Speech excitation signal recovering based on a novel error mitigation scheme under erasure channel conditions

Voice over IP (VoIP) communications are prone to transmission delays and data losses as they are carried out over packet-switched networks which are unable to guarantee real-time packet delivery. Speech codecs used in these channels strongly rely on Packet Loss Concealment (PLC) algorithms, the performance of which can be compromised as frame losses often occur in bursts. Thus, advanced PLC algorithms for erasure channels have already been proposed in the literature but these frequently focus on the speech envelope disregarding the excitation signal. In this paper we propose an error mitigation scheme focused on the estimation of this excitation signal whenever lost frames appear. These estimates are obtained by applying a minimum mean square error (MMSE) estimation technique based on the last correctly received frame. To this end an excitation signal's representation and quantization approach which compares the resulting synthesized signal with the original speech one is considered. In addition, we propose the combination of this approach with a recursive least squares (RLS) technique which provides a better excitation signal estimate for the first lost consecutive frames. The proposed error mitigation scheme has been tested on the iLBC codec, where objective and subjective tests have shown a noticeable improvement on speech quality for transmissions over erasure channels.