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

Chapter 3: Spectrum Analysis(a)(b)Figure 3.5. Effect of varying frequency grid resolution (FFT size) on spectra.The two single-frame spectra were taken at approximately 880 mS along thetime axis shown in Figure 3.4 and differ only in frequency grid resolution (FFTsize). In both spectra, filter bandwidth = 353 Hz (frame length = 256 points =11.5 mS), time grid resolution = 5.7 mS (frame overlap = 50%), window =Hamming. (a) Frequency grid resolution = 86.9 Hz (FFT size = 256 points). (b)Frequency grid resolution = 10.9 Hz (FFT size = 2048 points).Time grid resolution (frame overlap)The time interval (in mS) between successive gridpoints is the time betweenthe beginnings of successive frames. Frames can be overlapping, contiguous(0% overlap) or separated by time intervals that are omitted from the analysis(negative overlap).Frame overlap is usually expressed as percent of frame length. For example,an overlap of 50% means that each frame begins halfway through thepreceding fame. An overlap of -100% means that one frame of data is skippedbetween successive frames that are analyzed; -300% skips three frames, and soon. The relationship between time grid resolution and frame overlap is givenbytime grid resolution = frame length * (100% - overlap%)where frame length is measured in mS. The Time and Overlap choices in thedialog box are coupled so that you can specify time resolution either directly,by choosing a value from the Time pop-up menu, or indirectly, by specifyingan Overlap value. Using the units pop-up menu, you can specify themeasurement units for time resolution as either mS (the default) or points.Measurement units for overlap are percent (of frame length), mS or points.For computational efficiency, Canary requires that non-negative overlaps beequal to 100(1 - (1/2 n )), where n ≥ 0 (e.g., 0%, 50%, 75%, 87.5%, 93.8%,...).Negative overlaps must be equal to -100(2 n -1) (e.g., -100%, -300%, -700%,...),where n ≥ 0.48 Canary 1.2 User’s Manual