Due to the dispersion in the interstellar medium, the pulsar signal is shifted in time, with pulses in the higher frequency channels arriving earlier than the lower ones. This delay depends on the so-called dispersion measure DM of the pulsar, which can be taken as a measure of its distance from the Earth.
Shifts are applied to the time series in each frequency channel before adding all the channels to produced a dedispersed time series appropriate to a given DM. These data are subsequently folded with the help of the Fast Folding Algorithm (FFA). Each folding operation results in a pulse profile (green line in the animation). The signal-to-noise ratios (SNR) of each profile are then plotted as a function of folding period to form the so-called period spectrum which can be searched for maxima (yellow points in the animation). The pulsar signal is strongest if the pulses are situated one above the other in all eight channels which then add in phase. In the example, an observation of PSR 1758-23, this occurs at DM 1100 and a period of 415,78 msec. Harmonics of this signal can be also identified.
The maximum in the period spectrum corresponds to the folded profile
(green line). The pulsar in the example was found by
Manchester et al. 1985 in Parkes
(Australia).