After y = filter(d , x), y is exactly the same as x. Please help filter function NOT working.

I am doing a typical Noise Filtering DSP Project. I have done an Elliptic IIR LowPass Filter using filterDesigner. When i use y = filter(d,x), the output y is the same as x. No difference at all. why is the filter function not working ?

[x, Fs] = audioread("vuvuzela.mp3"); %read audio in vector x with Fs

N = length(x);

t = (0:N-1)/Fs; % time domain range for audio

duration = N/Fs % duration of audio is 97.8370 seconds

% Plot time-domain

plot(t,x)

grid on

xlabel('Time (s)')

ylabel('Amplt')

title('Signal in time-domain')

% Periodogram Power Spectrum shows power for each frequency

w = hanning(N, 'periodic'); % Periodic Hann Window with N samples

periodogram(x, w, N, Fs,'power')

%Compute FFT freq domain

Freq = (0:N-1).*(Fs/N); % Frequency axis Freq = Fs/N [Hz] of audio signal x

Xk = fft(x,N); % FFT audio signal

Real_Xk = abs(Xk); % FFT can give complex values. we just want magnitude

plot(Freq, Real_Xk);

title('Freq Domain Plot of Audio Signal');

xlabel('Frequency, Hz');

ylabel('Amplitude');

%Denoise using Filter. Look at Periodogram Power Spectrum Estimate

%at 11kHz is cut-off freq btwn message signal & noise signal

%Denoise the x audio signal using Filter

%Create filter using >>filterDesigner

d = EllipIIRlowpass;

y = filter(d,x);

plot(t,y)

grid on

xlabel('Time (s)')

ylabel('Amplt')

title('Filtered Signal in time-domain')

periodogram(y, w, N, Fs,'power')

%sound(y, Fs);

function Hd = EllipIIRlowpass

%ELLIPIIRLOWPASS Returns a discrete-time filter object.

% MATLAB Code

% Generated by MATLAB(R) 9.11 and Signal Processing Toolbox 8.7.

% Generated on: 25-Apr-2022 01:09:50

% Elliptic Lowpass filter designed using FDESIGN.LOWPASS.

% All frequency values are in Hz.

Fs = 48000; % Sampling Frequency

Fpass = 11000; % Passband Frequency

Fstop = 11500; % Stopband Frequency

Apass = 10; % Passband Ripple (dB)

Astop = 80; % Stopband Attenuation (dB)

match = 'passband'; % Band to match exactly

% Construct an FDESIGN object and call its ELLIP method.

h = fdesign.lowpass(Fpass, Fstop, Apass, Astop, Fs);

Hd = design(h, 'ellip', 'MatchExactly', match);

end

[Star Strider]

The filter appears to work as designed here.

d = EllipIIRlowpass;

figure
freqz(d.sosMatrix, 2^16, 48000)

Fs = 48000;
x = randn(1,Fs*5);
L = numel(x);

yf = filter(d,x);

FTx = fft(x)/L;
FTyf = fft(yf)/L;
Hf = FTyf ./ FTx;

Fv = linspace(0, 1, fix(L/2)-1)*Fs/2;
Iv = 1:numel(Fv);

figure
plot(Fv, mag2db(abs(Hf(Iv))))
grid
xlabel('Frequency (Hz)')
ylabel('Magnitude (dB)')

function Hd = EllipIIRlowpass
%ELLIPIIRLOWPASS Returns a discrete-time filter object.
% MATLAB Code
% Generated by MATLAB(R) 9.11 and Signal Processing Toolbox 8.7.
% Generated on: 25-Apr-2022 01:09:50
% Elliptic Lowpass filter designed using FDESIGN.LOWPASS.
% All frequency values are in Hz.
Fs = 48000;  % Sampling Frequency
Fpass = 11000;       % Passband Frequency
Fstop = 11500;       % Stopband Frequency
Apass = 10;          % Passband Ripple (dB)
Astop = 80;          % Stopband Attenuation (dB)
match = 'passband';  % Band to match exactly
% Construct an FDESIGN object and call its ELLIP method.
h  = fdesign.lowpass(Fpass, Fstop, Apass, Astop, Fs);
Hd = design(h, 'ellip', 'MatchExactly', match);
end

.

[Vishal Sathiaseelan]

Thank you so much for quick reply. What should i do to solve this problem ? is it the filterDesigner problem ? any specific changes I could do ?

[Star Strider]

My pleasure!

I do not see a problem. The filter appears to be working as desired. Compare the fft of your original signal with the filtered signal to see the difference. If the unfiltered signal has been filtered with a similar filter, the difference may not be easily apparent. I do not have your original signal, so I cannot determine that.

[Vishal Sathiaseelan]

I need to denoise the noisy signal. I just need all frequency components below 11kHz. How can I achieve that using IIR Elliptic filter (My Lecturer said must be IIR Elliptic Filter). Based on Periodogram Power Spectrum Estimate I just need the passband which is below 11Khz, but whenever i Filter it it gives both passband and stopband. I already set "MatchExactly" to "passband". Please help me. This is the link to my Google Drive to access Noisy Signal.

https://drive.google.com/file/d/1Lo3mI_ciypYHrTqm2D7ISD8ndR7qS_9_/view?usp=sharing

[Star Strider]

The signal energy in the stopband can never be entirely eliminated. The purpose of the filter is to force the energy in the stopband to be a low as practically attainable, usually -50 to -75 dB. (Filters of all types can have stability problems or not be able to be realised if the stopband requires extreme values of attenuation and the transition region is relatively short.)

So the filter you designed is doing exactly what it is supposed to do. It needs no changes. I would just use it as it is.

[Vishal Sathiaseelan]

Noted. Thank you very much for quick reply and helpful advice. I will try out better filter design specifications to filter out the noise.