Objective measurement of the speech transmission quality of vocoders by means of the Speech Transmission Index

Nearly all types of military speech communication involve the use of so-called (narrow band) voice coders or vocoders. Usually the Speech Transmission Index (STI) uses artificial test signals, which can not be reproduced by vocoders with the usual fidelity. Therefore the STI is not able to evaluate vocoders at this time. Although it is theoretically feasible to measure the Speech Transmission Index with natural speech instead of the usual artificial test signals, each of the various speech-based STI measurement methods proposed in the literature has its own shortcomings and inaccuracies. A new procedure is proposed for estimating a speech based modulation transfer function (MTF), on which the STI is based, that approaches the accuracy of conventional STI implementations based on artificial signals. The new method enables evaluation of vocoders by means of the STI. Applying the method to a voice coder database shows promising results, giving an average squared correlation coefficient R2 of 0.87 between the subjective CVC scores and the calculated STI for male speech