Int_t i;
const Int_t nbins = 1024;
Double_t xmin     = 0;
Double_t xmax     = nbins;
Double_t source[nbins];
gROOT->ForceStyle();

TH1F *d    = new TH1F("d","",nbins,xmin,xmax);

TString dir  = gROOT->GetTutorialDir();
TString file = dir+"/spectrum/TSpectrum.root";
TFile *f     = new TFile(file.Data());
TH1F *back = (TH1F*) f->Get("back1");
back->SetTitle("Estimation of background with decreasing window");
back->GetXaxis()->SetRange(1,nbins);
back->Draw("L");

TSpectrum *s = new TSpectrum();

for (i = 0; i < nbins; i++) source[i]=back->GetBinContent(i + 1);

s->Background(source,nbins,6,TSpectrum::kBackDecreasingWindow,
              TSpectrum::kBackOrder2,kFALSE,
              TSpectrum::kBackSmoothing3,kFALSE);

for (i = 0; i < nbins; i++) d->SetBinContent(i + 1,source[i]);
d->SetLineColor(kRed);
d->Draw("SAME L");

gROOT->GetListOfCanvases()->Draw()