Jaime,
The problem is in the ContourPlot and not in your Sinc function. I would report this as a bug to Support.
This works fine:
jnk = Flatten[
Table[{t, u, shannonIPB[t, u]}, {t, 1, 10, .1}, {u, 1, 10, .1}],
1];
ListContourPlot[jnk, GridLines -> {{1, 10}, None},
PlotLegends -> Automatic]
using any of your Sinc definitions. In fact even this works:
sincI[x_] := If[x == 0, 1, Sin[Pi *x]/(Pi * x)]
shannonIPI[v_, w_] =
Total[#3* sincI[(v - #1)/dDelta]*sincI[(w - #2)/dDelta] & @@@
interpolatedData];
jnk = Flatten[
Table[{t, u, shannonIPI[t, u]}, {t, 1, 10, .1}, {u, 1, 10, .1}], 1];
ListContourPlot[jnk, GridLines -> {{1, 10}, None},
PlotLegends -> Automatic]
I don't think you ever get close to zero in the 10th decimal place -- it either a big number or exactly zero.
I am not sure why the piecewise throws off the ContourPlot Algorithm.
Regards,
Neil