At the Institute of Telecommunications of the Technical University of Berlin, three speech assessment experiments were performed to compare the Modulated Noise Reference Unit (MNRU) [1] and the time and frequency warping (TFW) reference system [2]. To perform this evaluation, a category rating test procedure similar to ITU-Rec. P.85 [3] was implemented in an evaluation system developed at the Institute of Telecommunications [4]. This paper explains in the first part details of the reference systems and summarises the evaluation results of both systems individually (sections 1 to 3). Section 4 compares of the assessment results by a number of synthec speech samples that were commonly used in all experiments. Keywords: synthetic speech, category rating test, reference systems, modulated noise reference unit, time frequency warping