[1]^Robert K. Dueck. Digital Design with CPLD Applications and VHDL. A digital representation can have only specific discrete values.

[2]^Proakis, John G.; Manolakis, Dimitris G. (2007-01-01). Digital Signal Processing. Pearson Prentice Hall. ISBN 9780131873742..

[3]^"Digital Signal". Retrieved 2016-08-13..

[4]^Horowitz, Paul; Hill, Winfield (1989). The Art Of Electronics, 2nd Ed. Cambridge University Press. pp. 471–473. ISBN 0521370957..

[5]^B. SOMANATHAN NAIR, Digital electronics and logic design 2002: "Digital signals are fixed-width pulses, which occupy only one of two levels of amplitude.".

[6]^Joseph Migga Kizza, Computer Network Security 2005.

[7]^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..

[8]^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".

[9]^Fred Halsall, Computer Networking and the Internet: "In order to transmit a digital signal over an analog subscriber line, modulated transmission must be used; thas is the electrical signal that represents the binary bit stream of the source (digital) output must first be converted to an analog signal that is compatible with a (telephony) speech signal.".