How to insert new row after every 5 rows in Matlab? - frequency

Question

I get time-domain measurement data from my oscilloscope (the data is an array of size 200000x2, the first column is for time and the second column is for the measurement). The sample time is 0.1 microseconds, but after every 5 rows, it changes to 0.2 microseconds. Now I'd like to perform an FFT analysis. I'll need to insert a new row every 5 rows to accomplish this.
Could someone please show me how to create a program that inserts a new row after every 5 rows and fills the rows with the average value of the adjacent rows in Matlab?

Answer 1

Say you have data like the following:
t = [0.0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.6, 1.7, 1.8, 1.9];
data = [t.', cos(t.')];
You can use interp1 to resample the data at regular intervals as follows:
dt = 0.1; % desired sampling rate
t = t(1):dt:t(end); % desired sample locations
y = interp1(data(:,1), data(:,2), t);
plot(t, y, 'o-');

Answer 2

I don't know if there is an easier way or if the below code is is correct for all sizes and shapes.
function [mtxnew] = interleavedcopying(mtx, n1, n2)
if(nargin == 0)
clc;
% input data which has 20 rows and 3 columns
mtx = rand(15, 1);
n1 = 5;
n2 = 6;
end
nn = size(mtx, 1);
% averaging currently works for single intermediate point only
assert(n2 == n1+1);
% this code works only for the "perfect" input case
assert(rem(nn, n1) == 0);
% make into groups of n1 rows
ii = floor([0 : nn-1]/n1)
% find the new locations where the data fits into
% nn/n1 is the reason why this code will work only for the "perfect" input
iinew = ii*n2 + repmat([1:n1], [1, nn/n1])
% create a big enough matrix filled with NaNs
nnnew = nn*n2/n1 -1;
mtxnew = NaN * zeros(nnnew, size(mtx, 2))
% copy the raw data
mtxnew(iinew, :) = mtx
% now fill in the avearge
kk = n2 : n2 : nnnew
mtxnew(kk, :) = ( mtxnew(kk-1, :) + mtxnew(kk+1, :) )/2;
plot(1:size(mtxnew, 1), mtxnew, '-x', kk, mtxnew(kk, :), 's');
end