I am using the freqz command, and the second argument, n, is documented as N-point frequency response. Therefore, I take it to mean that as n gets larger, the resolution of the frequency response should get finer and more smooth. But when I run this command, this is not what happens. Instead, I see the magnitude values changing. This makes no sense, and the documentation is quite poor.
MATLAB: Does the magnitude response of freqz change with the value of the input argument n
freqzMATLAB
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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.
Try this:
D = load('100m.mat');EKG = D.val; % Signal
L = numel(EKG); % Signal Length (Samples)
Fs = 360; % Sampling Frequency
Fn = Fs/2; % Nyquist Frequency
t = linspace(0, L, L)/Fs; % Time Vector
b = fir1(12, (35/Fn)); % Order 12 FIR LPF Fco = 35 Hz
EKGfilt = filtfilt(b,1,EKG); % Filter Signal
figurefreqz(b,1, 2^12, Fs) % Display Filter Bode Plot
figureplot(t, EKG)hold onplot(t, EKGfilt)hold offgridThe filter does not appear to be very effective, however it is what you requested, and from the freqz plot, has approximately 6 dB attenuation at 35 Hz.
EDIT — (29 Nov 2019 at 13:54)
Changed filter to filtfilt, since there appears to be significant phase delay usiung filter, even thouigh this is a FIR filter.
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