User's Manual - Cornell Lab of Ornithology - Cornell University

Chapter 3: Spectrum AnalysisSpectrogramsSignificance of thegrayscale valuesContrast andbrightnessThe grayscale values shown in a spectrogram represent either the spectrumintensity level (in dB) or the spectrum intensity (in watts/m 2 /Hz) at eachpoint, depending on whether a Logarithmic or Quadratic amplitude axis wasselected. The numerical values for intensity level or intensity associated witheach point (which are displayed by the spectrogram measurement panel)depend on the current calibration settings. The intensity or intensity levelvalues in a spectrogram are meaningless unless the signal has been properlycalibrated according to the procedures outlined in Chapter 4. 1The basic operation of the spectrogram contrast and brightness controls isdiscussed in Chapter 1. This section provides a more detailed explanation ofhow these controls work. The information in this section is not needed to usethe controls effectively; it is provided for those who are interested in aquantitative explanation of how these controls affect the spectrogram image.To see exactly how the spectrogram and brightness controls work, first make alogarithmic spectrogram, then click anywhere on the waveform and make alogarithmic spectrum. Finally, click on the CURSORS button to turn selectioncursors on. The window should look something like Figure 3.14.Spectrogram amplitude ceilingSpectrogram amplitude floorFigure 3.14. The spectrum’s amplitude cursors delimit the amplitude “ceiling”and “floor” of the spectrogram. All amplitudes above the ceiling are displayedas black in the spectrogram; all amplitudes below the floor are displayed aswhite. The amplitude ceiling and floor can be manipulated together using thecontrast and brightness controls on the command panel, or independently bymoving the amplitude cursors in the spectrum pane. In this illustration thewaveform pane has been hidden.The amplitude (vertical axis) cursors in the spectrum pane delimit the range ofamplitudes represented by the scale of gray shades (from white to black)displayed in the spectrogram. The position of the lower cursor in the spectrumdetermines the amplitude “floor” of the spectrogram, below which all1 If you are interested only in differences in decibel levels between parts of one signal,you do not have to calibrate the signal.Canary 1.2 User’s Manual 59