How Sound Is Represented — Text, Sound and Images | IGCSE Computer Science

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 5% of your exam marks.

stable

Rare

Stable5%

ASCII/Unicode, sound sampling and pixel/colour depth appear regularly as 2 to 4 mark questions.

Sound in the real world is an analogue signal: a continuous wave of air pressure varying in time. A microphone turns the pressure into a continuous voltage. But a computer can only store digital values (discrete binary numbers), so the analogue signal has to be sampled before it can be stored.

Analogue-to-digital conversion

The process is called A/D conversion (or A2D):

The continuous analogue wave is measured at regular intervals. Each measurement is called a sample, and it records the wave's amplitude (height) at that moment.
Each sample's amplitude is rounded to the nearest value that can be stored in the available number of bits. The result is a binary number.
The binary numbers are stored on secondary storage in order.
To play the sound back, the binary numbers are read back and used to drive a speaker.

Two parameters control the quality of the digital recording: sample rate and sample resolution.

A smooth analogue sound wave with the sampled points marked at regular intervals along the time axis. A second panel shows the same wave sampled at a higher rate, with the staircase line that joins the samples appearing much closer to the original analogue curve

Sample rate

Sample rate is the number of samples taken per second, measured in hertz (Hz).

1 Hz means one sample per second. 1 kHz means 1000 samples per second.

Common sample rates:

8 kHz: telephone-quality voice. The reason phone calls used to sound thin and tinny.
22 kHz: low-quality compressed audio.
44.1 kHz: standard audio CD quality (44 100 samples per second).
48 kHz: standard for video soundtracks.
96 kHz or higher: studio recording.

The higher the sample rate, the more closely the digital version follows the original analogue wave, and the higher the maximum frequency that can be captured.

Sample resolution (bit depth)

Sample resolution (also called bit depth) is how many bits are allocated to record one sample.

With n bits per sample, there are 2ⁿ different amplitude levels that each sample can be rounded to:

8-bit audio has 256 amplitude levels per sample.
16-bit audio (the CD standard) has 65 536 levels.
24-bit audio (studio quality) has over 16 million levels.

A higher sample resolution means each sample is closer to the true amplitude of the original wave, so the recording sounds cleaner.

Effect on quality and file size

Parameter	If increased	Effect on quality	Effect on file size
Sample rate	More samples per second	Higher; captures higher frequencies	Larger
Sample resolution	More bits per sample	Higher; finer amplitude detail	Larger

Both improve quality but both also increase file size. Audio engineers choose the lowest values that give acceptable quality for the use case (phone calls can get away with 8 kHz; concert recordings need 44.1 kHz or higher).