Definition of Signals | Signals and Systems - Electronics and Communication Engineering (ECE) PDF Download

What is a Signal?

  • Anything that carries information can be called a signal. It can also be defined as a physical quantity that varies with time, temperature, pressure, or with any independent variables such as speech signal or video signal.
  • The process of operation in which the characteristics of a signal (Amplitude, shape, phase, frequency, etc.) undergoes a change is known as signal processing.

Note: Any unwanted signal interfering with the main signal is termed as noise. So, noise is also a signal but unwanted.

According to their representation and processing, signals can be classified into various categories details of which are discussed below.

Continuous Time Signals

  • Continuous-time signals are defined along a continuum of time and are thus, represented by a continuous independent variable. Continuous-time signals are often referred to as analog signals.
  • This type of signal shows continuity both in amplitude and time. These will have values at each instant of time. Sine and cosine functions are the best example of Continuous time signal.

Continuous Time SignalContinuous Time Signal

  • The signal shown above is an example of a continuous-time signal because we can get value of signal at each instant of time.

Discrete-Time signals

  • The signals, which are defined at discrete times are known as discrete signals. Therefore, every independent variable has distinct value. Thus, they are represented as sequence of numbers.
  • Although speech and video signals have the privilege to be represented in both continuous and discrete time format; under certain circumstances, they are identical. Amplitudes also show discrete characteristics. Perfect example of this is a digital signal; whose amplitude and time both are discrete.

Example of a Discrete Time SignalExample of a Discrete Time Signal

  • The figure above depicts a discrete signal’s discrete amplitude characteristic over a period of time. Mathematically, these types of signals can be formularized as;
  • Definition of Signals | Signals and Systems - Electronics and Communication Engineering (ECE)

Where, n is an integer.

  • It is a sequence of numbers x, where nth number in the sequence is represented as x[n].

Question for Definition of Signals
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Which type of signal is represented by a continuous independent variable and shows continuity in both amplitude and time?
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Analog and Digital Signals

  • Analog Signals: These are continuous signals that vary smoothly and can take on any value within a certain range. Examples include electrical voltages, sound waves, and temperature readings. Analog signals are represented as continuous waveforms and can have an infinite number of possible values.
  • Digital Signals: These are discrete signals that have a finite number of defined values. Digital signals are typically represented using binary code (0s and 1s). They are commonly used in digital electronics, computers, and communication systems. Digital signals are more immune to noise and distortion compared to analog signals.

Real and Complex Signals

  • Real Signals: Real signals are those that can be represented by a single real number at each point in time. These signals are physically measurable and correspond to real-world quantities. Examples include temperature, voltage, and pressure.
  • Complex Signals: Complex signals, on the other hand, involve both a real part and an imaginary part. They are often used in complex analysis and are essential in the study of electrical circuits and electromagnetic fields. Complex signals are commonly encountered in AC (alternating current) analysis.

Deterministic and Random Signals

  • Deterministic Signals: These are signals for which the values at any given time can be precisely predicted or determined based on a known mathematical function or algorithm. Examples include sine waves, square waves, and any signal with a well-defined pattern.
  • Random Signals: Random signals, also known as stochastic signals, have values that are not predictable with certainty. They exhibit randomness or uncertainty in their values over time. Examples include noise in communication systems and random voltage fluctuations in electronic components.

Even and Odd Signals

  • Even Signals: Even signals are symmetric with respect to the y-axis. Mathematically, f(-t) = f(t), meaning the signal is unchanged when reflected across the y-axis. An example of an even signal is a cosine wave.
  • Odd Signals: Odd signals are antisymmetric with respect to the y-axis. Mathematically, f(-t) = -f(t), meaning the signal changes sign when reflected across the y-axis. An example of an odd signal is a sine wave.

Energy and Power Signals

  • Energy Signals: These signals have finite energy over a specified time interval. They are typically associated with transient phenomena and can be visualized as impulses or pulses. Energy signals have zero power because they are of finite duration. Example: A unit step function.
  • Power Signals: Power signals have finite power over time, and they exist over an extended time interval. These signals are often periodic or non-periodic and have non-zero power. Example: A continuous sine wave.

Question for Definition of Signals
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Which type of signal has a finite number of defined values?
View Solution

Periodic and Non-Periodic Signals


  • Periodic Signals: These signals repeat their pattern or waveform at regular intervals. The fundamental property of a periodic signal is its period, which is the time it takes to complete one full cycle. Examples include sinusoidal waves and square waves.
  • Non-Periodic Signals (Aperiodic): Non-periodic signals do not exhibit a repetitive pattern. They have no discernible period, and their behavior over time is not predictable based on regular intervals. Examples include transient signals and random noise.

The document Definition of Signals | Signals and Systems - Electronics and Communication Engineering (ECE) is a part of the Electronics and Communication Engineering (ECE) Course Signals and Systems.
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FAQs on Definition of Signals - Signals and Systems - Electronics and Communication Engineering (ECE)

1. What is the difference between continuous-time signals and discrete-time signals?
Ans. Continuous-time signals are signals that vary continuously over time, meaning they have values at every instant of time within a certain range. Discrete-time signals, on the other hand, are signals that are only defined at specific time instances or intervals. These signals take on values only at discrete points in time, often represented as a sequence or a set of values.
2. How do analog signals differ from digital signals?
Ans. Analog signals are continuous signals that can take on any value within a certain range. They are characterized by their infinite precision and can represent a wide range of values. On the other hand, digital signals are discrete signals that can only take on a limited set of values. They are represented using binary digits (0s and 1s), and their precision is finite. Digital signals are commonly used in modern communication systems and electronic devices.
3. What is the difference between real signals and complex signals?
Ans. Real signals are signals that have real-valued amplitudes or values. They can be represented by a single waveform or a sequence of real numbers. Complex signals, on the other hand, have both real and imaginary components. They are often represented using complex numbers and can exhibit more complex behavior, such as phase shifts and frequency modulation. Complex signals are widely used in areas such as telecommunications and signal processing.
4. How do deterministic signals differ from random signals?
Ans. Deterministic signals are signals that can be precisely described by a mathematical equation or formula. They follow a specific pattern or function and can be completely predicted or determined. Random signals, on the other hand, do not follow a specific pattern and cannot be described by a deterministic equation. They exhibit randomness or unpredictability in their values, often resulting from noise or other stochastic processes.
5. What is the difference between periodic and non-periodic signals?
Ans. Periodic signals are signals that repeat their waveform or pattern over regular intervals of time. They have a fundamental period, which is the smallest time interval at which the signal repeats. Non-periodic signals, also known as aperiodic signals, do not exhibit any regular repetition or pattern. They are often characterized by a unique waveform that does not repeat. Non-periodic signals are commonly encountered in applications such as speech and music signals where each signal represents unique information.
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