bw_lowpass_size.m

来自「英文书《Digital Signal Processing with Examp」· M 代码 · 共 24 行

function L=bw_lowpass_size(vc,vs,dB)
% L=bw_lowpass_size(vc,vs,dB)
%
% L    =# Butterworth lowpass poles necessary to meet specs.
%       (NOTE: L is even so poles will be in conjugate pairs.)
% vc  =cutoff frequency of lowpass digital filter in Hz-s.
% vs  =frequency at beginning of stopband in Hz-s.
% dB   =maximum stopband power gain in dB. Must be <-6.
% See also: ch_lowpass_size

% Check for errors.
if(nargin~=3)
   error('iir_lowpass_bw_size requires 3 input arguments.');
elseif(min(vc,vs)<=0 | max(vc,vs)>=.5)
   error('Arguments vc and vs must be in within (0,0.5).');
elseif(dB>=-6)
   error('Stopband power gain spec. must be < -6 dB.');
end
% Ratio of warped frequencies.
r=tan(pi*vs)/tan(pi*vc);
% Butterworth size.
L=dB/(-20*log10(r));
% Make L an even integer.
L=2*ceil(L/2);