  The aim of this work is to show how it is possible to build an on line
whitening filter in an adaptive way. We have modeled the VIRGO noise spectrum
as an autoregressive stochastic process, after a pre-filtering of the
theoretical curve which flattens the low frequency part of the spectrum. We
have tested some very popular adaptive algorithms, based on the gradient
methods and on the least squares methods with a lattice structure filter.
