Selective conversion of a continuous signal with a periodic weight function of sinusoidal form permits realization of ideal selection of harmonic signal in a deterministic and random interference background in an infinite time interval given by an integral which incorporates the input and output signals and the weight function. However, difficulties in the apparatus realization of the conversion constrain its practical application. Selective conversion with a rectangular weight function is realized successfully considerably more accurately and simply by using a relay synchronized detector. The disadvantage of such a conversion is that it is responsive to high odd harmonics of the signal under investigation. A stochastic selective conversion is proposed that features simplicity of the technical realization and slight response to high harmonics.