The Classifications of Continuous-Time Signals

Periodicity

• Continuous-time signals can be periodic or nonperiodic. Periodic signals repeat themselves after a certain length of time. This period of time is called the period of the signal and is usually denoted with a "T." Nonperiodic signals do not repeat themselves over time.

Digital and Analog

• Continuous-time signals can be digital or analog. Digital signals must occupy one of a set of discrete values. Analog signals can have any real number value. This means that between a given band of possible values there are an infinite number of potential analog values, but a finite number of potential digital values. The distinction between digital and analog signals is similar to the distinction between discrete-time and continuous-time signals. However in the digital-analog distinction the relevant value is the Y-axis value, or the signal, whereas for the discrete-continuous time distinction the relevant value is the X-axis value, or the time.

Causality

• Continuous-time signals can be either causal, anti-causal or noncausal. Causal signals have a value of zero for all time less than zero. Anti-causal signals have a value of zero for all time greater than zero. Noncausal signals have nonzero values both before and after zero time. Another way of thinking about causality is to consider causal signals to be turned "on" at t = 0 seconds, anti-causal signals to be turned "off" at t = 0 seconds, and non-causal signals to be unaffected by passing through t = 0 seconds.

Randomness

• Continuous-time signals can be either deterministic or random. The exact value of a deterministic signal can be calculated for point any point in time. The calculation can be from a formula or set of tables. Random signals cannot be accurately calculated for any point in time and usually have to be found by averaging a set of experimental values.

Even and Odd

• Continuous-time signals can be even, odd or neither. Even signals are symmetric about the time t=0 seconds. Odd signals have rotational symmetry of 1 around the the time t=0 seconds and the 0 value of the signal. Some signals are neither odd nor even.

Handedness

• Continuous-time signals can be left-handed or right-handed. A right-handed signal has a value of zero between negative infinity and some point in time, and some nonzero values after that point in time. A left-handed signal has a value of zero between some point in time and positive infinity, and some nonzero values before that point in time.

Length

• Continuous-time signals can be of finite or infinite length. Finite signals have nonzero values over a finite interval, and zero values for all time outside of that finite interval. Infinite signals have nonzero values for all time.