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##

Function: `find_chirp()`

0
`void find_chirp(float* htilde, float* ch0tilde, float* ch90tilde, float* twice_inv_noise, float n0, float n90,
float* output0, float* output90, int n, int chirplen, int*
offset, float* snr_max, float* c0, float* c90, float
*var) `

This routine filters the gravity-wave strain through a pair of optimal
filters corresponding to the two phases of a binary chirp, then finds
the time at which the SNR peaks.
The arguments are:

`htilde`: Input. The FFT of the gravity-wave strain.
`ch0tilde`: Input. The FFT of the 0-degree chirp.
`ch90tilde`: Input. The FFT of the 90-degree chirp (assumed orthogonal to the 0-degree chirp).
`twice_inv_noise`: Input. Twice the inverse noise power spectrum, used for optimal filtering.
The array element `twice_inv_noise[0]` contains
the DC value, and the array element `twice_inv_noise[n/2]`
contains the value at the Nyquist frequency.
`n0:` Input. Normalization of the 0-degree chirp.
`n90:` Input. Normalization of the 90-degree chirp.
`output0:` Output. A storage array. Upon return, contains the filter output of the 0-degree
phase optimal filter.
`output90:` Output. A storage array. Upon return, contains the filter output of the 90-degree
phase optimal filter.
`n:` Input. Defines the lengths of the various arrays: ` ch0tilde[0..n-1]`, `ch90tilde[0..n-1]`, `output0[0..n-1]`,
`output90[0..n-1]`, and `twice_inv_noise[0..n/2]`.
`chirplen:` Input. The number of bins in the time domain occupied by the chirp that you
are searching for. This is necessary in order to untangle the wrap-around ambiguity explained earlier.
`offset:` Output. The offset, from 0 to `n-chirplen-1`,
at which the signal output (for an arbitrary linear combination of the two filters) peaks.
`snr_max:` Output. The maximum signal-to-noise ratio (SNR) found.
`c0:` Output. The coefficient of the 0-phase template which achieved the highest SNR.
`c90:` Output. The coefficient of the -phase
template which achieved the highest SNR. Note that
should be 1.
`var:` Output. The variance of the filter output. Would be 1 if the input to
the filter were colored Gaussian noise with a spectrum defined by .

- Author:
Bruce Allen, ballen@dirac.phys.uwm.edu
- Comments:
None.

** Next:** Function: freq_inject_chirp()
** Up:** GRASP Routines: Gravitational Radiation
** Previous:** Dirty details of optimal
** Contents**
Bruce Allen
2000-11-19