Sound representation (Cambridge International AS Computer Science)

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

• 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 bits and is recorded in stereo sound
• Using the graphic above to compare common sampling rates, the question, “Why does telephone hold music sound so bad?” can now be answered

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

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

Impact of sampling settings

What are the impacts of sampling settings?