WFDB Applications Guide - PhysioNet

COHERENCE(1) WFDB Applications Guide COHERENCE(1)
NAME
coherence − estimate coherence and cross-spectrum of two time series
SYNOPSIS
coherence -i file [ options ... ]
DESCRIPTION
coherence estimates the coherence and cross-spectrum of a pair of real-valued time series; as a byproduct
of its calculation of coherence, it also estimates the autospectra of each of its input time series. The file
argument specifies the name of a text file containing the samples of the series in two columns. The standard
output contains five columns of numbers (optionally preceded by column headings), which are frequency
(Hz), coherence, cross-spectral power (dB), autospectral power (dB) of the first time series, and
autospectral power (dB) of the second time series.
SEE ALSO
fft(1)
This program is based on a Fortran program by C.R. Arnold, G.C. Carter, and J.F. Ferrie, as described in ‘A
coherence and cross-spectral estimation program’, by G.C. Carter and J.F. Ferrie, in Programs for Digital
Signal Processing, edited by the Digital Signal Processing Committee of the IEEE ASSP Society (New
York: IEEE Press, 1979). The functions fft842() and its auxiliary functions r2tx(), r4tx(), and r8tx(), are
based on Fortran subroutines by G.D. Bergland and M.T. Dolan, as described by them in ‘Fast Fourier
transform algorithms’, also included in Programs for Digital Signal Processing.
Options are:
-f frequency
Specify the sampling frequency inHz(default: 250).
-n n Process the input in overlapping chunks of n samples (default: 1024). For best results, n should be
apower of two.
-v Print column headings.
-x sx sy Specify multiplicative scale factors for the two time series (defaults: 1). Areasonable choice is to
use the reciprocals of the standard deviations of the respective time series if these differ significantly.
Note that the scale factors generally have little or no visible effect on the coherence or on the shape of the
spectra. The choice of chunk size (using the -n option) will have a significant effect; some experimentation
may be needed to determine an appropriate chunk size in each case.
AUTHOR
George B. Moody (george@mit.edu)
SOURCE
http://www.physionet.org/physiotools/wfdb/psd/coherence.c
12 24 July 2002 WFDB 10.2.7