Digital Filtering (FIR and IIR)

One of the most powerful tools of DSP is digital filtering. The excellent performance obtained with digital filters is almost impossible to get with an analog filter since passive components fluctuactions over time and temperature and other effects of analog filters. So, digital filters are widely used for adaptive filtering, noise cancellation, speech recognition and etc…

The procedure for designing analog and digital filters are almost the same. The filter parameters are calculated according to desired filter response. As a result, amplitude and phase characteristics of the filter is derived. In digital filter design, capacitors and resistors in analog filters are replaced with numbers of filter coefficients.

The below listed the comparisons of digital and analog filters.

Digital Filters

  • High Accuracy
  • Linear Phase (FIR filters)
  • Adaptive filtering possible
  • Easy to simulate and design
  • Requires high performance ADC and DSP processing

Analog Filters

  • Less Accuracy
  • Nonlinear phase
  • Adaptive filtering difficult
  • Difficult to simulate and design
  • No ADC and DSP required

FIR vs. IIR Filters

There are two fundamental types of digital filtering: Finite Impulse Response (FIR) and Infinite Impulse Response (IIR).

Both filter has advantages and disadvantages compared to each other.

FIR filters have the following primary advantages:

  • They can have exactly linear phase.
  • They are always stable.
  • The design methods are generally linear.
  • They can be realized efficiently in hardware.
  • The filter startup transients have finite duration.

The primary disadvantage of FIR filters is that they often require a much higher filter order than IIR filters to achieve a given level of performance. Correspondingly, the delay of these filters is often much greater than for an equal performance IIR filter.

You can find a tutorial about designing and applying a FIR bandpass filter to an obtained signal with post named “Bandpass filter tutorial with STM32F4”.

References

1- Mixed-Signal and DSP Design Techniques, Analog Devices, Inc.
2- https://www.mathworks.com/help/signal/ug/fir-filter-design.html

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