June 1, 2023

What is an infinite impulse response?

What is an infinite impulse response?

An infinite impulse response (IIR) filter is a type of digital filter that reacts to abrupt changes or impulses in a signal. It is widely used in signal processing in various fields such as audio and image processing, telecommunications, and control systems. In this article, we will explore the different aspects of IIR filters, from their basic concepts to their design and implementation.

Understanding Infinite Impulse Response (IIR) Filters

Definition and Basic Concept

At the core of any digital filter is a mathematical algorithm that processes a signal to achieve a desired result. In an IIR filter, this algorithm involves feedback from the output of the filter to its input. This feedback allows the filter to have an infinite impulse response, meaning that it can potentially continue to produce output indefinitely even after the input has stopped.

One major advantage of IIR filters is their ability to achieve high filtering performance with relatively low computational complexity. By design, IIR filters are able to take advantage of the memory of past inputs and outputs, allowing them to achieve sharper filtering responses compared to other types of digital filters.

In addition, IIR filters can be designed to have a wide range of frequency responses, making them suitable for a variety of signal processing applications. For example, they can be designed to have a low-pass, high-pass, band-pass, or band-stop response, depending on the specific requirements of the application.

Difference between Finite Impulse Response (FIR) and IIR Filters

Another type of digital filter is the finite impulse response (FIR) filter. The main difference between IIR and FIR filters lies in their impulse responses. An FIR filter responds to an impulse by producing a finite sequence of output values, whereas an IIR filter can theoretically produce an infinite sequence of output values.

While FIR filters are easier to design and implement without stability concerns, they require a larger number of filter coefficients to achieve comparable filtering performance. On the other hand, IIR filters can achieve the same performance with fewer coefficients, but require careful design to ensure stability and avoid issues such as filter instability and oscillations.

Furthermore, IIR filters can be implemented using various structures, such as direct form, cascaded form, and parallel form, each with its own advantages and disadvantages. The choice of structure depends on factors such as computational efficiency, numerical stability, and ease of implementation.

Applications of IIR Filters

IIR filters are used in a wide range of applications, including speech processing, audio equalization, biomedical signal processing, and control systems. They are especially useful in applications where a real-time response to abrupt changes in signals is necessary.

One important application of IIR filters is in feedback control systems. In such systems, the filter is used to process sensor data and generate control signals that can modulate a physical process. By carefully designing the filter, it is possible to achieve stable and accurate control of the process.

In biomedical signal processing, IIR filters are used to remove noise and artifacts from signals such as electrocardiograms (ECGs), electroencephalograms (EEGs), and electromyograms (EMGs). These signals are often contaminated by various types of noise, such as power line interference, muscle artifacts, and electrode noise. IIR filters can be designed to selectively remove these unwanted components while preserving the underlying physiological information.

In audio equalization, IIR filters are used to shape the frequency response of audio signals to achieve a desired sound quality. For example, they can be used to boost or cut certain frequency bands to compensate for room acoustics, speaker characteristics, or personal preferences.

Finally, IIR filters are widely used in digital control systems, where they are used to implement various control algorithms such as proportional-integral-derivative (PID) control, lead-lag compensation, and state feedback control. These algorithms require the use of IIR filters to process sensor signals and generate control signals that can stabilize the system and achieve the desired performance.

Components of an Infinite Impulse Response Filter

Transfer Function

The transfer function of an Infinite Impulse Response (IIR) filter is a mathematical representation of the relation between the input and output of the filter. It is often expressed as a ratio of two polynomials in the complex variable z. The numerator represents the filter's output, while the denominator represents its input.

The transfer function is a fundamental property of the filter and can be used to calculate its frequency response and other key characteristics. Additionally, it plays a crucial role in filter design, as it determines the filter's overall behavior. The transfer function can be used to determine the stability of the filter, which is an important consideration in filter design.

It is important to note that IIR filters have infinite impulse responses, which means that the output of the filter can be affected by past inputs. This can result in a more complex transfer function compared to Finite Impulse Response (FIR) filters, which have a finite impulse response.

Feedback and Feedforward Structures

In an IIR filter, the input signal is combined with a delayed version of the output signal to form the filter's output. This feedback structure enables the filter to respond to abrupt changes in the input signal and maintain a consistent output.

Another important structural aspect of IIR filters is feedforward. In a feedforward IIR filter, some or all of the output signal is passed directly to the output without being fed back into the filter. This can result in a higher computational complexity but can also provide improved filtering performance in certain applications.

The choice between feedback and feedforward structures depends on the specific application and the desired filtering characteristics. In some cases, a combination of both structures may be used to achieve the desired performance.

Poles and Zeros

The poles and zeros of an IIR filter represent the locations in the complex plane where the transfer function is zero or infinite, respectively. They are key to understanding the filter's frequency response and overall behavior.

