Sound

Sound, like images, must first be digitised before it can be stored and processed by a computer system. At any one instant the amplitude (loudness) of a sound can be sampled by a microphone to produce an electrical signal that can be converted to a number by an analogue to digital converter. If the sampling rate is high enough then a reproduction of the original sound can be created by producing a series of sounds that follow the same amplitude pattern as the original. This is, in many ways, similar to recreating moving pictures by showing a rapid succession of still pictures.

Increasing the sampling rate will produce a more accurate representation of the original sound but will require faster and more expensive A to D converters and processing. The resulting sound will also take up more storage space since more data will be produced for the same sound sequence.

As well as recording sounds, computer systems can be used to create new sounds. A sound is represented by its loudness, its pitch and by the mixture of extra, unheard frequencies that provide the particular quality of the sound that you hear. It is these additional frequencies, called overtones combined with the way the note starts up and dies down that make a guitar sound different from a piano even when they are playing the same note. A computer system can be used to produce notes which have a particular mix of frequencies and which start up and die down in a way that mimics an existing instrument or which creates a previously unheard one. This is the function of a synthesiser.

Whether a computer system is creating a new sound or playing back a previously recorded one, digital to analogue converters will be needed to turn the stored digital data into the analogue electrical signal that can drive a loud speaker.

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