MATLAB: Numerator and denominator coefficients from designfilt function

digital signal processingfilter design

I am designing an lowpass IIR filter using designfilt function. I found coefficients for the filter which is 9×6 Double. Now, how can I find numerator and denominator from this?

Best Answer

The type of coefficients you encounter are called Second order section coefficients. Second-order section coefficients, specified as a matrix. sos is a K-by-6 matrix, where the number of sections, K, must be greater than or equal to 2. If the number of sections is less than 2, the function treats the input as a numerator vector. Each row of sos corresponds to the coefficients of a second-order (biquad) filter. The ith row of sos corresponds to [bi(1) bi(2) bi(3) ai(1) ai(2) ai(3)].

%Create a filter object using designfilt
filterObj = designfilt('lowpassiir', 'PassbandFrequency', .45, 'StopbandFrequency', .55, 'PassbandRipple', 1, 'StopbandAttenuation', 60);
%Convert the second order section coefficients to tranfer function format
[b,a] = sos2tf(filterObj.Coefficients);
%Here b is the numerator and a is the denominator

Related Solutions

MATLAB: How to design FIR lowpass filter with cutoff frequency, fc1 = 20 Hz and FIR bandpass filter with cutoff frequency fc2 = 10 Hz and fc3 = 35 Hz.

Déjà vu

how to design IIR highpass filter with cutoff frequency of 20Hz and FIR bandpass filter with cutoff frequency of 10Hz and 15Hz?

Make the appropriate changes to get the cascaded filter you want.

MATLAB: Filter design parameter: radian/sec or 1/sec

The passbands and stopbands for the Signal Processing Toolbox filter functions are actually normalised to the closed interval [0, pi] radians. It is calculated as the passband and stopband frequency in Hz divided by the Nyquist frequency.

Example for frequency calculations for the filters your signal:

Ts = 0.002;                                             % Sampling Interval (seconds)
Fs = 1/Ts;                                              % Sampling Frequency (Hz)

Fn = Fs/2;                                              % Nyquist Frequency

Note that the highest frequency you can design in your filter is the Nyquist frequency, here 250 Hz.

An example of a Chebyshev Type II filter design for filtering an EKG signal is here:

Fs = 250;                                               % Sampling Frequency (Hz)
Fn = Fs/2;                                              % Nyquist Frequency (Hz)
Wp = [1.0   100]/Fn;                                    % Passband Frequencies (Normalised)
Ws = [0.5   101]/Fn;                                    % Stopband Frequencies (Normalised)
Rp = 10;                                                % Passband Ripple (dB)
Rs = 50;                                                % Stopband Ripple (dB)
[n,Ws] = cheb2ord(Wp,Ws,Rp,Rs);                         % Filter Order
[z,p,k] = cheby2(n,Rs,Ws);                              % Filter Design
[sosbp,gbp] = zp2sos(z,p,k);                            % Convert To Second-Order-Section For Stability
figure(3)
freqz(sosbp, 2^17, Fs)
EKG_filt = filtfilt(sosbp, gbp, EKG_original);          % Filter EKG Signal

The filter passband goes from 1 Hz to 100 Hz.

Best Answer

Related Solutions

MATLAB: How to design FIR lowpass filter with cutoff frequency, fc1 = 20 Hz and FIR bandpass filter with cutoff frequency fc2 = 10 Hz and fc3 = 35 Hz.

MATLAB: Filter design parameter: radian/sec or 1/sec

Related Question