B. SOMANATHAN NAIR, Digital electronics and logic design 2002: "Digital signals are fixed-width pulses, which occupy only one of two levels of amplitude."
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.
J.S.Chitode, Communication Systems, 2008: "When a digital signal is transmitted over a long distance, it needs CW modulation."
