  We present a signal extraction utility written for the purposes of the
Arecibo Legacy Fast ALFA survey (ALFALFA). This survey, when completed, will
have covered 7000 square degrees of the high galactic latitude sky and should
detect over 20,000 extragalactic objects. It is the most sensitive blind HI
survey to date. The large size of the survey justifies in itself the need for
an automated way of identifying signals in the data set. The matched-filtering
signal extractor proposed is based on convolutions in the Fourier domain of
templates of varying widths with each spectrum. The chosen templates are built
from a simple combination of Hermite functions to mimic the shape of typical
galactic HI profiles of varying widths. The main advantages of this
matched-filtering approach are a sensitivity to the total flux of the signals
(and not only to peak flux), robustness against instabilities and short
computing times. The details of the algorithm are given here, as well as
results of simulations that assess the reliability and completeness of the
process.
