.TH SPECTRUM1D l "1 Jan 2004"
.SH NAME
spectrum1d \- compute auto\- [and cross\- ] spectra from one [or two] timeseries.
.SH SYNOPSIS
\fBspectrum1d\fP [ \fIx[y]file\fP ] \fB\-S\fP\fIsegment_size\fP]
[ \fB\-C\fP[\fBxycnpago\fP] ] [ \fB\-D\fP\fIdt\fP ]
[ \fB\-N\fP\fIname_stem\fP ] [ \fB\-V\fP ] [ \fB\-W\fP ]
[ \fB\-bi\fP[\fBs\fP][\fIn\fP] ] [ \fB\-bo\fP[\fBs\fP][\fIn\fP] ]
.SH DESCRIPTION
\fBspectrum1d\fP reads X [and Y] values from the first [and second] columns on standard input
[or \fIx[y]file\fP]. These values are treated as timeseries X(t) [Y(t)] sampled at equal intervals
spaced \fIdt\fP units apart. There may be any number of lines of input. \fBspectrum1d\fP will create
file[s] containing auto\- [and cross\- ] spectral density estimates by Welch's method of ensemble '
averaging of multiple overlapped windows, using standard error estimates from Bendat and Piersol.
.sp
The output files have 3 columns: f or w, p, and e. f or w is the frequency or wavelength, p is the
spectral density estimate, and e is the one standard deviation error bar size. These files are named
based on \fIname_stem\fP. If the \fB\-C\fP option is used, up to eight files are created; otherwise only one
(xpower) is written. The files (which are ASCII unless \fB\-bo\fP is set) are as follows:
.TP
\fIname_stem\fP.xpower
Power spectral density of X(t). Units of X * X * \fIdt\fP.
.TP
\fIname_stem\fP.ypower
Power spectral density of Y(t). Units of Y * Y * \fIdt\fP.
.TP
\fIname_stem\fP.cpower
Power spectral density of the coherent output. Units same as ypower.
.TP
\fIname_stem\fP.npower
Power spectral density of the noise output. Units same as ypower.
.TP
\fIname_stem\fP.gain
Gain spectrum, or modulus of the transfer function. Units of (Y / X).
.TP
\fIname_stem\fP.phase
Phase spectrum, or phase of the transfer function. Units are radians.
.TP
\fIname_stem\fP.admit
Admittance spectrum, or real part of the transfer function. Units of (Y / X).
.TP
\fIname_stem\fP.coh
(Squared) coherency spectrum, or linear correlation coefficient as a function of frequency.
Dimensionless number in [0, 1]. The Signal-to-Noise-Ratio (SNR) is coh / (1 - coh). SNR = 1 when coh = 0.5.
.SH REQUIRED ARGUMENTS
.sp
.TP
\fIx[y]file\fP
ASCII (or binary, see \fB\-bi\fP) file holding X(t) [Y(t)] samples in the first 1 [or 2] columns. If no file is specified, \fBspectrum1d\fP
will read from standard input.
.TP
.B \-S
\fIsegment_size\fP is a radix-2 number of samples per window for ensemble averaging. The smallest frequency
estimated is 1.0/(\fIsegment_size\fP * \fIdt\fP), while the largest is 1.0/(2 * \fIdt\fP). One standard error
in power spectral density is approximately 1.0 / sqrt(\fIn_data\fP / \fIsegment_size\fP), so if \fIsegment_size\fP
= 256, you need 25,600 data to get a one standard error bar of 10%. Cross-spectral error bars are larger and more
complicated, being a function also of the coherency.
.SH OPTIONS
.sp
.TP
.B \-C
Read the first two columns of input as samples of two timeseries, X(t) and Y(t).
  Consider Y(t) to be the
output and X(t) the input in a linear system with noise. Estimate the optimum f
requency response function
by least squares, such that the noise output is minimized and the coherent outpu
t and the noise output are
uncorrelated.
Optionally specify up to 8 letters from the set { \fBx y c n p a g o\fP }
in any order to create only those output files instead of the default [all].
\fBx\fP = xpower, \fBy\fP = ypower, \fBc\fP = cpower, \fBn\fP = npower,
\fBp\fP = phase, \fBa\fP = admit, \fBg\fP = gain, \fBo\fP = coh.
.TP
.B \-D
\fIdt\fP Set the spacing between samples in the timeseries [Default = 1].
.TP
.B \-N
\fIname_stem\fP Supply the name stem to be used for output files [Default = "spectrum"].
.TP
.B \-V
Selects verbose mode, which will send progress reports to stderr [Default runs "silently"].
.TP
.B \-W
Write Wavelength rather than frequency in column 1 of the output file[s] [Default = frequency, (cycles / \fIdt\fP)].
.TP
.B \-bi
Selects binary input. Append \fBs\fP for single precision [Default is double].
Append \fIn\fP for the number of columns in the binary file(s).
[Default is 2 input columns].
.TP
.B \-bo
Selects binary output. Append \fBs\fP for single precision [Default is double].
.SH EXAMPLES
Suppose data.g is gravity data in mGal, sampled every 1.5 km. To write its power spectrum, in mGal**2-km,
to the file data.xpower, try
.sp
spectrum1d data.g \fB\-S\fP256 \fB\-D\fP1.5 \fB\-N\fPdata
.sp
Suppose in addition to data.g you have data.t, which is topography in meters sampled at the same points as
data.g. To estimate various features of the transfer function, considering data.t as input and data.g as
output, try
.sp
paste data.t data.g | spectrum1d \fB\-S\fP256 \fB\-D\fP1.5 \fB\-N\fPdata \fB\-C\fP
.SH "SEE ALSO"
.IR gmt (1gmt),
.IR grdfft (1gmt)
.SH REFERENCES
Bendat, J. S., and A. G. Piersol, 1986, Random Data, 2nd revised ed., John Wiley & Sons.
.br
Welch, P. D., 1967, "The use of Fast Fourier Transform for the estimation of power spectra:  a method 
based on time averaging over short, modified periodograms", IEEE Transactions on Audio and Electroacoustics,
Vol AU-15, No 2.