Estimation of ‘quality per call’ in modelled telephone conversations

We present a method to estimate the perceived listening quality by a subscriber at the end of a common voice telephony conversation. This method was recently introduced in ETSI STQ mobile and was approved as TR 102 506 'Speech Quality per Call'. The idea is to calculate this "speech-quality-per-call" value based on short-term listening quality scores (so-called Mean Opinion Scores, MOS), as they are usually derived by subjective listening-only tests, or based on predictions of short-term scores by means of objective measures. It is shown that a pure linear averaging of short-term scores will not predict the perceived quality of the entire call sufficiently well in case of a non-stationary quality over the call. Mainly the "recency effect" and the out-weighting of very bad parts in a call have to be considered in an adequate way. An algorithm was developed that allows the obtained "speech-quality-per-call" score to be predicted on the basis of the MOS of the individual utterances. The algorithm can be applied for various lengths of call and numbers of individual utterances. Since speech quality is usually objectively predicted in real networks the approach was also proven and confirmed for objectively obtained quality scores. This paper follows widely the work and the decisions taken within ETSI STQ mobile.