  We investigate the information processing of a linear mixture of independent
sources of different magnitudes. In particular we consider the case where a
number $m$ of the sources can be considered as ``strong'' as compared to the
other ones, the ``weak'' sources. We find that it is preferable to perform
blind source separation in the space spanned by the strong sources, and that
this can be easily done by first projecting the signal onto the $m$ largest
principal components. We illustrate the analytical results with numerical
simulations.
