SIGNAL CLASSIFICATION
Signals are mathematically classified into pairs based on two main characteristics: Value and Time.
By Value (Amplitude)
An analog signal has a continuous amplitude and the signal's value can be any value within a continuous range.
A digital signal has a discreet (quantized) amplitude and the signal's value can only be one of a finite set of specific values (usually just two: high/1 or low/0).
By Time
A continuous-time signal is defined and exists for every single instant of time. The signal is continuous in time and its graph is a solid line.
A discreet-time signal is defined only at specific, distinct points in time, usually at uniform intervals. The signal is discreet in time and its graph is a series of dots/samples.
SIGNAL PROCESSING
Beyond the amplitude and time pairs, signals can be categorized in several other ways, often used in advanced Signal Processing and System Analysis: Deterministic vs. Random, Periodic vs. Aperiodic, and Energy vs. Power.
