An analog signal divided into time frames is encoded and a synthetic signal is formed on the model thereof in a time frame manner via a synthesis filter which is excited by an excitation signal. The excitation signal is formed by at least one adaptive code list containing a plurality of scanning values provided with a defined scanning space. For the actual excitation signal, a segment corresponding to the time frame length is selected from the plurality of scanning values via a speech-based frequency parameter which can take non-integer values and, in such a case, the values intermediate to the scanning values defined by the speech-based frequency parameter are formed in such a way that the time space between the intermediate values and the scanning values is reduced and the totality of the intermediate and the scanning values is used for forming the excitation signal.