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OutlierMoni -- Quick reference (draft!)

by Eric Chassande-Mottin — last modified 2007-06-05 18:20

OutlierMoni -- Quick reference

OutlierMoni

Summary

OutlierMoni monitors short transients of large amplitude in a set of channels. OutlierMoni computes a whitened version of each input channel and it looks for outliers (i.e., points larger than a given threshold) in the resulting stream.

OutlierMoni, basic scheme

The output of OutlierMoni can be accessed with dataDisplay by opening raw.ffl. It is also available by a web interface.

OutlierMoni's home page: http://wwwcascina.virgo.infn.it/MonitoringWeb/Noise/outlierMoni

Principle

Whitening

whitening = make the spectrum flat. in the frequency domain, this amounts to dividing by 1/sqrt(PSD).
since the PSD is unknown a priori, PSD has to be estimated : using running Welch. averaging time can be tuned by the

Thresholding
computation and output frame wise (one per GPS second)
output homogeneous to a SNR.

Detailed description

Several parameters are required OutlierMoni
The OutlierMoni's configuration is defined by /virgoData/Noise/outlierMoni.cfg
We detail a typical configuration.

Vetoing
a condition can be put on OutlierMoni execution. for instance, OutlierMoni can be enabled only in Science Mode.

/* keyword     veto_channel   threshold */
#VETO_CHANNEL    Qc_Moni..ScienceMode  1  1

Whitening
since the PSD is unknown a priori, PSD has to be estimated : using running Welch. averaging time can be tuned by the

parameter "Average" (see Configuration below).
this is an adaptive procedure: follows changes in the PSD with typical time longer than averaging time.

/* nsamples for FFT  nAverage  min/max frequency (percent of Nyquist frequency) Nb  Nt  */
/* keyword  Nf    a_max  fmin  fmax  Nb  Nt  */
OUTLIER     70000  10     0.   0.5   0   0

when OutlierMoni enabled, whitening stream not available immediately: require "warming cycles" (number depends on averaging time).

/* choose single whitening */
OUTLIER_SINGLE 1

if so configured, the whitening stream is sent also to the output.

/* channel for which whitened signal will be put in the data */
OUTLIER_WHITENED_CHANNEL Pr_B1_ACp

Thresholding

/* keyword  channel  threshold */
MONITOR   Pr_B1_ACp    6

all computation of OutlierMoni is based on the frame.
compute the max in the current frame, threshold the max.
1 flag per frame.

Output
detection flag, SNR (calibrated)

Configuration
The monitor load its parameters from a configuration file.

Software

The source code of OutlierMoni can be found in /virgoDev/outlierMoni.

Sections

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