Signal is not getting filtered properly

Question

Giggs B. 2021-7-23

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/884244-signal-is-not-getting-filtered-properly

编辑： Chunru 2021-7-23

Hi,

I am trying to implement 4th order high pass elliptical filter with cut-off frequency at 500/510 Hz at 20 dB passband and 25 dB stopband attenuation. I have attached the plot for my original data and data after filtering. But if you see the filtered data, there is almost no data after 500

[b,a]=ellip(4,20,25,500/510,'high');
fvtool(b,a)
y2=audioread('fr1.mp3');
y_b2=filter(b,a,y2);
fs=44100;
N=length(y_b2);
time=N/fs;
t = linspace(0, time, N);
referenceLength=length(y_b2);
fref=fs*(1:referenceLength)/referenceLength;
R=fft(y_b2,referenceLength);
stor=R(1);
R(1)=0;
absR=abs(R);
plot(fref,absR);
xlim([0 2000]); 
ylim([0 200]); 
xticks([0 200 400 600 800 1000 1200 1400 1600 1800 2000]); 
yticks([0 25 50 75 100 125 150 175 200 300 400 500 600 700 800 900 1000]); 
grid on;
xlabel('Frequency(Hz)');
ylabel('Amplitude');

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Giggs B. 2021-7-23

My desired freq is variable since I am still in developng stage of my project and finding the best cut-off frequency. But for now my desired frequencies are above 500 Hz and undesired frequencies are below 500 Hz. My desired filter response is also variable but considering 1 dB passband and 25 dB stopband attenution, I want the transition phase to be as small as possible so as to prevent frequencies below 500 Hz as much as possible, also I don't mind attenuating my signal by some amount (<20 dB for example).

But I don't understand why the transition phase is ~2.5 KHz instead of 500 Hz.

Chunru 2021-7-23

编辑：Chunru 2021-7-23

the cut off is at (500/510) in normalized freq and (500/510)*fs/2=21618Hz (not ~2.5kHz) while you want a cut-off at 500Hz.

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Answer 1

Chunru 2021-7-23

1
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https://ww2.mathworks.cn/matlabcentral/answers/884244-signal-is-not-getting-filtered-properly#answer_752114

在 MATLAB Online 中打开

Try design the filters as follows. The filter will attenuate the frequency content below 500Hz. When plotting spectrum, you may consider log scale of the ampiltude (20*log10(abs(filtered_date))).

fs=44100;

% The filter to have 1dB ripples in passband, 40 dB attenuation in stop

% band, the cut-off freq of 500Hz. (you may adjust the value as you want)

[b,a]=ellip(4, 1, 40, 500/(fs/2),'high');

%fvtool(b,a)

freqz(b, a, 8192, fs);