  The packet loss problem seriously affects the quality of service in Voice
over IP (VoIP) sceneries. In this paper, we investigated online receiver-based
packet loss concealment which is much more portable and applicable. For
ensuring the speech naturalness, rather than directly processing time-domain
waveforms or separately reconstructing amplitudes and phases in frequency
domain, a flow-based neural vocoder is adopted to generate the substitution
waveform of lost packet from Mel-spectrogram which is generated from history
contents by a well-designed neural predictor. Furthermore, a waveform
similarity-based smoothing post-process is created to mitigate the
discontinuity of speech and avoid the artifacts. The experimental results show
the outstanding performance of the proposed method.
