After all allowable matches have been made between the two frames, there may be some peaks in each frame that were not linked. Some peaks in the earlier frame may have frequency drift ranges that exclude every peak in the later frame. Some peaks in the later frame may lie outside the frequency drift range of every peak in the earlier frame. Since the short-time spectra are computed and searched for peaks independently for each frame, and since no harmonicity is assumed, the number of peaks may change from frame to frame, in which case it is impossible to link every peak in each frame. Peaks in the earlier frame which are not linked forward are considered to mark the "death" of a track. When a track dies at a peak in one frame, a null (zero amplitude) peak of the same frequency is inserted in the next frame, and these two peaks are linked. Death peaks mark the termination of active sinusoidal components. Similarly, when a peak has no link in the preceding frame, it is considered to mark the "birth" of a track. A null peak of the same frequency is inserted in the preceding frame, and these two peaks are linked. Birth peaks mark the appearance of new active sinusoidal components (McAulay and Quatieri 1985, Maher 1989, Serra 1989).
The Frequency Drift for a Lemur analysis may be set directly in the Peak Selection Parameters panel, accessed from Lemur's Parameters menu.