A processing method of an alternating signal (Sv) for detecting the rotation speed of a shaft of an aircraft engine, produced by a variable reluctance sensor and affected by parasitic measurement noise, from a raw window signal (S CI) switching toward a low voltage level (U 1), respectively toward a high voltage level (U 2), in correspondence with the passage of the voltage of the alternating signal (Sv) below a restart threshold (U R), respectively above a reset threshold (Uv), the method comprising the following steps: - calculation of a processed window signal (Sc), based on the raw window signal (S CI), and using a value of the switching continuation duration below which a non-noisy portion of the alternating signal (Sv) will not cause a switching, the switching continuation duration being calculated as a function of a rotation speed value of the shaft obtained for the previous period (P n-1) by means of a predetermined maximum gradient law expressing a maximal gradient (G N1max) of rotation speed of the shaft for the previous period (P n-1), - measurement of the rotation speed of the shaft, by measurement of the frequency of the processed window signal (Sc), wherein each switching toward the low level (U 1) or toward the high level (U 2) of the processed window signal (Sc) triggers the locking of the processed window signal (Sc) for a duration exactly equal to the switching continuation duration previously calculated, so that the switchings of the un-processed window signal (S CI) are not translated into switchings of the processed window signal (Sc) during said locking, the processed window signal (Sc) no longer being locked once the switching continuation duration has expired. - calculation of a processed window signal (Sc), based on the raw window signal (S CI), and using a value of the switching continuation duration below which a non-noisy portion of the alternating signal (Sv) will not cause a switching, the switching continuation duration being calculated as a function of a rotation speed value of the shaft obtained for the previous period (P n-1) by means of a predetermined maximum gradient law expressing a maximal gradient (G N1max) of rotation speed of the shaft for the previous period (P n-1), - measurement of the rotation speed of the shaft, by measurement of the frequency of the processed window signal (Sc), wherein each switching toward the low level (U 1) or toward the high level (U 2) of the processed window signal (Sc) triggers the locking of the processed window signal (Sc) for a duration exactly equal to the switching continuation duration previously calculated, so that the switchings of the un-processed window signal (S CI) are not translated into switchings of the processed window signal (Sc) during said locking, the processed window signal (Sc) no longer being locked once the switching continuation duration has expired. wherein each switching toward the low level (U 1) or toward the high level (U 2) of the processed window signal (Sc) triggers the locking of the processed window signal (Sc) for a duration exactly equal to the switching continuation duration previously calculated, so that the switchings of the un-processed window signal (S CI) are not translated into switchings of the processed window signal (Sc) during said locking, the processed window signal (Sc) no longer being locked once the switching continuation duration has expired.