As the names suggest, this classification is determined by whether or not the time axis (x-axis) is discrete (countable) or continuous. A continuous signal will contain a value for all real numbers along the time axis. In contrast to this, a discrete signal is often created by using the sampling theorem to sample a continuous signal, so it will only have values at equally spaced intervals along the time axis.

Figure 1

The difference between analog and digital is similar to the difference between continuous-time and discrete-time. In this case, however, the difference is with respect to the value of the function (y-axis). Analog corresponds to a continuous y-axis, while digital corresponds to a discrete y-axis. An easy example of a digital signal is a binary sequence, where the values of the function can only be one or zero.

Figure 2

Periodic signals repeat with some period T, while aperiodic, or nonperiodic, signals do not. We can define a periodic function through the following mathematical expression, where t t can be any number and T T is a positive constant:

Figure 3

(a) A periodic signal with period T 0 T 0 (b) An aperiodic signal

Causal signals are signals that are zero for all negative time, while anitcausal are signals that are zero for all positive time. Noncausal signals are signals that have nonzero values in both positive and negative time.

Figure 4

(a) A causal signal (b) An anticausal signal (c) A noncausal signal

An even signal is any signal ff such that f⁢t=f⁢−t f t f t . Even signals can be easily spotted as they are symmetric around the vertical axis. An odd signal, on the other hand, is a signal ff such that f⁢t=−f⁢−t f t f t .

Figure 5

(a) An even signal (b) An odd signal

Using the definitions of even and odd signals, we can show that any signal can be written as a combination of an even and odd signal. That is, every signal has an odd-even decomposition. To demonstrate this, we have to look no further than a single equation.

Example 1

Figure 6

(a) The signal we will decompose using odd-even decomposition (b) Even part: e⁢t=12⁢(f⁢t+f⁢−t) e t 1 2 f t f t (c) Odd part: o⁢t=12⁢(f⁢t−f⁢−t) o t 1 2 f t f t (d) Check: e⁢t+o⁢t=f⁢t e t o t f t

A deterministic signal is a signal in which each value of the signal is fixed and can be determined by a mathematical expression, rule, or table. Because of this the future values of the signal can be calculated from past values with complete confidence. On the other hand, a random signal has a lot of uncertainty about its behavior. The future values of a random signal cannot be acurately predicted and can usually only be guessed based on the averages of sets of signals.

Figure 7

(a) Deterministic Signal (b) Random Signal

A right-handed signal and left-handed signal are those signals whose value is zero between a given variable and positive or negative infinity. Mathematically speaking, a right-handed signal is defined as any signal where f⁢t=0 f t 0 for t< t 1 <∞ t t 1 , and a left-handed signal is defined as any signal where f⁢t=0 f t 0 for t> t 1 >−∞ t t 1 . See the figures below for an example. Both figures "begin" at t 1 t 1 and then extends to positive or negative infinity with mainly nonzero values.

Figure 8

(a) Right-handed signal (b) Left-handed signal

As the name applies, signals can be characterized as to whether they have a finite or infinite length set of values. Most finite length signals are used when dealing with discrete-time signals or a given sequence of values. Mathematically speaking, f⁢t f t is a finite-length signal if it is nonzero over a finite interval t 1 <f⁢t< t 2 t 1 f t t 2 where t 1 >−∞ t 1 and t 2 <∞ t 2 . An example can be seen in the below figure. Similarly, an infinite-length signal, f⁢t f t , is defined as nonzero over all real numbers: ∞≤f⁢t≤−∞ f t

Figure 9: Finite-Length Signal. Note that it only has nonzero values on a set, finite interval.