**What is signal?**

A signal is an electrical or electromagnetic current that is used for carrying data from one device or network to another. The simplest form of signal is a direct current (DC) that is switched on and off; this is the principle by which the early telegraph worked. More complex signals consist of an alternating-current (AC) or electromagnetic carrier that contains one or more data streams.

**Signals are classified into the six categories:**

- Continuous Time and Discrete Time Signals
- Deterministic and Non-deterministic Signals
- Even and Odd Signals
- Periodic and A periodic Signals
- Energy and Power Signals
- Real and Imaginary Signals

Continuous Time and Discrete Time Signals

A signal is said to be continuous when it is defined for all instants of time.

A signal is said to be discrete when it is defined at only discrete instants of time/

Deterministic and Non-deterministic Signals

A signal is said to be deterministic if there is no uncertainty with respect to its value at any instant of time. Or, signals which can be defined exactly by a mathematical formula are known as deterministic signals.

A signal is said to be non-deterministic if there is uncertainty with respect to its value at some instant of time. Non-deterministic signals are random in nature hence they are called random signals. Random signals cannot be described by a mathematical equation. They are modelled in probabilistic terms.

Even and Odd Signals

A signal is said to be even when it satisfies the condition x(t) = x(-t)

**Example 1:** t2, t4… cost etc.

Let x(t) = t2

x(-t) = (-t)2 = t2 = x(t)

∴,∴, t2 is even function

**Example 2:** As shown in the following diagram, rectangle function x(t) = x(-t) so it is also even function.

A signal is said to be odd when it satisfies the condition x(t) = -x(-t)

**Example:** t, t3 … And sin t

Let x(t) = sin t

x(-t) = sin(-t) = -sin t = -x(t)

∴,∴, sin t is odd function.

Any function ƒ(t) can be expressed as the sum of its even function ƒ_{e}(t) and odd function ƒ_{o}(t).

ƒ(*t* ) = ƒ_{e}(*t* ) + ƒ_{0}(*t* )

where

ƒ_{e}(*t* ) = ½[ƒ(*t* ) +ƒ(*-t* )]

Periodic and Aperiodic Signals

A signal is said to be periodic if it satisfies the condition x(t) = x(t + T) or x(n) = x(n + N).

Where

T = fundamental time period,

1/T = f = fundamental frequency.

The above signal will repeat for every time interval T_{0} hence it is periodic with period T_{0}.

Energy and Power Signals

A signal is said to be energy signal when it has finite energy.

EnergyE=∫∞−∞x2(t)dtEnergyE=∫−∞∞x2(t)dt

A signal is said to be power signal when it has finite power.

PowerP=limT→∞12T∫T−Tx2(t)dtPowerP=limT→∞12T∫−TTx2(t)dt

NOTE:A signal cannot be both, energy and power simultaneously. Also, a signal may be neither energy nor power signal.

Power of energy signal = 0

Energy of power signal = ∞

Real and Imaginary Signals

A signal is said to be real when it satisfies the condition x(t) = x*(t)

A signal is said to be odd when it satisfies the condition x(t) = -x*(t)

Example:

If x(t)= 3 then x*(t)=3*=3 here x(t) is a real signal.

If x(t)= 3j then x*(t)=3j* = -3j = -x(t) hence x(t) is a odd signal.

**Note:** For a real signal, imaginary part should be zero. Similarly for an imaginary signal, real part should be zero.