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Convert FIR Type I lowpass to FIR Type 1 lowpass with inverse bandwidth


g = firlp2lp(b)


g = firlp2lp(b) transforms the Type I lowpass FIR filter b with zero-phase response Hr(w) to a Type I lowpass FIR filter g with zero-phase response [1 - Hr(π-w)].

When b is a narrowband filter, g will be a wideband filter and vice versa. The passband and stopband ripples of g will be equal to the stopband and passband ripples of b.


collapse all

Create a narrowband lowpass filter to use as prototype. Display its zero-phase response.

b = firgr(36,[0 0.2 0.25 1],[1 1 0 0],[1 5]);


Convert the prototype filter to a wideband lowpass filter. Add to the plot the zero-phase response of the new filter.

h = firlp2lp(b);

hold on

Convert the previous filter back to a narrowband lowpass filter. Add to the plot the zero-phase response of the new filter.

g = firlp2lp(h);

[gr,w] = zerophase(g);
hold off


Saramaki, T, Finite Impulse Response Filter Design, Handbook for Digital Signal Processing. S.K. Mitra and J.F. Kaiser Eds. Wiley-Interscience, N.Y., 1993, Chapter 4.

Introduced in R2011a

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