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Digital communications, including digital transmission and digital reception, is the transfer of either a digitized analog signal or a born-digital bitstream. [1]
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.
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.
Digital modulation schemes are possible because the transmitter-receiver pair has prior knowledge of how data is encoded and represented in the communications system. In all digital communication systems, both the modulator at the transmitter and the demodulator at the receiver are structured so that they perform inverse operations.
Phase modulation (analog PM) and phase-shift keying (digital PSK) can be regarded as a special case of QAM, where the amplitude of the transmitted signal is a constant, but its phase varies. This can also be extended to frequency modulation (FM) and frequency-shift keying (FSK), for these can be regarded as a special case of phase modulation ...
Calculated digital samples are converted to voltages with a digital-to-analog converter, typically at a frequency less than the desired RF-output frequency. The analog signal must then be shifted in frequency and linearly amplified to the desired frequency and power level (linear amplification must be used to prevent modulation distortion). [9]
Mixed-signal ICs contain both digital and analog circuitry on the same chip, and sometimes embedded software. Mixed-signal ICs process both analog and digital signals together. For example, an analog-to-digital converter (ADC) is a typical mixed-signal circuit.
Analogue electronics (American English: analog electronics) are electronic systems with a continuously variable signal, in contrast to digital electronics where signals usually take only two levels. The term analogue describes the proportional relationship between a signal and a voltage or current that represents the signal.