.TH COHERENCE 1  "24 July 2002" "WFDB 10.2.7" "WFDB Applications Guide"
.SH NAME
coherence \- estimate coherence and cross-spectrum of two time series
.SH SYNOPSIS
\fBcoherence -i\fR \fIfile\fR [ \fIoptions ...\fR ]
.SH DESCRIPTION
.PP
\fBcoherence\fR 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 \fIfile\fR 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.
.PP
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 \fIPrograms for Digital Signal
Processing\fP, 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 \fIPrograms for Digital Signal
Processing\fP.
.PP
\fIOptions\fR are:
.TP
\fB-f\fR \fIfrequency\fR
Specify the sampling frequency in Hz (default: 250).
.TP
\fB-n\fR \fIn\fR
Process the input in overlapping chunks of \fIn\fR samples (default: 1024).
For best results, \fIn\fR should be a power of two.
.TP
\fB-v\fR
Print column headings.
.TP
\fB-x\fR \fIsx sy\fR
Specify multiplicative scale factors for the two time series
(defaults: 1).  A reasonable choice is to use the reciprocals of the
standard deviations of the respective time series if these differ
significantly.
.PP
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 \fB-n\fR option) will have a significant effect;  some
experimentation may be needed to determine an appropriate chunk size
in each case.
.SH SEE ALSO
.PP
\fBfft\fR(1)
.SH AUTHOR
George B. Moody (george@mit.edu)
.SH SOURCE
http://www.physionet.org/physiotools/wfdb/psd/coherence.c