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

Chapter 3: Spectrum AnalysisAnalysisresolution:Filter Bandwidthand Frame LengthThe Filter Bandwidth and Frame Length parameters provide two alternative(and equivalent) ways to control the tradeoff between analysis resolution inthe time and frequency dimensions. Filter bandwidth is the bandwidth (in Hz)of the individual analysis filters in the filterbank simulated by the short-timeFourier transform (STFT) with the selected frame length (see Appendix B). Thefilter bandwidth is also influenced by the choice of window function (seediscussion of the Window Function parameter below). Frame length is theduration (measured in mS or points 1 ) of each successive interval over whichthe spectral composition of a signal is estimated. The Filter Bandwidth andFrame Length fields in the dialog box are coupled to each other: changing oneof them automatically changes the other. Improved analysis resolution ineither dimension (time or frequency) can only be obtained at the expense ofresolution in the other dimension.For computational efficiency, Canary requires that the number of individualpoints (digital samples) in each frame be a power of 2 (i.e., 16, 32, 64, ...).Pressing the mouse button on the frame length field pops up a menu fromwhich you can choose the desired frame length. You can choose whether tohave the frame length displayed in points or milliseconds using the pop-upmenu that initially reads Points. The Frame Length parameter cannot be set toa value larger than the FFT Size.In a spectrum, where time is not represented, you may want to use a narrowfilter bandwidth (large frame length) to achieve greater frequency resolution.Keep in mind, however, that the spectrum that is calculated is the averagespectrum over the duration of a frame. Long frame lengths may thus “blur”spectral peaks in a sound when frequency is changing rapidly.In a spectrogram, where you are interested in frequency variations with time,the “best” choice of frame length depends in part on the nature of the signal,and on what features you are most interested in observing or measuring. Ifyou are most concerned with precise frequency measurements, you willprobably want to choose a small filter bandwidth (hence long frame lengthand poorer time resolution). If you want better time resolution, select a shorterframe length; the filter bandwidth will then be larger (poorer frequencyresolution).Figures 3.2 and 3.3 show pairs of spectrograms and spectra that differ only inanalysis resolution. See Appendix B for further discussion and more examplesof the effect of varying filter bandwidth and frame length.1 A point is a single sample taken by the A/D converter.Canary 1.2 User’s Manual 45