Definition - 6.1.1.4.1 | Module 6: Time Domain Analysis of Discrete-Time Systems | Signals and Systems
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6.1.1.4.1 - Definition

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Introduction & Overview

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Quick Overview

The **step response**, denoted as $s[n]$, is defined as the **output sequence of a Discrete-Time Linear Time-Invariant (DT-LTI) system** when the **discrete-time unit step function $u[n]$** is applied as its input. In simpler terms, if the input $x[n]$ is $u[n]$, then the resulting output $y[n]$ of the LTI system is $s[n]$. It characterizes the system's reaction to a suddenly applied, sustained, or constant input.

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Step Response Definition

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6.1.1.4.1 Definition: The step response, denoted as $s[n]$, is defined as the output sequence of a DT-LTI system when the discrete-time unit step function $u[n]$ is applied as its input. Thus, if the input is $x[n]=u[n]$, then the corresponding output of the system is $y[n]=s[n]$.

Detailed Explanation

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Examples & Analogies

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Definitions & Key Concepts

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Key Concepts

  • Input: Always the unit step function ($u[n]$).

  • Output: Defined as $s[n]$.

  • Purpose: Shows system's reaction to sudden, sustained input.

  • Visualization: Good for observing transient behavior and steady-state.

  • Complementary to h[n]: Offers intuitive view, while h[n] is for mathematical completeness.


  • Examples

  • Example 1: Step Response of a Unit Delay System

  • System: y[n] = x[n-1]

  • Input: $x[n] = u[n]$

  • Output ($s[n]$): $y[n] = u[n-1]$

  • Definition Applied: The step response $s[n]$ for this system is u[n-1]. This means the output is zero for $n \< 1$, and then becomes 1 for $n \ge 1$. It graphically shows the delayed "turn-on" effect.

  • Example 2: Step Response of a Two-Point Averaging System

  • System: y[n] = (x[n] + x[n-1]) / 2

  • Input: $x[n] = u[n]$

  • Output ($s[n]$):

  • For $n \< 0$, $u[n]=0$ and $u[n-1]=0$, so $s[n]=0$.

  • For $n = 0$, $u[0]=1$ and $u[-1]=0$, so $s[0] = (1+0)/2 = 0.5$.

  • For $n \ge 1$, $u[n]=1$ and $u[n-1]=1$, so $s[n] = (1+1)/2 = 1$.

  • Definition Applied: The step response $s[n]$ is ${0, \underline{0.5}, 1, 1, 1, ...}$ where the underline is at $n=0$. This graphically shows that the system gradually transitions to the steady-state value of 1, taking one sample to reach it fully.


  • Flashcards

  • Term: Step Response ($s[n]$)

  • Definition: The output of a DT-LTI system when the discrete-time unit step function ($u[n]$) is applied as its input.

  • Term: Unit Step Function ($u[n]$)

  • Definition: The specific input signal used to define the step response; 0 for negative time, 1 for non-negative time.

  • Term: Transient Behavior

  • Definition: The initial, changing part of the step response as the system adapts to the sudden input.

  • Term: Steady-State Output

  • Definition: The final, constant value that the step response settles to after the transient phase.


  • Memory Aids

  • "Step Input = Step Output": Just like the impulse response is triggered by an impulse, the step response is triggered by a step input. It's a direct mapping.

  • "Switch On": Think of the step input $u[n]$ as literally "switching on" a constant input. The step response $s[n]$ then shows how the system "powers up" and stabilizes in response to that switch being thrown.

  • "S" for Step and Sustained: The "S" in $s[n]$ reminds you it's the response to a Step input, which is a Sustained input, showing the Settling behavior.

Examples & Real-Life Applications

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Examples

  • Example 1: Step Response of a Unit Delay System

  • System: y[n] = x[n-1]

  • Input: $x[n] = u[n]$

  • Output ($s[n]$): $y[n] = u[n-1]$

  • Definition Applied: The step response $s[n]$ for this system is u[n-1]. This means the output is zero for $n \< 1$, and then becomes 1 for $n \ge 1$. It graphically shows the delayed "turn-on" effect.

  • Example 2: Step Response of a Two-Point Averaging System

  • System: y[n] = (x[n] + x[n-1]) / 2

  • Input: $x[n] = u[n]$

  • Output ($s[n]$):

  • For $n \< 0$, $u[n]=0$ and $u[n-1]=0$, so $s[n]=0$.

  • For $n = 0$, $u[0]=1$ and $u[-1]=0$, so $s[0] = (1+0)/2 = 0.5$.

  • For $n \ge 1$, $u[n]=1$ and $u[n-1]=1$, so $s[n] = (1+1)/2 = 1$.

  • Definition Applied: The step response $s[n]$ is ${0, \underline{0.5}, 1, 1, 1, ...}$ where the underline is at $n=0$. This graphically shows that the system gradually transitions to the steady-state value of 1, taking one sample to reach it fully.


  • Flashcards

  • Term: Step Response ($s[n]$)

  • Definition: The output of a DT-LTI system when the discrete-time unit step function ($u[n]$) is applied as its input.

  • Term: Unit Step Function ($u[n]$)

  • Definition: The specific input signal used to define the step response; 0 for negative time, 1 for non-negative time.

  • Term: Transient Behavior

  • Definition: The initial, changing part of the step response as the system adapts to the sudden input.

  • Term: Steady-State Output

  • Definition: The final, constant value that the step response settles to after the transient phase.


  • Memory Aids

  • "Step Input = Step Output": Just like the impulse response is triggered by an impulse, the step response is triggered by a step input. It's a direct mapping.

  • "Switch On": Think of the step input $u[n]$ as literally "switching on" a constant input. The step response $s[n]$ then shows how the system "powers up" and stabilizes in response to that switch being thrown.

  • "S" for Step and Sustained: The "S" in $s[n]$ reminds you it's the response to a Step input, which is a Sustained input, showing the Settling behavior.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🧠 Other Memory Gems

  • Just like the impulse response is triggered by an impulse, the step response is triggered by a step input. It's a direct mapping.
    - "Switch On"

🧠 Other Memory Gems

  • The "S" in $s[n]$ reminds you it's the response to a Step input, which is a Sustained input, showing the Settling behavior.

Flash Cards

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Glossary of Terms

Review the Definitions for terms.

  • Term: SteadyState Output

    Definition:

    The constant or repeating value that a system's output eventually reaches after the transient behavior has subsided, usually in response to a constant or periodic input.

  • Term: Complementary to `h[n]`

    Definition:

    Offers intuitive view, while h[n] is for mathematical completeness.

  • Term: Definition Applied

    Definition:

    The step response $s[n]$ is ${0, \underline{0.5}, 1, 1, 1, ...}$ where the underline is at $n=0$. This graphically shows that the system gradually transitions to the steady-state value of 1, taking one sample to reach it fully.

  • Term: Definition

    Definition:

    The final, constant value that the step response settles to after the transient phase.

  • Term: "S" for Step and Sustained

    Definition:

    The "S" in $s[n]$ reminds you it's the response to a Step input, which is a Sustained input, showing the Settling behavior.

6.1.1.4.1 Definition Step Response (s[n])

This section provides the precise definition of the discrete-time step response, denoted as $s[n]$. Understanding this specific output is important for visualizing how a DT-LTI system behaves when subjected to a sudden and sustained input.