Error in butter command

Hello,

I wrote the following function:

function [output_signal] = AddDirectivityError (bat_loc_index, butter_deg_vector, sound_matrix)

    global chirp_initial_freq ;
    global chirp_end_freq;
    global sampling_rate;
    global num_of_mics; 
    global sound_signal_length;

    for (i=1 : num_of_mics)

        normalized_co_freq = (chirp_initial_freq + chirp_end_freq)/ (1.6* sampling_rate);
        A=sound_matrix  ( i, :  )  ;

        peak_signal=max(A);
        B=find(abs(A)>peak_signal/100);

        if (butter_deg_vector(i)==0)
            butter_deg_vector(i)=2;
        end

        [num, den] = butter ( butter_deg_vector(i),  normalized_co_freq, 'low');   // HERE!!!

        filtered_signal=filter(num,den, A );

        output_signal(i, :)=filtered_signal;
    end

This functions runs many-many times without any error. However, when I reach the line: [num, den] = butter ( butter_deg_vector(i), normalized_co_freq, 'low');

AND the local variables are: i=3, butter_deg_vector(i)=1, normalized_co_freq=5.625000e-001

MATLAB prompts an error says:

??? Error using ==> buttap Expected N to be integer-valued.

"Error in ==> buttap at 15 validateattributes(n,{'numeric'},{'scalar','integer','positive'},'buttap','N');

Error in ==> butter at 70 [z,p,k] = buttap(n);"

I don't understand why this problem occurs especially in this iteration. Why does this function prompt an error especially in this case?

Answer 1

[Star Strider]

Check to see that butter_deg_vector is a positive integer greater than zero. Use floor, round, ceil, or fix to be sure.

[Star Strider]

I can’t reproduce the error.

This (that I believe are the arguments you supplied) works:

[num,den] = butter(1, 5.625000e-001, 'low')

and produces:

num =

     549.2457e-003   549.2457e-003

den =

       1.0000e+000    98.4914e-003

The only thing I can think of is that somehow butter_deg_vector rounds to zero as you see it but may actually be 1+eps or something similar, the reason I suggested fix and friends.

[Image Analyst]

MB's "Answer" moved here:

Yes, it consists of positive values only.

Answer 2

[Jan]

Matlab tells you unequivocally that butter_deg_vector(3) is not a positive integer. How do you test if it is one? Note that the numbers are rounded if you display them in the standard format short f. See also:

doc format

Please try this:

d = butter_deg_vector - round(butter_deg_vector);
fprintf('%.16g\n', d)