Complexity reduction using frequency masking technique for FIR filter design




Kumar, Sanjeev

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A digital filter is an essential part of many present day electronics devices and a common type of digital filter used is Finite Impulse Response (FIR) filters. FIR filters have several advantages over Infinite Impulse Response (IIR) filters, such as easy design of linear phase FIR filters, inherently stability and low round off noise sensitivity. On the other hand, FIR filters have the disadvantage that when the design specifications require a narrow transition band, the order of FIR designs increases rapidly. One of the common techniques to design narrow transition band FIR filter is the Frequency Masking (FRM) technique which is based on up-sampling and the use of masking filters. This leads to filters with lower order and decreased arithmetic complexity of the filter implementation. In this project the design of linear phase FIR filters using the FRM technique is compared with some of the common ways to design linear phase FIR filters using windows. Design of linear phase low pass FIR filters using conventional window method technique is highlighted with different design examples implemented using MATLAB. The window technique has limitation as it lacks flexibility because, in design both the peak passband (δp) and stopband (δs) ripples are considered approximately equals, so that the designer can’t make a passband ripple very small or a stopband attenuation very large. Further, the order of FIR filters designed using the window technique tends to increase fast when a narrow transition band is required. These design limitations can be overcome by using the frequency masking techniques which provides high selectivity with reduced arithmetic complexity. Linear phase low pass narrow bandwidth FRM FIR filter design are implemented in MATLAB and compared with the window method design in terms of filter order. Two masking filter techniques are being considered. In the first one, a masking filter technique is being used in cascade with the up-sampled model filter to design the linear phase low pass narrow transition bandwidth FIR filter. In the second, two masking filters are being used in a two channel configuration to achieve more general passbands. The comparison of these FIR filter designs indicates that the FRM technique leads to lower order FIR filters compared with the FIR filter designs using window methods at the cost of a slightly more complex design process. This reduction of filter order implies that the FRM techniques can be used to reduce arithmetic complexity for FIR filter implementations in applications requiring linear phase narrow transition bandwidth FIR filters.



Frequency Masking technuique, FIR filter design, Window method of designing FIR filter, Gibbs phenomena, Complexity reduction