PERIODINDEX Returns time index in increments of instantaneous period. INDEX=PERIODINDEX(DT,OMEGA,N) returns a time index INDEX that skips every N times the instantaneous period 2*pi/OMEGA. OMEGA is a column vector of instantaneous frequency in radians per unit time as computed by INSTMOM. DT is the sample time, a scalar. The units of 2*pi/OMEGA should be the same as the units of DT. Then PERIODINDEX constructs an index into a vector of the same length as OMEGA that skips every N times the instantaneous period 2*pi/OMEGA. Thus N=1 returns an index with one sample per period, etc. The first and last N periods are omitted from the index. Specifically, the first value of INDEX is N times first value of 2*pi/OMEGA, and the last value of INDEX is not larger than the length of OMEGA minus N times the last value of 2*pi/OMEGA. If NaNs are found within the OMEGA, this is interpreted as being separate ridges output by RIDGEWALK. Then INDEX will be a cell array with the period index for each ridge of OMEGA in a separate cell. INDEX=PERIODINDEX(OMEGA,N) also works, in which case DT is taken to be as unity. Thus OMEGA must have units of radians per sample interval. PERIODINDEX is useful with ELLIPSEPLOT for plotting ellipses a specified number of periods apart. ___________________________________________________________________ Cell array input / output PERIODINDEX returns cell array output given cell array input. That is, if OMEGA is a cell arrays of length K, containing K different numerical arrays, then INDEX will also be cell arrays of length K. In this case DT may be either a scalar, or an array of length K. If the cells of OMEGA are found to contain NaNs, these are interpreted as separating individual ridges, and INDEX will in this case be a cell array of length OMEGA with each element containing one cell per ridge. ___________________________________________________________________ See also ELLIPSEPLOT. Usage: index=periodindex(dt,omega,N); __________________________________________________________________ This is part of JLAB --- type 'help jlab' for more information (C) 2011--2016 J.M. Lilly --- type 'help jlab_license' for details