# ListConvolve returns only one value

Generating my first function

num = 1000;
Amp = 0.05;
time = 1.5;
width = 0.1;
T = 1.5;
(*declaration of continous function*)
With[{w = width, T = time},
pulse[x_] :=
Cos[2*Pi*x/(w*T)]*(UnitStep[x + w*T/4] - UnitStep[x - w*T/4])]
(*funciton sampling*)
funX = Table[i, {i, -T/2, T/2, T/(num - 1)}];
fun1 = pulse /@ funX + Amp*RandomReal[{-0.5, 0.5}, num];
ListPlot[Transpose[{funX, fun1}], PlotRange -> All, Filling -> Axis,
Frame -> True, FrameLabel -> {"Time [s]", "Amplitude [V]", "Pulse"},
PlotLegends -> {"Pulse"}, ImageSize -> Large] and the second one

With[{\[Delta] = 0.1},
ImpulseResponse[t_] := (1/(2.0*Pi*10.0*Sqrt[1 - \[Delta]*\[Delta]]))*
Exp[-\[Delta]*2.0*Pi*10.0*t]*
Sin[2.0*Pi*10.0*Sqrt[1 - \[Delta]*\[Delta]]*t]*HeavisideTheta[t]]
funTF = ImpulseResponse /@ funX;
ListPlot[Transpose[{funX, funTF}], Frame -> True, PlotRange -> All,
ImageSize -> Large] Now the plan was to follow this discussion using

    konv = ListConvolve[Transpose[{funX, funTF}], Transpose[{funX, fun1}]]
(*{{0.152085}}*)


but for some reason this doesn't work. The ListConvolve[] only returns one value instead of list. Any ideas why?

You need to tell ListConvolve how to handle "end conditions". Since your sequences are about the same length, everything is an end condition. Hence:

ListPlot[ListConvolve[funTF, funX, {1, -1}]]


will give the circular convolution of the sequences funX and funTF. There are several other choices for the pair of numbers (like {1,-1}) or you can directly specify the desired length of the output. You can read more about these optional arguments in the Details section of the help file.

From your comment, I guess you are expecting linear (rather than circular) convolution. You can do this by zeropadding. For example:

zeropad = Flatten[{ConstantArray[0, Length[funTF]], funX,
ConstantArray[0, Length[funTF]]}];
ListPlot[ListConvolve[funTF, zeropad, {1, -1}], PlotRange -> All]

• I think you meant ListConvolve[funTF, fun1, {1,-1}]; or num instead of {1,-1}.... but anway... I am not very satisfied with the result, because if you plot it ListPlot[ListConvolve[funTF, fun1, {1,-1}], PlotRange -> All, ImageSize -> Large, Filling -> Axis] you see an unexpected rising at the end which (in my opinion) should not be there and also isn't if you convolve continuous functions using Convolve[ImpluseResponse[t],pulse[t],t,x]. – skrat Dec 16 '16 at 14:47