Manual Reference Pages - RACF (1)
racf - recursive autocorrelation function of contiguous frames.
racf [arguments] [file]
Autocorrelation function using a recursive "leaky integrator" filter. For the mth lag, the nth recursive update is:
y[n] = a.y[n-1] + (1-a).f[n].f[n-m]
where decay constant a = exp(-Ts/T), Ts is the sample interval in seconds, and T is the decay time constant in seconds.
The recursion computes the mean value of the product f[n].f[n-m] within an exponential window which weights past values. The window width parameter (eg. corresponding with the width of the fft analysis window) is the time constant parameter. The half life of the exponential window is given by -T.ln(0.5) = 0.693T ie. for a given time constant, T secs, the window decays to half of its current value in a period of 0.693T secs. Similarly, the window decays by 99% of its initial value in about 4.6T secs. So T can be set to accomodate an expected period.
The routine generates the autocovariance function. An optional normalization (dividing each coefficient by the zeroth coefficient) produces the autocorrelation function, (and this is then scaled up to the range [0-1000]).
The output framewidth is the given maximum acf lag.
options acf acgram
Copyright (c) Applied Psychology Unit, Medical Research Council, 1995
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|SunOS 5.6||RACF (1)||1 September 1993|