Optimum Frequency Response Characteristics for Wideband Terminals

In ETSI standard ES 202 739 and ES 202 740 a new testing technique for the measurement of wideband terminals is introduced. Tolerance masks are given for sending and receiving frequency response characteristics. As an important new concept in this standard no longer the ear reference point (ERP) but the free field reference point is used for determining the response characteristics in receiving. Subjective tests have been carried out in order to derive the impact of non optimum receiving frequency response characteristics on the perceived speech sound quality. Different experiments as conducted are introduced and the results are shown. Based on these frequency response characteristics a tolerance mask based on diffuse field equalization is proposed. The tolerance mask guarantees (based on subjective testing) a minimum speech sound quality in receiving with respect to impairments introduced by different frequency response characteristics only. In sending direction different subjective experiments were conducted with different types of background noises and different types of wideband terminals. The aim of this investigation was to find desirable sending frequency response characteristics with and without background noise at the near end. Furthermore general recommendations concerning frequency responses in case of speech with near end background noise are desired. The subjective experiments are introduced and the results will be discussed.