The location of poles and zeros in the complex plane can affect the stability of the filter and its ability to achieve certain filtering characteristics. As such, they play an important role in filter design and analysis.

For example, if the poles of the transfer function are located outside the unit circle in the complex plane, the filter will be unstable. On the other hand, if the poles are located inside the unit circle, the filter will be stable.

The number and location of poles and zeros also affect the frequency response of the filter. For instance, if the transfer function has a zero at a certain frequency, the filter will not attenuate signals at that frequency. Similarly, if the transfer function has a pole at a certain frequency, the filter will amplify signals at that frequency.

Overall, the poles and zeros of an IIR filter are a crucial consideration in filter design and analysis, as they have a significant impact on the filter's performance and behavior.

Designing an IIR Filter

Designing an infinite impulse response (IIR) filter is a critical task in signal processing. IIR filters are widely used in audio processing, image processing, and biomedical signal processing applications. They are used to remove unwanted noise from signals and to extract useful information from them.

Filter Specifications

The design of an IIR filter begins with the specification of its frequency response and other key characteristics. These specifications can include cutoff frequency, passband ripple, stopband attenuation, and transition bandwidth, among others. The cutoff frequency is the frequency at which the filter begins to attenuate the signal. The passband ripple is the maximum deviation of the filter's gain from its ideal value in the passband. The stopband attenuation is the amount of attenuation in the stopband, which is the range of frequencies that are attenuated by the filter. The transition bandwidth is the range of frequencies between the passband and stopband where the filter's gain changes from the passband value to the stopband value.

By specifying these characteristics, the designer can determine the necessary coefficients and other parameters for the filter transfer function. The transfer function describes the relationship between the input and output signals of the filter.

Design Techniques

There are different techniques for designing IIR filters, including Butterworth, Chebyshev, and elliptic filters. Each technique has its own strengths and weaknesses and is suited to different applications and design requirements.

The Butterworth filter is a type of low-pass filter that has a maximally flat frequency response in the passband. It is commonly used in applications where a flat frequency response is desired. The Chebyshev filter is a type of filter that has a steeper rolloff in the stopband than the Butterworth filter. It is commonly used in applications where a sharp transition between the passband and stopband is required. The elliptic filter is a type of filter that has a steeper rolloff than the Chebyshev filter and a flatter response in the passband than the Chebyshev filter. It is commonly used in applications where both a sharp transition and a flat passband response are required.

One common approach to filter design is to use MATLAB or other software tools to perform frequency analysis and generate filter coefficients automatically. However, this approach requires careful consideration of filter stability and other issues to avoid filter instability and oscillations.

Stability Considerations

Ensuring the stability of an IIR filter is crucial to its successful operation. Unstable filters can produce wild oscillations and other unpredictable behaviors that can render the filtered signal useless for its intended purpose.

Designers use a range of techniques to ensure filter stability, such as ensuring that all poles lie within the unit circle in the complex plane. The poles are the values of the transfer function that cause the filter to oscillate. If any pole lies outside the unit circle, the filter will be unstable. Other methods include pole-zero cancellation, adding damping to the feedback loop, and using filters with a fixed-point arithmetic.

In conclusion, designing an IIR filter requires careful consideration of the filter specifications, design techniques, and stability considerations. By taking these factors into account, designers can create filters that effectively remove noise from signals and extract useful information from them.

Implementing IIR Filters

Direct Form I and II

There are different ways to implement an IIR filter in software or hardware. One common approach is to use direct form I or direct form II structures.

Direct form I first replicates each pole of the transfer function in one feedback loop and each zero in one feedforward path. Direct form II, on the other hand, groups together conjugate pole-zero pairs to reduce computational complexity.

Cascade and Parallel Structures

Another approach to IIR filter implementation is to use cascade or parallel structures. Cascade structures involve implementing the filter as a series of smaller sub-filters, each with its own transfer function and coefficients. Parallel structures involve implementing the filter using multiple identical sub-filters operating in parallel.

Software and Hardware Implementations

IIR filters can be implemented in software using programming tools such as Collimator and Modelica. Hardware implementations can range from simple microcontroller circuits to dedicated application-specific integrated circuits (ASICs).

Designers must consider issues such as computational complexity, data format, and real-time constraints when choosing an implementation method.


An infinite impulse response filter is a powerful tool for processing digital signals and achieving desired effects such as noise reduction, smoothing, or control. By understanding the basic concepts of IIR filters, their components, and design considerations, designers can create filters that meet their specific needs and applications.

Whether designing a speech processing algorithm, a control system, or a multimedia application, an IIR filter can help achieve higher performance, lower computational complexity, and better signal quality.

Learn more about how Collimator’s signal processing solutions can help you fast-track your development. Schedule a demo with one of our engineers today.

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