Representation of sound

Digital storage of sound

Computers work in . All must be converted into binary in order for a computer to process it. Sound is no exception. To do this, sound is captured - usually by a microphone - and then converted into a digital signal.

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How computers process and represent sound

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An analogue-to-digital converter will capture a sound wave at regular time intervals. This is known as a .

For example, a sound wave like this can be sampled at each time sample point.

The sound recorded at each sample point is converted to its nearest numeric equivalent:

Sample	1	2	3	4	5	6	7	8	9	10
Denary	8	3	7	6	9	7	2	6	6	6
Binary	1000	0011	0111	0110	1001	0111	0010	0110	0110	0110

Sample	Denary
1	8
2	3
3	7
4	6
5	9
6	7
7	2
8	6
9	6
10	6

Sample	Binary
1	1000
2	0011
3	0111
4	0110
5	1001
6	0111
7	0010
8	0110
9	0110
10	0110

This data is then stored in a file for later use.

The effect of different sampling rates on the quality of sound

Sample rate

is the number of samples recorded in any given period of time. The higher the sample rate, the closer the recorded signal is to the original. Sample rate is measured in .

If the samples recorded above were plotted on a graph, the resulting representation of the sound wave would not be too accurate:

However, if the sample rate is doubled - twice as many samples in the same time period - the resulting representation would be closer:

The higher the sample rate, the larger the resulting file. As a result, sound files are often a compromise between quality and size of file. An audio file is usually recorded at 44.1 kilohertz (KHz). This is high enough for good sound quality while keeping file size down to sensible levels.

Bit depth

refers to the number of used to record each sample. Just as with images, the higher the bit depth, the more accurately a sound can be recorded, but the larger the file size. Typical bit depths are 16 bit and 24 bit.

Bit rate

is a measure of how much data is processed for each second of sound. Bit rate is calculated by:

Sample rate × bit depth

As with sample rate, the higher the bit rate, the better the quality of the recorded sound.

Metadata

Files contain extra data called . A sound file includes metadata about itself such as:

track number
bit rate
sampling rate

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