Towards iLBC speech coding at lower rates through a new formulation of the start state search

The Internet Low Bit-rate Coder (iLBC) has emerged as a candidate for Voice over Internet Protocol (VoIP) applications. By avoiding the inter-frame coding dependencies endemic to many speech coders such as those based on Code Excited Linear Prediction, iLBC is able to achieve superior robustness to packet loss. In addition to robustness to packet loss, a VoIP codec should possess the agility to adjust its source coding rate in order to react to network congestion and to be amenable to joint source channel coding for wireless channels. Towards this end, we develop a new formulation of the iLBC encoding process that allows for a variable rate iLBC. In particular, we demonstrate how the LP excitation signal is constructed from a much shorter vector of 'start state' samples through a non-square synthesis matrix that captures the effects of the Adaptive Codebook operations. With this new framework, the search and quantization of the start state is re-formulated as an Analysis by Synthesis matching problem. We demonstrate how a Multi-Pulse (MP) approach can be utilized to effect a variable rate coding solution for this new framework. A variable rate coder with the MP start state achieves better performance than the Adaptive Multi-Rate (AMR) coder at 12.2 and 10.2 kbps for packet loss rates greater than 4 %.