How does resample exactly work?

Hi,

I try to find out, what resample is exactly doing.

Therefore, I calculate the filter coefficients with resample.

Then, I construct a constant signal and calculate the steady state zi with the filter, the constant signal and the filter coefficients. After that, I use the filter with the data and zi to get the filtered signal.

I assumed, that the filtered signal and the resampled signal should be the same, but it is not the case. What do I wrong?

                    [y,b] = resample(data', 1, factor);
                    const_signal = data(1)*ones(1,length(b));  
                    [~,zi] = filter(b,1.0,const_signal);   
                    x = filter(b,1.0,data,zi);     
                    x = x(1:factor:end);

In this code, x and y are not the same. But I do not understand, why?

Best regards :)

Answer 1

[Mathieu NOE]

hello

everything is explained if you type help resample

resample Resample uniform or nonuniform data to a new fixed rate.

Y = resample(X,P,Q) resamples the values, X, of a uniformly sampled

signal at P/Q times the original sample rate using a polyphase

antialiasing filter. If X is a matrix, then resample treats each

column as an independent channel.

have you tried the examples ?

   % Example 1:
    %   Resample a sinusoid at 3/2 the original rate.
 
    tx = 0:3:300-3;         % Time vector for original signal
    x = sin(2*pi*tx/300);   % Define a sinusoid
    ty = 0:2:300-2;         % Time vector for resampled signal
    y = resample(x,3,2);    % Change sampling rate
    plot(tx,x,'+-',ty,y,'o:')
    legend('Original','Resampled');
    xlabel('Time')
 
    % Example 2:
    %   Resample a non-uniformly sampled sinusoid to a uniform 50 Hz rate.
 
    Fs = 50;
    tx = linspace(0,1,21) + .012*rand(1,21);
    x = sin(2*pi*tx);
    [y, ty] = resample(x, tx, Fs);
    plot(tx,x,'+-',ty,y,'o:')
    legend('Original','Resampled');
    xlabel('Time')