A digital signal is a type of signal that can take on a set of discrete values (a quantized signal).〔(Digital Design with CPLD Applications and VHDL By Robert K. Dueck )〕 A logic signal is a digital signal with only two possible values, that can represent an arbitrary bit stream.〔()〕〔Art of Electronics, Horowitz and Hill.〕 Other types of digital signals can represent three-valued logic or higher valued logics.
Digital signals can represent a discrete set of values using any discrete set of waveforms. Digital signals can be optical, electrical, acoustic, or others. Digital signals are present in all digital electronics, notably computing equipment and telecommunications.
Although digital signals are strictly a subset of analog signals, they are created and processed differently. With digital signals, system noise, provided it is not too great, will not change the quantification whereas with signals handled using analog processing, noise always degrades the operation to some degree.
In digital signal processing, digital signals are often described by an abstraction where the signal's value is sampled at regular intervals. These values can be used to describe and process analog as well as digital signals.
The term ''digital signal'' has related definitions in different contexts:
# In digital electronics a digital signal is a pulse train (a pulse amplitude modulated), i.e. a sequence of fixed-width square-wave electrical pulses or light pulses, each occupying one of a discrete number of levels of amplitude.〔B. SOMANATHAN NAIR, (Digital electronics and logic design ) 2002: "Digital signals are fixed-width pulses, which occupy only one of two levels of amplitude."〕〔Joseph Migga Kizza, (Computer Network Security ) 2005〕 A special case is a logic signal or a binary signal, which varies between a low and a high signal level.
# In digital communications and computer networking, a digital signal is a continuous-time signal, alternating between a discrete number of waveforms (the shape of the waveforms depend on the line coding).〔( Analogue and Digital Communication Techniques ): "A digital signal is a complex waveform and can be defined as a discrete waveform having a finite set of levels"〕 In this context, it may either be:
## baseband transmission, used in serial communications, (often but not always fulfulling definition 1 above), or
## passband transmission, also known as digital carrier wave modulation〔J.S.Chitode, (Communication Systems ), 2008: "When a digital signal is transmitted over a long distance, it needs CW modulation."〕
# In digital signal processing, a digital signal is an abstract signal that is considered discrete in time and amplitude, i.e. a sequence of codes, 〔Vinod Kumar Khanna, (Digital Signal Processing ), 2009: A digital signal is a special form of discrete-time signal which is discrete in both time and amplitude, obtained by permitting each value (sample) of a discrete-time signal to acquire a finite set of values (quantization), assigning it a numerical symbol according to a code ... A digital signal is a sequence or list of numbers drawn from a finite set.〕 representing a digitized analog signal signal. It may be stored or transmitted physically as a pulse code modulation (PCM) signal.
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