Multiple descriptions coding in MELP coder for voice over IP

Abstract

In VoIP systems, CELP coders, such as G.729, are commonly used as they offer good speech quality in the absence of packet loss. However, harmonic coders such as MELP may be a good alternative for VoIP due to their higher resilience to packet loss. In this paper we examine the problem of the packet loss in the VoIP application using MELP coders. The proposed packetization scheme based on Multiple Description Coding (MDC) applied to the MELP coder is presented. A packet will contain information on two MELP coders operating at 2.4 and 1.2 Kbps respectively. The packetization is achieved using 135 bits in 22.5 ms corresponding to a total rate of 6 kbps. The results show that under typical VOIP operating conditions, the method performs well and outperforms CELP coders operating without MDC at 8 kbps.