Polyphase decimation matlab I am new to DSP, and currently trying to implement a simple polyphase decimation program in matlab.

M is decimation factor, x is input (for now it is a square wave 8096 samples), y_polyDec is result from polyphase, output is result from regular filter then decimation.

code :

FilterBank_LPF = fir1(127, 1/M);
y_polyDec = zeros(8096/M, 1);

for i = 1:M

    p(:, i) = FilterBank_LPF(i:M:end);
    x(:, i) = input(M-i+1:M:end);
    y_polyDec = filter(p(:, i), 1, x(:, i))+ y_polyDec;

end

out_LPF = filter(FilterBank_LPF, 1, input);
output = downsample(out_LPF, M);

%===========================================


If I compare "output" and "y_polyDec" (either comparing the decimated sample values or frequency spectrum using fvtool) I notice there are differences in the values. I was wondering if my code is wrong, or am I still missing something.

Thank you
The problem is in the input signals to your polyphase filters. They should be delayed and subsampled versions of the input signal, like in the following code fragment:

M=8;    % downsampling factor
L=256;  % length of input signal, integer multiple of M
x=randn(L,1);  % input signal
h = fir1(127, 1/M);
yp = zeros(L/M, 1);

for i = 1:M,
    xtmp = [zeros(i-1,1); x];  % delayed input signal
    tmp = filter( h(i:M:end), 1, xtmp(1:M:L) );  % polyphase filtering
    yp = yp + tmp;  % accumulate outputs of polyphase filters
end

% compare with standard filtering
y = filter ( h, 1, x);
yM = downsample( y, M );

n=1:L/M;
plot(n,(yM-yp)/max(yM))