Filtering a signal out of multiple frequencies without for loop

Hello MATLABers,

I am trying to filter out a humming noise from my recordings. The noise has a frequency of 20kHz and it's harmonics (40,60,80 kHz).

I want to filter out all the noisy parts with one pass over the signal and the only solution I have managed to come up with is by removing each frequency with a for loop (see code). This takes a lot of time, I need to filter about 2 hours of audio recordings with 250k sampling rate.

My question is, is there a method to filter out all the desired frequencies in one pass instead of multiple passes to save computational time.

Fs = 250000; %Sampling rate

filtered =sig; %sig is a vector of the signal to be filtered

%The frequency of the noise to be filtered
onset = 19700; 
offset = 20000; 
times = [];
n=4; %Number of harmonics (n=1: 20, n=2: 40, n=3: 60, m=4: 80)
for k = 1:n
    tic
    %Is there a filter that can remove multiple bands at onces?
    d = designfilt('bandstopiir','FilterOrder',2, ...
        'HalfPowerFrequency1',onset*k,'HalfPowerFrequency2',offset*k, ...
        'DesignMethod','butter','SampleRate',Fs);
    filtered = filtfilt(d,filtered);
    toc
    times = [times,toc];
end

Answer 1

[Raunak Gupta]

Hi,

There is a similar question which is nicely explained by Star Strider, this can help you implement the multiband filter required here. It uses filtfilt.