code: def preview(self):
          
          msg = self.ui.trackview.model().verify()
          if msg:
              answer = QtGui.QMessageBox.warning(self, "Bummer", 'Problem: {}.'.format(msg))
              return
          stim_signal, atten, ovld = self.ui.trackview.model().signal()
          fig = SpecWidget()
          fig.setWindowModality(2) # application modal
          fig.updateData(stim_signal, self.ui.trackview.model().samplerate())
          fig.setTitle('Stimulus Preview')
          fig.show()
          self.previewFig = fig
docs: Assemble the current components in the QStimulusModel and generate a spectrogram 
          plot in a separate window