Sound Representation (Cambridge (CIE) A Level Computer Science) : Revision Note

Sound encoding

How is sound encoded in a computer system?

• 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, analogue sound must be sampled and stored

Analogue sound

• Sound waves begin as analogue and for a computer system to understand them they must be converted into a digital form

• Measurements of the original sound wave are captured and stored as binary on secondary storage

• This process is called Analogue to Digital conversion (A2D)

• The process begins by measuring the loudness (amplitude) of the analogue sound wave at a point in time, this is called sampling

• The higher the amplitude, the louder the sound

• 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 (black line) the sampling rate, sampling resolution and sample interval can be changed

Sampling rate vs sampling resolution

Sampling rate

• The sampling rate is the amount of samples taken per second of the analogue wave

• Samples are taken for the duration of the sound

• The sampling rate is measured in Hertz (Hz)

• 1 Hertz is equal to 1 sample of the sound wave

• In the example above, the higher the sample rate, the closer to the original sound wave the digital version looks

Sampling resolution

• Sampling resolution is the number of bits used to represent each sound sample

• Sampling resolution is closely related to the bit depth of a bitmap image, they measure the same thing in different contexts

• The sample rate of a typical audio CD is 44.1kHz (44,100 Hertz or 44,100 samples per second), a sampling resolution of 16 and is recorded in stereo sound

• Using the graphic above to compare common sampling resolutions, the question, “Why does telephone hold music sound so bad?” can now be answered

Impact of sampling settings

What are the impacts of sampling settings?