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

Chapter 3: Spectrum AnalysisBoxy vs. smoothCanary can display spectrograms in either of two styles, “boxy” or “smooth.”In a boxy spectrogram, each actual data point on the spectrogram grid isrepresented by a rectangular gray box. The width and height of the boxesdepend on the grid resolution in the time and frequency dimensionsrespectively. Grid resolution is determined in the time dimension by framelength and frame overlap and in the frequency dimension by FFT size, asdiscussed in the section above on “Grid resolution.” The left edge of each boxrepresents the beginning of a frame; the top edge is located at the analysisfrequency whose amplitude is given by the darkness of the box. The size andvisibility of the boxes on the screen depends on the size of the entirespectrogram on the screen, which in turn depends on the window size and thesignal length (which affects the time dimension only), and on the degree ofzoom invoked (by using the zoom and stretch controls). Figure 3.15 shows aboxy spectrogram of the entire time and frequency range of a 1.5-secondsignal and a portion of the spectrogram after a zoom; individual boxes areindistinguishable in the full-scale spectrogram, but are clearly visible in themagnified portion.(a)(b)Figure 3.15. A boxy spectrogram at two different magnifications. (a) Song ofa western meadowlark, digitized at 22.3 kHz. Filter bandwidth = 176 (framelength = 512 points = 23.0 mS), time resolution = 5.8 mS (overlap = 75%),frequency resolution = 43.5 Hz (FFT size = 512 points), window = Hamming.(b) Magnified view of section outlined in (a).In a smooth spectrogram, the darkness of each individual screen pixel isdetermined by bilinear interpolation between the amplitude (or logamplitude) values calculated at the grid points. Each time the spectrogram isresized, the grayscale values for individual screen pixels are recalculated.Thus no matter how much you stretch a smooth spectrogram, you will not seesharp-edged boxes as you would with a boxy spectrogram. Displaysmoothing is not a substitute for finer grid resolution, however. Both maymake a spectrogram more esthetically pleasing, but only finer grid resolutionCanary 1.2 User’s Manual 61