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

Chapter 3: Spectrum Analysis(a)Hamming(b)Blackman(c)RectangularFigure 3.9. Effect of window function on spectra. The single-frame spectrawere made at a point approximately 0.30 sec along the time axis of the signalshown in Figure 3.8. All three spectra have the same clipping level, filterbandwidth = 353 Hz (frame length = 256 points = 11.5 mS), and frequencyresolution = 10.9 Hz (FFT size = 2048). Filter bandwidths: (a) 353 Hz, (b) 538Hz, (c) 174 Hz.The appearance of sidelobes in spectra of finite signals, and the use of windowfunctions to reduce their magnitude, is discussed further in Appendix B.Clipping LevelThe Clipping Level parameter allows you to specify a “noise floor” belowwhich any amplitude value is ignored. In a spectrum, any frequencycomponent less than the clipping level is displayed as being equal to theclipping level. In a spectrogram, any value below the clipping level isdisplayed as white. The Clipping Level parameter is in units of intensity dBrelative to the dB reference value specified in the Set Calibration dialog. Whenyou first use Canary, the factory default clipping level is -80 dB.There are several reasons for specifying a noise floor. First, because of thefinite precision of the digitizing process, a digitized sound always containssome error and has a limited dynamic range. For signals digitized with 8-bitsamples, the dynamic range is limited to 48 dB; for 16-bit signals, the dynamicrange is 96 dB. 1 Thus, any amplitude value in an 8-bit spectrum orspectrogram that is more than 48 dB below the highest peak in the signal must1 The dynamic range of a digitized sound is 6 dB/bit.52 Canary 1.2 User’s Manual