Representing Sound (Cambridge (CIE) O Level Computer Science): Revision Note

How Sound is Sampled & 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 & 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?