Representing Sound (AQA GCSE Computer Science)
Revision Note
Written by: Robert Hampton
Reviewed by: James Woodhouse
How Sound is Sampled & Stored
Computers represent all data in binary, including sound that we record using a microphone (input) or sound that we playback from a speaker (output)
For this to happen, sound must be sampled and stored
How is sound sampled & stored?
Measurements of the original sound wave are captured and stored as binary on secondary storage
Sound waves begin as analogue and for a computer system to understand them they must be converted into a digital form
This process is called Analogue to Digital conversion (A2D)
The process begins by measuring the amplitude of the analogue sound wave at a point in time, called samples
Each measurement (sample) generates a value which can be represented in binary and stored
Using the samples, a computer is able to create a digital version of the original analogue wave
The digital wave is stored on secondary storage and can be played back at any time by reversing the process
In this example, the grey line represents the digital wave that has been created by taking samples of the original analogue wave
In order for the digital wave to look more like the analogue wave the sample rate and bit depth can be changed
Sample Rate, Duration & Sample Resolution
What is sample rate?
Sample rate is the amount of samples taken per second of the analogue wave
Samples are taken each second for the duration of the sound
The sample rate is measured in Hertz (Hz)
1 Hertz is equal to 1 sample of the sound wave
In this example you can see that the higher the sample rate, the closer to the original sound wave the digital version looks
The sampling rate of a typical audio CD is 44.1kHz (44,100 Hertz or 44,100 samples per second)
Using the graphic above helps to answer the question, “Why does telephone hold music sound so bad?”
What is sample resolution?
Sample resolution is the number of bits stored per sample of sound
Sample resolution is closely related to the colour depth of a bitmap image, they measure the same thing in different contexts
What effect do sample rate and sample resolution have?
| Sample rate | Sample resolution | ||
---|---|---|---|---|
| High | Low | High | Low |
Playback quality | ⇑ | ⇓ | ⇑ | ⇓ |
File size | ⇑ | ⇓ | ⇑ | ⇓ |
Calculate Sound File Sizes
How do you calculate the size of a sound file?
Calculating the size of a sound file is carried out with the following formula:
Sample rate x duration x sample resolution
Example
Sound Files | |||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
(Sample Rate) x (Duration in seconds) x (Sample Resolution) | |||||||||||||||
|
Worked Example
A student records a podcast about computer science for a school project.
Describe how an analogue sound wave is converted into digital form [3]
Answer
1 mark per bullet to max 3
(analogue) sound wave is sampled
...amplitude is measured
...at a point in time
Each sample/measurement is stored as a binary value
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