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366 Chapter 7 FIR FILTER DESIGN<br />

Local maximum errors are determined over a finer grid, and the extremal<br />

frequencies {ω i } are adjusted at these new extremal values. A new Lthorder<br />

polynomial is fit through these new frequencies, and the procedure<br />

is repeated. This iteration continues until the optimum set {ω i } and the<br />

global maximum error δ are found. The iterative procedure is guaranteed<br />

to converge, yielding the polynomial P (ω). From (7.46) coefficients β(n)<br />

are determined. Finally, the coefficients a(n) aswell as the impulse response<br />

h(n) are computed. This algorithm is available in MATLAB as<br />

the firpm function, which is described shortly.<br />

Since we approximated M, the maximum error δ may not be equal to<br />

δ 2 .Ifthis is the case, then we have to increase M (if δ>δ 2 )ordecrease<br />

M (if δ

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