Does the Content of Speech Influence its Perceived Sound Quality?

From a user’s perspective, the speech quality of modern telecommunication systems often differs from that of traditional wireline telephone systems. One aspect is a changed sound of the interlocutor’s voice – introduced by an expansion of the transmissionbandwidth to wide-band, by low-bitrate coding and/or by the acoustic properties of specific user-interfaces. In order to quantify the effect of transmission on speech quality, subjective data to be correlated to transmission characteristics have to be collected in auditory tests. In this paper, a study is presented investigating in how far the content of specific speech material used in a listening-only test impacts its perceived sound quality. A set of French speech data was presented to two different groups of listeners: French native speakers and listeners without knowledge of French. The speech material consists of different text types, such as everyday speech or semantically unpredictable sentences (SUS). The listeners were asked to rate the sound quality of the transmitted voice on a onedimensional category rating scale. The French listeners’ ratings were found to be lower for SUS, while those of the non-French listeners did not show any major dependency on text material. Hence, it can be stated that if a given speech sign is understood by the listeners, they are unable to separate form from function and reflect content in their ratings of sound.