Constrained least square, FIR multiband filter design
b = fircls(n,f,amp,up,lo)
f is a vector of transition frequencies in the range from 0 to 1, where 1 corresponds to the Nyquist frequency. The first point of f must be 0 and the last point 1. The frequency points must be in increasing order.
amp is a vector describing the piecewise constant desired amplitude of the frequency response. The length of amp is equal to the number of bands in the response and should be equal to length(f)-1.
up and lo are vectors with the same length as amp. They define the upper and lower bounds for the frequency response in each band.
fircls always uses an even filter order for configurations with a passband at the Nyquist frequency (that is, highpass and bandstop filters). This is because for odd orders, the frequency response at the Nyquist frequency is necessarily 0. If you specify an odd-valued n, fircls increments it by 1.
'trace', for a textual display of the design error at each iteration step.
'plots', for a collection of plots showing the filter's full-band magnitude response and a zoomed view of the magnitude response in each sub-band. All plots are updated at each iteration step. The O's on the plot are the estimated extremals of the new iteration and the X's are the estimated extremals of the previous iteration, where the extremals are the peaks (maximum and minimum) of the filter ripples. Only ripples that have a corresponding O and X are made equal.
'both', for both the textual display and plots.
Design an order 150 lowpass filter:
n=150; f=[0 0.4 1]; a=[1 0]; up=[1.02 0.01]; lo =[0.98 -0.01]; b = fircls(n,f,a,up,lo,'both'); % Display plots of bands % The Bound Violations indicate iterations as % the design converges. fvtool(b) % Display magnitude plot
 Selesnick, I.W., M. Lang, and C.S. Burrus, "Constrained Least Square Design of FIR Filters without Specified Transition Bands," Proceedings of the IEEE® Int. Conf. Acoust., Speech, Signal Processing, Vol. 2 (May 1995), pp.1260-1263.
 Selesnick, I.W., M. Lang, and C.S. Burrus. "Constrained Least Square Design of FIR Filters without Specified Transition Bands." IEEE Transactions on Signal Processing, Vol. 44, No.8 (August 1996).