How to perform forward and backward lowpass filtering with filter command?

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Kalasagarreddi Kottakota 2023-11-2

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编辑： Kalasagarreddi Kottakota 2023-11-2

I need to perform forward and backward filtering with filter commnad in matlab. I was able to perform forward filtering, but not backward. In this regard, could you help me with the following code?

I used convolution with rectangular window for lowpass filtering
In second case I used filter matlab comand with boxcar for lowpass filtering
In case three, I used filtfilt with boxcar for lowpass filetering

The first case with conv is correct and I have analytically validated. But other two approaches have issues, so how to correct them? *Please dont chnage the filter type,as I need only boxcar.

Needs: 1. Using filter creates a forward delay. How to perform backward filtering with filter command?

2. Is it possible to perform same operation with filtfilt matlab command?

clear all; close all; clc;

fs=8192*2; % sampling frequency

dt = 1/fs; % sample time

T=8; % duration of the signal

Nt = T*fs; % total number of samples

t = 0:dt:T-dt; % time vector

f0 = 500;

fT = 4000;

finst = linspace(f0,fT, length(t)).';

phi = 2*pi*cumsum(finst)*dt;

fc = 10; % cutoff

N=round(fs/fc);

% source 1

a1 = 1;

b1 = 6000;

c1 = 2;

c2 = 1.5;

A1 = ((a1 * exp(-b1 * (t - T/c1).^2 + 1j)));

q1 = A1.'.*exp(1j*phi);

%% using convolution for lowpass fltering

Afiltconv = conv(q1.*exp(-1j*phi),ones(1,N)./N, 'same');

%% using filter command for filtering

fc = fc/(fs);

N = 1/fc;

h = boxcar(N);

Warning: Rounding order to nearest integer.

h = h/sum(h);

AfiltFilter = filter(h, 1, q1.*exp(-1j*phi));

%% using filtfilt command

Afiltfiltfilt = filtfilt(h, 1, q1.*exp(-1j*phi));

figure()

plot(t,abs(A1),'LineWidth',2)

hold on

plot(t,abs(AfiltFilter),'--k','LineWidth',2)

plot(t,abs(Afiltfiltfilt),'r','LineWidth',2)

plot(t,abs(Afiltconv),'LineWidth',2)

legend('True','filter','filtfilt','conv')

set(gca,'Fontsize',14); xlabel('s'); ylabel('Amplitude')

xlim([3.5 4.5])

title('filter bandwidth of fc = 10Hz')

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Bruno Luong 2023-11-2

编辑：Bruno Luong 2023-11-2

You have not explained why it doesn't meet your expectation.

Filter is causal so it has delay. filtfilt squares the amplitude-response and has zero-phase delay, in case of rectanguar input it becomes triangular kernel convolution.

Your plots show exactly that.

I seems that you expect something that these commands do not suppose to do, but you not expressed it.

Kalasagarreddi Kottakota 2023-11-2

编辑：Kalasagarreddi Kottakota 2023-11-2

Hi @Bruno Luong,

Regarding just forward filtering, I know why it is delayed in time, which is reflected in the simulation.

Second, I had the doubt in filtfilt (forward and backward filtering), when I had rectangular input, the output is traingular. This is where I was bit confused. Thanks for your answer, but can you explain little bit more about this traingular conversion?

As you said, that filtfilt squares the amplitude, then the magnitude result, which is shown in the plot by filtfilt is wrong? Is it fc = fc/(fs); or fc = fc/(fs/2)?

Can you help me with backward filtering using filter command?

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