How is analogue sound converted into binary, and how do sample rate and bit depth affect quality and file size?
Representing sound in binary by sampling an analogue wave, the meaning of sample rate and sample resolution (bit depth), and how they affect sound quality and file size.
A focused answer to the WJEC GCSE Computer Science Unit 1 content on representing sound, covering analogue to digital conversion by sampling, sample rate and sample resolution (bit depth), how each affects sound quality and file size, and calculating the file size of a sound recording.
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What this topic is asking
WJEC wants you to know how analogue sound is turned into binary by sampling, what sample rate and sample resolution (bit depth) mean, how each affects quality and file size, and how to calculate a sound file's size. This is part of the Data representation and data types content in Unit 1 of WJEC GCSE Computer Science (3500).
Analogue sound and sampling
Sample rate
Sample resolution (bit depth)
Calculating file size
The uncompressed file size of a recording, in bits, is the sample rate multiplied by the duration in seconds multiplied by the bit depth (and by the number of channels for stereo, though questions usually use one channel).
Quality versus file size
Both a higher sample rate and a higher bit depth improve the quality of a recording but increase its file size, so designers choose values that balance acceptable quality against storage and bandwidth. This is also why sound is often compressed (for example to MP3) to make files smaller for streaming or storage.
Try this
Q1. State what is meant by the sample rate of a sound recording. [1 mark]
- Cue. The number of samples (amplitude measurements) taken each second.
Q2. A -second sound is sampled at with a bit depth of bits. Calculate the file size in bytes. [2 marks]
- Cue. bits bytes.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC-style Unit 13 marksA sound is recorded for seconds at a sample rate of samples per second using a sample resolution of bits. Calculate the file size in bytes. Show your working.Show worked answer →
A Unit 1 sound file-size calculation. Total samples (1 mark). Bits bits (1 mark). Bytes bytes (1 mark). Markers reward sample rate times duration times bit depth, then converting to bytes. A common error is to forget the duration or to leave the answer in bits rather than dividing by .
WJEC-style Unit 14 marksExplain how analogue sound is converted into a digital form, and describe how increasing the sample rate affects sound quality and file size.Show worked answer →
A Unit 1 explain question. Sound is an analogue wave, so it is converted to digital by sampling: the height (amplitude) of the wave is measured at regular intervals and each measurement is stored as a binary number (1 mark for sampling, 1 mark for recording amplitude as binary). The sample rate is the number of samples taken per second; taking more samples per second captures the wave more accurately, so a higher sample rate gives a more faithful, higher-quality recording (1 mark). However, more samples means more values to store, so a higher sample rate also increases the file size (1 mark). Markers reward sampling, the definition of sample rate and the trade-off between quality and file size. A common error is to confuse sample rate with sample resolution.
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