Windowing In Lemur Analysis
The first step in the Lemur analysis is windowing the signal to be analyzed. Time-domain windowing produces
scaled replicas of the window spectrum at each point in the analysis spectrum (Oppenheim and Schafer 1989,
Serra 1989).
The choice of window function is a trade-off between time and frequency resolution. Short analysis windows
provide better time resolution, but poorer frequency resolution than long windows. This trade-off is further
governed by the shape of the window spectrum, specifically the width of the main lobe and the height of the side
lobes relative to the height of the main lobe. A window with a wide main lobe makes it difficult to resolve spectral
peaks that are close in frequency. A window with high sidelobes makes it difficult to resolve low amplitude
spectral peaks in the presence of high amplitude peaks. We use the Kaiser window (Kaiser 1974), which has a shaping parameter
to control the balance between main lobe width and side lobe magnitude.
The Main Lobe Width and Sidelobe Attenuation for the Lemur analysis window may be set directly in the
Spectral Analysis Parameters panel on Lemur's Parameters menu. Lemur will notify you if your
window specifications correspond to a window that is longer than the FFT you have selected (the FFT Length is also
specfied on the Spectral Analysis Parameters panel). In this case, the analysis window will be truncated,
and is likely to produce a bad analysis, so you should halt the analysis process (using the Halt Process
option on the File menu) and select a longer FFT or a shorter window.
We have found that 18dB of Sidelobe Attenuation (the minimum for a Kaiser window) is usually sufficient, and that
the Main Lobe Width needs to be tailored to the sound being analyzed.
Examples
A cello tone at approximately 98 Hz (G2) analyzed with a Main Lobe Width of 60 Hz and a Sidelobe Attenuation of
18dB:
A cello tone at approximately 98 Hz (G2) analyzed with a Main Lobe Width of 100 Hz and a Sidelobe Attenuation
of 18dB:
Forward to Lemur Analysis Frame Length
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