An optimization method is proposed to find mask parameters of a brass player coming from a one degree of freedom lip model, with only constant mouth pressure and periodic mouthpiece pressure as input data, and a cost function relying on the waveform and the frequency of the signal. It delivers a set of parameters called C-admissible, which is a subset of all mask parameters that allow the inverse problem to be well defined up to an acceptable precision. Values for the mask parameters are found that give a good aproximation of real signals, with an error on the playing frequency of less than 5 cents for some notes. The evolution of the mask parameters is assessed during recordings with real musicians playing bend notes and their effects on the playing frequency are compared to the theoretical change on a model.