22 Mar 2012

Difference b/w Digital & Analog filters [Digital Signal Processing]

Difference between Digital and Analog filters [Digital Signal Processing].

As the name says, the basic unit of the course “digital signal processing” is signal more specifically a digital signal. The signal propagation process consists of one important step, which is filtering out the noise signal from the original signal. At this stage, we need filters.

Well in this post I am going to tell you about 2 kinds of filters whose names are already mentioned in the title of this post:


  1. Digital filters
  2. Analog filters

So, Let us check out the differences between both kinds of filters.

Digital Filters:
  1. It operates on the digital samples of the signals.
  2. These kinds of filters are defined using linear difference equations.
  3. While implementing the digital filters in hardware or software (for simulation), we need adders, subtractors, delays, etc which are classified under digital logic components.
  4. In this filter, the filter coefficients are designed to meet the desired or expected frequency response.
  5. Mathematically the transfer function H(z) is required to be a rational function of z, where the coefficients of z are real to meet the stability and causality requirement.
  6. In order to be stable and causal, the poles of the transfer function should lie inside the unit circle in z-plane.

Analog Filters:
  1. Unlike digital, analog filters works on analog signals or the so called actual signals.
  2. It is defined by linear differential equations.
  3. While implementing the analog filters in hardware or software simulation, electrical components like resistors, capacitors and inductors are used.
  4. To achieve the required frequency response, approximation problem is solved.
  5. To be stable and causal, the transfer function H(s) must be a rational function of s, whose coefficients are real.
  6. For stability and causality, the poles should lie on the left half of s-plane.
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