  In the hierarchical search for periodic sources of gravitational waves, the
candidate selection, in the incoherent step, can be performed with Hough
transform procedures. In this paper we analyze the problem of sensitivity loss
due to discretization of the parameters space vs computing cost, comparing the
properties of the sky Hough procedure with those of a new frequency Hough,
which is based on a transformation from the time - observed frequency plane to
the source frequency - spin down plane. Results on simulated peak maps suggest
various advantages in favor of the use of the frequency Hough. The ones which
show up to really make the difference are 1) the possibility to enhance the
frequency resolution without relevantly affecting the computing cost. This
reduces the digitization effects; 2) the excess of candidates due to local
disturbances in some places of the sky map. They do not affect the new analysis
because each map is constructed for only one position in the sky. Pacs.
numbers: 04.80Nn,07.05Kf,97.60Jd 1.
