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Telephony and voice communication was originally primarily analog in nature, as was most television and radio transmission. Early telecommunication devices utilized analog-to-digital conversion devices called modulator/demodulators, or modems , to convert analog signals to digital signals and back.
Data transmission, digital transmission or digital communications is the transfer of data over a point-to-point or point-to-multipoint communication channel. Examples of such channels include copper wires, optical fibers, wireless communication channels, storage media and computer buses.
Transmission of a digital message, or of a digitized analog signal, is known as data transmission. Examples of transmission are the sending of signals with limited duration, for example, a block or packet of data, a phone call, or an email.
An analog signal is any continuous signal for which the time-varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. For example, in an analog audio signal, the instantaneous voltage of the signal varies continuously with the sound pressure.
Since each tone (i.e., symbol) represents a message consisting of two digital bits in this example, the bit rate is twice the symbol rate, i.e. 2000 bits per second. According to one definition of digital signal, [3] the modulated signal is a digital signal. According to another definition, the modulation is a form of digital-to-analog conversion.
Such a carrier-modulated sine wave is considered a digital signal in literature on digital communications and data transmission, [11] but considered as a bit stream converted to an analog signal in electronics and computer networking. [12] In communications, sources of interference are usually present, and noise is frequently a significant problem.
Analog discrete-time signal processing is a technology based on electronic devices such as sample and hold circuits, analog time-division multiplexers, analog delay lines and analog feedback shift registers. This technology was a predecessor of digital signal processing (see below), and is still used in advanced processing of gigahertz signals.
ADCs are required in digital signal processing systems that process, store, or transport virtually any analog signal in digital form. TV tuner cards, for example, use fast video analog-to-digital converters. Slow on-chip 8-, 10-, 12-, or 16-bit analog-to-digital converters are common in microcontrollers.