what is the correct formula of (sum of ...from...to...) in matlab

    snrin=(-5:5:55);                %initial snr
    N=length(ecg);                  % length of ecg signal 
    n=(0:N-1);
    e=sum (ecg^2) from 0 to N-1;    **% I want to write this formula correctly**
    f=sum (W^2) from 0 to N-1;      *% also this*
    A=sqrt(e/(10^(snrin/10))*f);    *% also this one*

[James Tursa]

How is the indexing from 0 to N-1 related to the variable "ecg"? I.e., what would an index of 0 mean? Same question for the other sums.

[Good mind]

ECG is a signal sampled with 360hz. 0 to N-1 i.e means along the length of ecg signal

Answer 1

[Walter Roberson]

e = sum( ecg(1+n).^2 );
f = sum( W(1+n).^2 );

[Good mind]

thank you for the answer. how about A ??

[Walter Roberson]

      A=sqrt(e./(10^(snrin/10))*f);    *% also this one*

[Good mind]

thank you sir. i don't know how to deal with this error

(Error using  * 
Inner matrix dimensions must agree.

Error in noisyecg (line 26)
    ecgnoisy=ecg+A*(W);

This is my code. can you help me please

%ecg and noisy ecg

clc;
    clear all;

      load m105a.m;
      ecg=m105a;
      subplot(3,2,1);
      plot(ecg); title ('ECG');
      xlabel('samples'); ylabel('magnitude');

      subplot(322);
      load nnoise.m;
      plot(nnoise,'k');
   % using shaping filter
      subplot(323);
      W = filtfilt(60,120,nnoise);
      plot (W,'k');
   % noisy ecg  

      snrin=(-5:5:55);                %initial snr
      N=length(ecg);
      n=(0:N-1);
     e = sum( ecg(1+n).^2 );
     f = sum( W(1+n).^2 );
      A=sqrt(e./(10.^(snrin/10))*f);   %the factor A in function of e and f
      subplot(324);
      ecgnoisy=ecg+A*(W);
      plot(ecgnoisy,'k');title('Noisy signal');

[Walter Roberson]

e and f will be scalars, snrin is 1 x 13, so A is going to be 1 x 13. W is an unknown size because it derives from nnoise.m which is a "black box" to the volunteers here.

In order to be use algebraic matrix multiplication, A*W, the "inner dimensions" of A and W must match: size(A,2) == size(W,1) . We know that size(A,2) is 13, so size(W,1) would have to be equal to 13. But is it?

I would suggest that you want

ecgnoisy = repmat(ecg(:), 1, size(A)) + repmat(W(:), 1, size(A)) .* repmat(A, numel(W), 1) );

[Good mind]

thank you sir. i try with your last suggestion but also i get this error ""Error using .* Matrix dimensions must agree"".

[Walter Roberson]

ecgnoisy = repmat(ecg(:), 1, length(A)) + repmat(W(:), 1, length(A)) .* repmat(A, numel(W), 1);

[Good mind]

thank you so much