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Digital signal can mean two different things:
Digital signals are digital representations of discrete-time signals, which are often produced by changing an analog signal.
An analog signal is a datum that changes over time (for instance the temperature at a given place; the depth of a certain point in a pond; or the amplitude of the voltage at some node in a circuit). A discrete-time signal is a sampled version of an analog signal: the value of the datum is noted at fixed intervals (for example, every microsecond) rather than continuously.
In short, a digital signal is a quantized discrete-time signal; a discrete-time signal is a sampled analog signal.
In the digital revolution the usage of digital signals has increased significantly. Many modern media devices, especially the ones that connect with computers use digital signals to represent signals that were traditionally represented as analog signals; cell phones, music and video players, personal video recorders, and digital cameras are examples.
Waveforms in digital systems
The clock signal is a special digital signal that is used to synchronize digital circuits. The image shown can be considered the waveform of a clock signal. Logic changes are triggered either by the rising edge or the falling edge.