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# Questions about lowpass filters using 'butter' function in Matlab

By : Hamedsbt
Date : November 16 2020, 06:23 AM
This might help you By default, butter produces a discrete-time filter design. Therefore the transfer function is defined in terms of z (z-transform), not s (Laplace transform).
A discrete-time Butterworth filter of order n has an n-order zero at z=-1 and n poles within the unit circle. This is in accordance with your results. (In contrast, a continuous-time Butterworth filter would have an n-order zero at infinity and n poles in the left-hand unit semicircle).
code :

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## Where can I find good tutorials on writing audio DSP filters (lowpass, etc)?

Date : March 29 2020, 07:55 AM
I hope this helps . The link you really want from MusicDSP is http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt
I also recommend getting Lyon's Understanding Digital Signal Processing. I am a bit biased, though, since I was a reviewer for the second edition (but I think a third edition came out recently).

## Matlab 3 dB 12 hertz lowpass filter

By : Henke van den Boer
Date : March 29 2020, 07:55 AM
With these it helps For your application I would strongly recommend trying a plain butterworth filter, the Matlab syntax is:
code :
``````[b,a]=butter(n,Wn)
``````
``````% assume x is time and y is speed
Ts = mean(diff(x));
Fs = 1/Ts;
% for butter, we need Wn, which is the cutoff frequency
% where 0.0 < Wn < 1.0, where 1.0 is half the sample rate
% The cutoff is the -3 dB point of the filter
% Wn = fCutOff/(Fs/2)
% for a cutoff of 12 Hz
fCutOff = 12/(Fs/2);
[b,a] = butter(1,fCutoff);
% plot the filter frequency response to see what it looks like
% use 512 points to plot it
freqz(b,a,512,Fs)
``````

## Equivalent function in python for MATLAB's lowpass() function?

By : user3422657
Date : March 29 2020, 07:55 AM
wish help you to fix your issue Scipy is probably the best tool for this. You can use scipy's signal processing libraries.
From their docs, they even offer matlab style filter desing.

## what is the theory behind matlab butter function?

By : pavan kumarSp
Date : March 29 2020, 07:55 AM
like below fixes the issue I assume you've looked at the Matlab help page.
Background

## Issues with MATLAB butter

By : P.Gaur
Date : March 29 2020, 07:55 AM
I wish did fix the issue. Just a guess but is there any chance you've defined your own poly function? which('poly') should point to some Matlab directory unless you've defined it elsewhere, potentially as a variable?. For me on a newer version that path is something like \$MATLAB/toolbox/matlab/polyfun/poly.m You can also edit the poly function edit poly and verify that there is one output argument for the function.
Also, I'd advise against ever writing decimal numbers without a leading zero. It took me way to long to figure out what [.15,.3] was. Instead write [0.15,0.3] or even just [0.15 0.3] Edit: I just realized that is an example in Matlab ... my point stands but the toolbox author should know better ...