Intelligibility evaluation of speech coding standards in severe background noise and packet loss conditions

Speech intelligibility is an important aspect of speech transmission but often when speech coding standards are compared only the quality is evaluated using perceptual tests. In this study, the performance of three wideband speech coding standards, adaptive multi-rate wideband (AMR-WB), G.718, and enhanced voice services (EVS), is evaluated in a subjective intelligibility test. The test covers different packet loss conditions as well as a near-end background noise condition. Additionally, an objective quality evaluation in different packet loss conditions is conducted. All of the test conditions extend beyond the specification range to evaluate the attainable performance of the codecs in extreme conditions. The results of the subjective tests show that both EVS and G.718 are better in terms of intelligibility than AMR-WB. EVS attains the same performance as G.718 with lower algorithmic delay.

1. Introduction

In modern speech transmission systems, the intelligibility of the communication can be jeopardized by many factors. Phone calls are increasingly moving to IP-based networks which creates new challenges, such as varying delay and lost packets, for speech coding and processing. This can also happen to such a degree that the intelligibility of the speech is severely compromised. Another common problem is environmental noise in one or both ends of the communication channel for which many pre-and post-processing stages can be used in the mobile devices. For instance, in the speaker's end, the effects of the far-end noise can be diminished by utilizing noise suppression as a pre-processing step. Additionally, in the receiving device, the intelligibility of the speech can be increased over the near-end noise in the listener's surroundings with the utilization of post-processing techniques. However, it depends highly on the phone manufacturer whether additional enhancement techniques are used. Therefore, the performance of the speech codec alone in the presence of degradations is very important.