Representing Sound (AQA GCSE Computer Science)

Revision Note

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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

analogue-to-digital-image
  • 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

image2---sample-rate-comparisons

In this example you can see that the higher the sample rate, the closer to the original sound wave the digital version looks

image-of-sample-rate-comparisons
  • 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) (Sample Resolution)

Size of sound file = 

 

 

Sample rate

10

Samples per second

Duration

30

Seconds

Sample resolution

4

Number of bits stored per sample

10 x 30 x 4

=

1200 bits = 150 bytes

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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