Representing Sound (Edexcel GCSE Computer Science)

How Analogue Sound is Represented

• 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

How is analogue 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 loudness (amplitude) of the analogue sound wave at a point in time, these are called samples

• 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 sample rate, bit depth and sample interval can be changed

Sample Rate, Bit Depth & Sample Interval

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

What is sample interval?

• Sample interval is the amount of time between samples being taken

• Sample interval is the direct opposite of sample rate, when one is high the other is low and vice versa

• For example, to increase the recording quality of a digital recording we increase the sample rate, which reduces the sample interval

• If the sample interval is increased the sample rate is reduced, thus making the quality worse

What is bit depth?

• Bit depth is the number of bits used to represent each sound sample

• Bit depth is closely related to the colour 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 bit depth of 16 and is recorded in stereo sound

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

What effect do sample rate, bit depth & sample interval have?