Filter() command is not passing through sine wave

Hello, I am having a bit of trouble getting the proper output from the filter() function in matlab. I am using two filter() commands in cascade to pass through a sine function input into a given transfer function at different n values, however, the output seems to be always coming out as 0.

After messing about a bit I figured out that it is due to the first coefficient of the numerator of the second fraction in my transfer function being 0, however I do not know what I can do to fix this as I cannot change my transfer function. Is there anything I can do?

(1 - a0) (1 - b1 + b2)z^-1

Transfer function : K* ------------------ ------------------------------------

1 - (a0)z^-1 1 - (b1)z^-1 + (b2)z^-2

This is my code for passing the input through to the fiilter functions, cascaded

The calculation of the variables is pretty long but this is one set of the variable values;

K = a = 0.9589

(1 - a0) = b = 0.8137

a0 = c = 0.1863

(1 -b1 + b2) = d = 3.2863

b1 = e = -1.517

b2 = f = 0.7697

input = sin(2*pi*1000*n/48000) (n equals 1 for this set of variables)

function dissy = displacement_output(a,b,c,d,e,f,input)

    lpf_n = [a*b];
    lpf_d = [1 -c];

    rf_n= [0 d];   % first coefficent is zero
    rf_d = [1 -e f];

    z = filter(lpf_n,lpf_d,input);
    dissy = filter(rf_n, rf_d, z);

end

Currently dissy is producing the value 0 for all values of n, which shouldn't be the case,

If anybody could clarify any of what is happening to me that would be great ahahah, thanks

Note: All the variables are values that i have calculated and double checked

[Paul]

Hi Sajith,

Instead of screenshots, just copy and paste all of your code into your question. Preferably, click the left-most icon in the code bar in the toolstrip to format as code. That way people can copy exactly what you did and be more effective in providing help.

[Sajith Sen]

yupp sorry about that, I've edited it now

Answer 1

[Askic V]

You do understand that "input = sin(2*pi*1000*n/48000) (n equals 1 for this set of variables) " means input is not sine wave but only a number, in this case (0.1305), do you?

Without going into details of your logic about the calculating the coefficients, based on your current code, I would test it like this:

clear 
clc
close all
a = 0.9589;
b = 0.8137;
c = 0.1863;
d = 3.2863;
e = -1.517;
f = 0.7697;
% make this a sine wave
n = 1:100;
input = sin(2*pi*1000*n/48000);  

subplot(2,1,1)
plot(n,input);

dissy = displacement_output(a,b,c,d,e,f,input);
subplot(2,1,2)
plot(n,dissy);

function dissy = displacement_output(a,b,c,d,e,f,input)
    lpf_n = [a*b];
    lpf_d = [1 -c];
    rf_n= [0 d];   % first coefficent is zero
    rf_d = [1 -e f];
    z = filter(lpf_n,lpf_d,input);
    dissy = filter(rf_n, rf_d, z);
end