matlab calculating delta between plotted lines - matlab

Question

I am producing this graph (reduced code version) :
k = 1000
r = [100 220 470 1*k 2200 4700 10*k 22*k 47*k 100*k 220*k 470*k 1000*k ]
unModNB = [0.72 0.746 0.801 0.92 1.16 1.69 2.78 4.6 6.45 9.1 11.2 12.4 13.2]
unModWB = [1.124 1.17 1.23 1.48 1.84 2.65 4.2 7.6 11.8 15.4 18.6 20.01 21.7]
ModNBdB = 20*log10( ModNB)
ModWBdB = 20*log10( ModWB)
semilogx( r, ModNBdB, r, ModWBdB )
grid
legend(Line 1 ', 'Line 2')
How am I able to produce another graph of the delta between line 1 and line 2 across intervals of delta across the full scale?
I want to avoid simply using the reading deltas because of the 'jumpy' graph that this meathod generates.
Many many thanks for any help,
Ed.

Answer 1

I believe what you want to do is interpolate between the unModNB and unModWB points and calculate the differences (deltas, as you say) between those. For that you can use interp1(). Try this:
k = 1000;
r = [100 220 470 1*k 2200 4700 10*k 22*k 47*k 100*k 220*k 470*k 1000*k ];
unModNB = [0.72 0.746 0.801 0.92 1.16 1.69 2.78 4.6 6.45 9.1 11.2 12.4 13.2];
unModWB = [1.124 1.17 1.23 1.48 1.84 2.65 4.2 7.6 11.8 15.4 18.6 20.01 21.7];
ModNBdB = 20*log10( unModNB );
ModWBdB = 20*log10( unModWB );
semilogx( r, ModNBdB, r, ModWBdB )
grid
legend('Line 1', 'Line 2')
sampled_x_vals = linspace(r(1),r(end),1000);
unModNB_sampled = interp1(r, unModNB, sampled_x_vals, 'pchip');
unModWB_sampled = interp1(r, unModWB, sampled_x_vals, 'pchip');
deltas = unModWB_sampled-unModNB_sampled;
figure, semilogx(sampled_x_vals,deltas), grid;
If, by 'deltas' you mean something other than difference, please clarify.