Wonderful! Digital Filter (Part Three)
I told the filter explained in the previous page is an Infinite Impulse Response (IIR) digital filter but there is another typical type of digital filtering. It is called Finite Impulse Response (FIR) digital filters and the configuration is as follows.
What is the biggest difference from IIR filters is that FIR filters do not return any calculation result to the input, that is, there is no feedback loop in FIR filters. The name FIR is derived from the nature that the response converges with in a finite time span when an impulse(*) is inputted to the filter.
(*) Impulse is a signal that takes value one at zero in the horizontal axis and value zero for all other horizontal values.
The excellence of this method is to easily design a stable filter since there is no feedback loop. However, it is the drawback that FIR filters with sharp frequency characteristics require many steps. In era when semiconductors had lower integration density and lower speed, filters with many steps required tremendous cost but now it does not cost so much owing to the dramatic improvement of semiconductors.
The following figure is the frequency characteristics of a low pass filter (LPF) realized by TA Signal Processings FIR Design Tool. The maximum frequency of the passband is defined at 2KHz, the minimum frequency of stopband at.4KHz, the stopband level at -100dB and the sampling frequency is 44.1KHz. The number of step is 299 but the desired characteristic is not realized yet.
Arranging the parameters, you can improve the filter characteristics and the easiest one is to increase the number of steps. Another way is to allow somewhat ripples in the passband to obtain a steep slope between the passband and the stopband. Using this method, you may be able to drastically improve the cutoff characteristics.
FIR filter has another significant characteristic but I will explain it in the next page.
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