This paper discusses a model-based synthesis approach focused on the estimation of model parameters. For the treated approach, tube models are used for analysis and synthesis of speech units. In comparison to the standard lossless tube model, an extended tube model is used which includes the frequency dependent vocal tract losses. The parameters of the tube models are estimated by minimizing the spectral error between the tube model and a speech segment. For the analysis of speech units, the time evolution of the parameters is taken into account. For that purpose, the speech segments are analyzed jointly which ensures smooth parameter trajectories. The investigations show that, especially for extended tube models, the joint analysis of frames improves the quality of the synthesized speech signals. Additionally, the differences of the results obtained by the standard and the extended tube model are discussed.