The perceptibility of degradations caused to signals transmitted over a transmission medium is measured by generating one or more predetermined transmission degradation conditions (e.sub.t) and subjecting a test signal (S.sub.t) to the transmission degradation conditions (e.sub.t) in a network simulation device. The degree M(e) to which the or each transmission degradation condition (e.sub.t) is perceptible to the human perceptual system is measured and a data set is generated and stored for converting one or more transmission degradation conditions (e.sub.t) to respective values of perceptibility M(e). The data set may be a look-up table or an empirically determined formula. The data set may then be used on live traffic (S'), by identifying objectively measured transmission degradation conditions (e) in the received signal (S') and retrieving from the data set in the data storage means (7) a value of perceptibility (M) associated with the transmission degradation conditions (e) so identified. In this way a measure of the subjective significance of degradation in the signal (S') can be derived from objective measures of degradation.
A method is provided for optimizing an objective measure used to estimate mean opinion score or naturalness of synthesized speech from a speech synthesizer. The method includes using an objective measure that has components derived directly from textual information used to form synthesized utterances. The objective measure has a high correlation with mean opinion score such that a relationship can be formed between the objective measure and corresponding mean opinion score. The objective measure is altered to provide a different function of textual information derived from the utterances so as to improve the relationship between the scores of the objective measure and subjective ratings of the synthesized utterances.
