1. A method of converting a voice signal as spoken by a source speaker into a converted voice signal the acoustic characteristics thereof resemble those of a target speaker, the method comprising: a determination step of determining a function for transforming acoustic characteristics of the source speaker into acoustic characteristics close to those of the target speaker on the basis of samples of the voices of the source and target speakers, and a transformation step of transforming acoustic characteristics of the source speaker voice signal to be converted by applying said transformation function, wherein said determination step comprises a step of determining a function for conjoint transformation of characteristics of the source speaker relating to the spectral envelope and of characteristics of the source speaker relating to the pitch and wherein said transformation step comprises applying said conjoint transformation function, wherein said step of determining a conjoint transformation function comprises, a step of analyzing source and target speaker voice samples grouped into frames to obtain for each frame information relating to the spectral envelope and to the pitch, a step of concatenating information relating to the spectral envelope and information relating to the pitch for each of the source and target speakers, a step of determining a model representing common acoustic characteristics of source speaker and target speaker voice samples, and a step of determining said conjoint transformation function from said model and the voice samples, and wherein said steps of analyzing the source and target speaker voice samples are adapted to produce said information relating to the spectral envelope in the form of cepstral coefficients.