In this paper we propose a new method for a generalized model representing the time-varying spectral characteristics of quasi harmonic instruments. This approach comprises a linear sourcefilter model, a parameter estimation method and a model evaluation based on the prototype’s variance. The source-filter-model is composed of an excitation source generating sinusoidal parameter trajectories and a modeling resonance filter, whereas basic-splines (B-Splines) are used to model continuous trajectories. To estimate the model parameters we apply a gradient decent method to a training database and the prototype’s variance is being estimated on a test database. Such a model could later be used as a priori knowledge for polyphonic instrument recognition, polyphonic transcription and source separation algorithms as well as for resynthesis.