Perceptual dimensions of speech sound quality in modern transmission systems

A study is reported which was carried out to determine factors governing the perception of sound quality of human, natural speech transmitted over telephone systems of different transmission-channel bandwidths, and their impact on perceived overall quality. The data collected in the described auditory tests is supposed to form the basis for instrumental modelling approaches of overall and especially speech-sound quality in wide-band systems. Measurement parameters of the listening-only test (LOT) were the lower and upper frequency limits of the transmission-channel as well as the terminal equipment used for speech presentation, with the aim of investigating the role of expectation and psychoacoustic reference for speech-sound quality perception. In this way, the terminal equipment best suitable for the assessment of speechsound quality with respect to the channel frequency band was identified. With this approach, the test subjects’ expectation towards different terminal equipment was taken into consideration, while the acoustical properties of the terminal were adjusted to an ideal reference. In the tests, speech samples were presented over an electrostatic headphone, monotic and diotic, as well as telephone handsets modified to allow wide-band presentation. Results show that an interdependence exists between the presentation method (handset vs. headphone) and the transmission bandwidth with respect to overall quality. The paper discusses reasons for this effect, which is assumed to be caused by the expectation listeners show towards specific types of terminal equipment.