Representing Sound (OCR GCSE Computer Science)

• 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)
• It may be useful to understand the science of ‘Sound Waves in the Ear’ to help with this concept
• The process begins by measuring the height (amplitude) of the analogue sound wave, these are 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

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

• 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 bit depth?

• Bit depth is the number of bits stored per sample of sound
• Bit depth 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 bit depth have?

Factor	Effect on playback quality	Effect on file size
Sample rate	⬆️higher = more detail, better sound quality	⬆️higher = more data, larger file size
Bit depth	⬆️higher = bigger range, better sound quality	⬆️higher = more data per sample, larger file size