot point is asking
AQA wants you to explain how a bitmap image is represented with pixels, colour depth and resolution, how analogue sound is converted to digital by sampling, and how sample rate, sample resolution and metadata affect quality and file size, including the relevant calculations.
Representing images (bitmaps)
Higher resolution captures finer spatial detail (more pixels per object), while higher colour depth captures smoother colour gradients (more distinct shades). Both increase quality and file size, so a real format chooses a balance and usually applies compression on top of the raw pixel data. A common exam point is that doubling the resolution in both dimensions quadruples the pixel count (and so the file size), because area scales with the square of the linear dimension, whereas adding one bit of colour depth increases the size only by the ratio of the new to old bit count.
Representing sound (sampling)
Worked calculations
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksA bitmap image is 800 pixels wide by 600 pixels high and uses a colour depth of 24 bits. Calculate the file size of the image in mebibytes (MiB), ignoring metadata. Show your working.Show worked answer →
Number of pixels . Bits per pixel , so total bits bits.
Convert to bytes: bytes.
Convert to mebibytes: MiB.
Markers award marks for the pixel count, multiplying by the colour depth, dividing by 8 for bytes, and dividing by for MiB. Carrying an early slip correctly still earns the method marks.
AQA 20224 marksA 30 second mono sound clip is recorded at a sample rate of 44 100 Hz with a sample resolution of 16 bits. Calculate the file size in bits, then explain how the file size and quality change if the recording is made in stereo. Show your working.Show worked answer →
File size in bits bits.
Stereo records two channels, so the file size is multiplied by 2, giving bits (twice the size). Quality, in the sense of frequency range and amplitude precision per channel, is unchanged because sample rate and resolution are the same; the benefit of stereo is spatial separation of the sound, not higher fidelity per channel.
Markers reward the correct mono calculation, doubling for stereo, and recognising that the per-channel sample rate and resolution (so the captured quality) are unchanged.
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Sources & how we know this
- AQA A-level Computer Science (7517) specification — AQA (2015)