Using the lowpass function

In using the lowpass function, how can you suppress the display after? (The default implementation outputs plots of the time-domain signal and power spectrum, comparing the original signal to the filtered one.) Also, are there any good rules of thumb for selecting the passband frequency and steepness? I feel like the latter should be as high as possible.

Answer 1

[Star Strider]

The lowpass function does not display anything unless you leave the trailing seimcolon off of the lowpass call. It will design a very good elliptic filter if you inclued the name-value pair 'ImpulseResponse','iir.

[L'O.G.]

Thank you so much! And regarding the passband frequency and steepness, are there any rules of thumb?

[Star Strider]

My pleasure!

It is possible to design those specifically with the designfilt funciton, however not with lowpass. It designs a very good generic elliptic filter if you ask it to. The passband frequency is entirely dependent on the signal being filtered, and the best way to determine that is with a Fourier transform.

Example —

t = linspace(0, 20, 250);                           % Time Vector
s = sum(sin([1:5]'*2*pi*t));                        % Signal Vector
Fs = 1/(t(2)-t(1));                                 % Sampling Frequency
Fn = Fs/2;                                          % Nyquist Frequency

figure
plot(t, s)
grid
xlabel('t')
ylabel('s')
title('Original Signal')

L = numel(t);
NFFT = 2^nextpow2(L);
FTs = fft(s, NFFT)/L;
Fv = linspace(0, 1, NFFT/2+1)*Fn;
Iv = 1:numel(Fv);

figure
plot(Fv, abs(FTs(Iv))*2)
grid
xlabel('Frequency')
ylabel('Amplitude')

s_filt = lowpass(s, 1.5, Fs, 'ImpulseResponse','iir');  % Set Stopband = 1.5 Hz

figure
plot(t, s_filt)
grid
xlabel('t')
ylabel('s')
title('Lowpass Filtered Signal')

.

Answer 2

[Paul]

The only way to suppress the plot output is to specify at least one output argument (or use lowpass() in an expression, like 1*lowpass(...) )

If calling lowpass() alone without output arguments, why would you want the plots suppressed?