Analog Signals and Digital Signals

Because most physical quantities can be described by measurements, and because any measurement can be represented by a sufficiently long series of 0s and 1s, it is possible to transfer some of the information in any analog signal to a digital signal. In fact, according to the Nyquist sampling theorem, sufficiently precise measurements of an analog waveform made at twice or more the maximum frequency present in that waveform will preserve all its information. Digital signals can, conversely, be converted to analog signals by using them as inputs to a device whose output shifts smoothly between a series of voltages (or other physical states) as directed by a changing set of input bits. Analog-to-digital conversion is performed, for example, when a digital compact disc (CD) is recorded, and digital-to-analog conversion is performed when a CD is played back over an audio system.