void mergeNoise(string input_file = "mergeNoise.txt", const int kNMerge=788,
		const char *output_file="mergeNoise.root") 
{
  const int kNChan = 32;

  // help variables
  char dummy[20], infostr[50];

  // variables to be stored in tree
  int iChip = 0;
  int nChan = 0;
  int iNent[kNChan];
  float hMean[kNChan];
  float hRMS[kNChan];
  float gMean[kNChan];
  float gSigma[kNChan]; // all in ADC

  float avehRMS;
  float convFactor; // to electrons

  char desc[20], filename[200];

  // root stuff
  TFile *f = new TFile(output_file,"RECREATE");
  TTree* tree = new TTree("tree", "noise info");

  tree->Branch("iChip",&iChip,"iChip/I");
  tree->Branch("desc",&desc,"desc/C");
  tree->Branch("filename",&filename,"filename/C");

  tree->Branch("nChan",&nChan,"nChan/I");
  sprintf(infostr,"iNent[%d]/I", kNChan);
  tree->Branch("iNent", iNent, infostr);
  sprintf(infostr,"hMean[%d]/F", kNChan);
  tree->Branch("hMean", hMean, infostr);
  sprintf(infostr,"hRMS[%d]/F", kNChan);
  tree->Branch("hRMS", hRMS, infostr);
  sprintf(infostr,"gMean[%d]/F", kNChan);
  tree->Branch("gMean", gMean, infostr);
  sprintf(infostr,"gSigma[%d]/F", kNChan);
  tree->Branch("gSigma", gSigma, infostr);

  tree->Branch("avehRMS",&avehRMS,"avehRMS/F");
  tree->Branch("convFactor",&convFactor,"convFactor/F");

  ifstream input;
  input.open(input_file);

  int imV = 20; // same for all files in standard calib/mass testing

  int nfiles = 0;
  for (int i = 0; i < kNMerge; i++) {
    input >> desc >> iChip >> filename;

    convFactor = 2.14 * 6241.5 / imV; // 6241.5 is approx # of electrons per fC, 2.14 mV / ADC

    ifstream in;
    in.open( filename);
    // skip header line
    in >> dummy >> dummy >> dummy >> dummy >> dummy >> dummy;

    avehRMS = 0;
    int iChan = 0;

    for (iChan=0; iChan<kNChan; iChan++) {
      iNent[iChan] = 0; 
      hMean[iChan] = 0; 
      hRMS[iChan] = 0;  
      gMean[iChan] = 0; 
      gSigma[iChan] = 0;
    }

    nChan = 0;
    while (in.good() && nChan<kNChan) {

      in >> iChan;
      if (!in.good() && iChan<kNChan) break;

      in >> iNent[iChan] >> hMean[iChan] >> hRMS[iChan] >> gMean[iChan] >> gSigma[iChan];

      avehRMS += hRMS[iChan];
      nChan++;
    }

    if (nChan > 0) {
      avehRMS /= nChan;
      tree->Fill();
    }

    in.close();
    nfiles++;
  }
  input.close();

  cout << " merged " << nfiles << " files " << endl;

  // Save root file also
  f->Write();
}