Sound
How sound can be sampled and stored in digital form
Computers work in A number system that contains two symbols, 0 and 1. Also known as base 2.. All Units of information. In computing there can be different data types, including integers, characters and Boolean. Data is often acted on by instructions. 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.
How computers process and represent sound
An analogue-to-digital converter will capture a sound wave at regular time intervals. This recording is known as a A digitally recorded fragment of sound, taken from an existing track or sound environment..
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 sample rate, duration and bit depth on the playback quality and size of a sound file
How many samples of data are taken per second. This is normally measured in hertz, eg an audio file usually uses samples of 44.1 kHz (44,100 audio samples per second). 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 The unit of frequency, symbol 'Hz'. 1 Hz is 1 wave or cycle per second.(Hz).
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:
However, 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
The number of bits available to store an audio sample. refers to the number of The smallest unit of data in computing represented by a 1 in binary. 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
In computing, the number of bits processed per second. is simply a measure of how much data is processed for each second of sound.
Bit rate = sample rate x bit depth
As with sample rate, the higher the bit rate, the better quality of the recorded sound.
File size
File size depends on the How many samples of data are taken per second. This is normally measured in hertz, eg an audio file usually uses samples of 44.1 kHz (44,100 audio samples per second). (samples per second), sample resolution (bits per sample), length (in seconds) and the number of channels (1 for mono, 2 for stereo.)
File size = sample rate x sample resolution x time x number of channels.
Example
A 10-second stereo clip sampled at 44,100 Hz using 16-bit resolution:
10 x 2 x 44,100 x 16 = 14,112,000 bits
To work out the file size in bytes, divide the answer by 8, as there are 8 bits in a byte.
Therefore, 14,112,000 / 8 = 1,764,000 bytes or 1.76 MB.
