THE DETERMINATION OF A PERCEPTUAL SPACE FOR THE QUALITY OF FILTERED SPEECH
Authors: McGee Victor E. 
Published: August 2014

Studies in auditory perception have met with considerable difficulty in coming to grips with the concept of timbre or tonal quality. It continues to defy accurate definition and as a concession to its indeterminate nature it has been described as a multidimensional dimension.

In the field of speech perception the concept is important for a number of reasons, especially as it relates to the evaluation of the fidelity of speech processing devices. For many purposes it is desirable to have a criterion of speech quality which is predictable from a purely physical specification of the communication system. So far only one satisfactory criterion has been established–the Articulation Index–and it refers to the fidelity with which the information bearing elements of speech are preserved over any particular communication system. It is referred to in this study as the AI theory of intelligibility. While several studies have outlined the need for a physically predictable criterion of naturalness only one group of investigators has attempted to define naturalness from both a physical and a psychological point of view. This work is referred to as the VV theory of quality since it is concerned with stationary “vocalic voices”.

The present study was designed to investigate further the relationships between physically specifiable speech signals and psychologically perceived speech quality. A major objective was to examine the role of naturalness in the perception of quality and to relate physical properties of the speech signals to this factor.

In order to exercise physical control over the speech signals (called “voices”) only one talker was used and his voice was subjected to controlled frequency distortion. A sample of male and female listeners provided three basic kinds of data: (a) preference judgments and (b) similarity judgments over all pairs of voice distortions, and (c) semantic judgments made on each voice over a set of semantic scales.

The preference data were used to determine a unidimensional quality continuum and the similarity data were analysed as interstimulus distances to produce a multidimensional quality space. In the latter analysis allowance was made for individual differences by employing the Eckart‐Young factoring procedure, whereby the original rectangular data matrix (interstimulus distances‐by‐listeners) is approximated by the product of three matrices, in the form

S = UβW

Matrix U was called the “voice” matrix, β the “principal roots” matrix, and W the “points of view” matrix.

In order to appreciate individual differences in the semantic judgments a critical analysis of each scale was undertaken by the same Eckart‐Young procedure. In practically all cases, it was found that differences of interpretation were attributable to the same, small, culturally‐distinct, group of listeners. For the average listener a “semantic space” was determined by factoring the intercorrelations between the scale values of the voices on each of the scales. It was found that two correlated dimensions were necessary to describe the voices and two factors were identified as THE NATURALNESS FACTOR and THE INTELLIGIBILITY FACTOR. The latter referred to the physical determination of intelligibility in terms of the AI theory.

The multidimensional quality space was in fact a two‐dimensional space for the average listener and was characterized by two parallel, linear distributions of the voices. The one linear distribution was interpreted as the quality continuum under a response set “fundamental frequency–YES” and the other as that obtained under a response set “fundamental frequency–NO.” The directions of the parallel distributions coincided with the direction of the unidimensional quality continuum derived from the preference judgments and in terms of the semantic space approximately bisected the identified factors.

Using the “points of view” matrix resulting from the Eckart‐Young analysis of the similarity data, four slightly different perceptual spaces corresponding to four “idealized” listeners were identified and interpreted within the framework of the hypothesized response sets. The different points of view were not especially significant and the perceptual space for the average listener was accepted as an adequate representation of the speech quality space.

Finally an attempt was made to determine importance curves across the frequency scale for both naturalness and intelligibility and for the unidimensional quality scale. This represents the outcome of a psychophysical correspondence analysis based on known frequency characteristics of the voice sample and the scaling of the voices on the various scales.