Re: Butter Command - Maximum Order and Cut Off Frequency - Issues

[Przemek Klosowski]

On 01/18/2018 06:56 PM, Shadders wrote:
> Code that replicates failure - simple
> =================================
> nc = 5;   % Number of cycles of the waveform
> fs = 192000;
> ts = 1/fs;
> n = 0:192000;               % Variable used for indexing
> x = zeros(1, length(n));  % Input signal initialisation
> [b,a] = butter(12, 0.98);        % Create the butterworth response
> k1 = fs/1000;
> k1n=0:(nc*k1);
> x1k = sin(2*pi*k1n/k1);
> x(1, 1000:1000+length(x1k)-1) = x1k;
> y = filter(b, a, x);            % Use the filter function
> figure(1);
> plot(n, y)
> =================================
There's an interesting numerical instability around 0.972:

 for d=0.96+[.010:.0001:.015]; [b,a] = butter(12,d );     y = filter(b, 
a, x); plot(n(500:3600), y(500:3600)),title(d); d,pause(.5); end

I have no idea what is going on but it's kind of pretty :)