Vibrations in the air are perceptible as sound but when stored on a phonographic disc they are physical ups and downs on a vinyl track. This physical representation of sound (phono), using mechanical graphical techniques, lead to the word phonograph coming into popular use. In sound reproduction the process has to be reversed to enable the ups and downs stored on the phonographic disc to be transformed back into sound waves. Three principal media have been developed for sound recording and reproduction: - Mechanical systems (phonographic disc)
- Magnetic systems (such as cassette tape)
- Optical systems (such as CD and DVD) | 
