Error executing harmonic product spectrum - matlab

Question

I've been trying to find the fundamental notes using Harmonic Product Spectrum in MATLAB. I came across an algorithm and tried using it. I tested it with C Major scale (piano) with the notes
C4 D4 E4 F4 G4 A4 B4 C5 B4 A4 G4 F4 E4 D4 C4
I get the correct (almost close) frequency values for all the notes except the last C4 note because at that point i get the error
Matrix dimensions must agree.
This is the line that shows the error
f_ym = (1*seg_fft) .* (1.0*seg_fft2) .* (1*seg_fft3) .* (1*seg_fft4);
I'm not quite sure what's wrong here..
I found the note onsets and performed FFT on each onset and used that for the harmonic product spectrum. This is the part that does the HPS
h = 1;
for i = 2:No_of_peaks
song_seg = song(max_col(i-1):max_col(i)-1);
L = length(song_seg);
NFFT = 2^nextpow2(L); % Next power of 2 from length of y
seg_fft = fft(song_seg,NFFT);%/L;
%HPS
seg_fft = seg_fft(1 : size(seg_fft,1) / 2);
seg_fft = abs(seg_fft);
%HPS: downsampling
for i = 1:length(seg_fft)
seg_fft2(i,1) = 1;
seg_fft3(i,1) = 1;
seg_fft4(i,1) = 1;
% f_x5(i,1) = 1;
end
for i = 1:floor((length(seg_fft)-1)/2)
seg_fft2(i,1) = (seg_fft(2*i,1) + seg_fft((2*i)+1,1))/2;
end
for i = 1:floor((length(seg_fft)-2)/3)
seg_fft3(i,1) = (seg_fft(3*i,1) + seg_fft((3*i)+1,1) + seg_fft((3*i)+2,1))/3;
end
for i = 1:floor((length(seg_fft)-3)/4)
seg_fft4(i,1) = (seg_fft(4*i,1) + seg_fft((4*i)+1,1) + seg_fft((4*i)+2,1) + seg_fft((4*i)+3,1))/4;
end
%HPS, PartII: calculate product
f_ym = (1*seg_fft) .* (1.0*seg_fft2) .* (1*seg_fft3) .* (1*seg_fft4);
%HPS, PartIII: find max
f_y1 = max(f_ym);
for c = 1 : size(f_ym)
if(f_ym(c, 1) == f_y1)
index = c;
end
end
% Convert that to a frequency
f_y(h) = (index / NFFT) * FS
h=h+1;
f_y = abs(f_y)';
end
Well I'm trying to find the fundamental frequency in the presence of harmonics. Harmonic product spectrum is one way of doing it and that is what is being implemented in the above code. When I do the multiplication the size of seg_fft seems to be half the size of seg_fft2, seg_fft3, seg_fft4.
I don't know how I can make the dimensions to be of the same size.. That is where I need some help..
Would really appreciate some quick help. Thanx in advance :)

Answer 1

It is hard to understand exactly what you are trying to do without more explanation. But you get an error while trying to do element-wise multiplication of several matrices. This error message means that not all matrices have the same size, which is required for element-wise operations. To debug this kind of errors, the easiest is usually to check the sizes of the various matrices. So add these lines just before the line that gives the error:
disp(size(seg_fft))
disp(size(seg_fft2))
disp(size(1*seg_fft3))
disp(size(seg_fft4))
This should probably show you that one of them has a size that is different from what you expect. After this, try to fix your for-loops so that they all have the same size.