Representing sound

Seite 1

Representing sound

Resources - Learn more about this topic:

https://www.bbc.co.uk/education/guides/zpfdwmn/revision/3 (basic level)

https://teachwithict.weebly.com/binary-representation-of-sound.html (intermediate level)

https://www.bbc.co.uk/education/guides/z7vc7ty/revision/1 (advanvced level)

Analogue sound

Analogue Sound - Vibration of the air that travels as sound waves

Sound is vibrations travelling through a medium (normally, air).

Analogue sound changes constantly.

Amplitude - The height of a sound wave

The higher is the amplitude, the louder is the sound.

The amplitude of an analogue sound changes constantly over time, and can have any value between its minumum and maximum value.

This means that in an analogue sound, there are infinite instants of time to consider, and infinite possible values for the amplitude.

A computer has limited memory and storage, therefore it cannot deal with infinite values - it would run out of memory.

This is why a sound must be digitized in order to be processed by and stored on a computer system.

Sound digitization transforms an analogue sound, into a digital sound.

Frequency - How fast the sound waves changes in a second. It is measured in Hertz (Hz)

The higher is the sound frequency, the higher is the pitch.

Frequency is measured in Hertz (Hz) - how many times the sound wave "goes up and down" in a second.

The voice of an adult male has a basic frequency of around 150 Hz

The voice of an adult female has a basic frequency of around 300 Hz

Sound digitization

Sound digitization - The process of converting analogue sound into digital sound

The analogue sound's amplitude is measured at regular intervals, not constantly.

Each measurement, called a sample, is stored as a binary number.

Amplitude measurements are approximated, only a limited number of values, not all, are possible.

Each sample is stored in the sound file as a binary number of fixed length (i.e. a fixed number of bits for each sample)

Sample - A single measurement of sound's amplitude

Each sample is stored as a binary number of fixed length (i.e. a fixed number of bits).

Sampling rate - The number of samples taken in one second. Measured in Hertz (Hz)

The higher is the sampling rate, the better quality is the digital sound.

The higher is the sampling rate, the larger is the sound file size.

Typical values:

44.1KHz (44.1 thousand samples per second, CD quality)

8 KHz (8 thousand samples per second, telehone quality)

Bit depth - The number of bits used for each sample

The higher is the bit depth, the better quality is the digital sound.

The higher is the bit depth, the larger is the sound file size.

Typical values:

8 bits (256 possible values, low-quality)

16 bits (65536 possible values, CD quality)

Bit rate - The number of bits used for one second of digital sound

Can be calculated as:

SamplingRate x BitDepth x NumnerOfChannels (1 channels is a 'mono' sound, 2 channels is a 'stereo' sound)

High bit rate gives better sound quality.

High bit rate means larger sound file size.

Digital sound

Digital Sound - A sound wave represented as binary numbers

Digital sound can be processed by a computer.

Digital sound is stored on a computer system as a sound file.

A digital sound is obtained by digitizing an analogue sound.

A digital sound is a long list of binary numbers.

Audio file - A computer file that stores a digital sound

There are many types of sound files.

Some common examples are:

WAV - Type of high-quality sound file used in Windows systems

AIFF - Type of high-quality sound file used in Mac systems

AAC - Type of compressed, low-quality sound file used in mobile phones

MP3 - Type of compressed, low-quality sound file used on any system