Appendix III to ITU-T Recommendation G.722 specifies a high-quality packet loss concealment (PLC) algorithm for G.722. The algorithm performs the packet loss concealment in the 16‑kHz output domain of the G.722 decoder. Periodic waveform extrapolation is used to fill in the waveform of lost packets, mixing with filtered noise according to signal characteristics prior to the loss. The extrapolated 16-kHz signal is passed through the QMF analysis filter bank, and the subband signals are passed to partial subband ADPCM encoders to update the states of the subband ADPCM decoders. Additional processing takes place for each packet loss in order to provide a smooth transition from the extrapolated waveform to the waveform decoded from the received packets. Among other things, the states of the subband ADPCM decoders are phase aligned with the first received packet after a packet loss, and the decoded waveform is time-warped in order to align with the extrapolated waveform before the two are overlap-added to smooth the transition. For protracted packet loss, the algorithm gradually mutes the output.
The algorithm operates on an intrinsic 10-ms frame size. It can operate on any packet or frame size that is a multiple of 10 ms. The longer input frame becomes a super frame, for which the packet loss concealment is called an appropriate number of times at its intrinsic frame size of 10 ms. It results in no additional delay when compared with regular G.722 decoding using the same frame size.
The PLC algorithm described in this appendix meets the same complexity requirements as the PLC in G.722 Appendix IV. At an additional complexity of 2.8 WMOPS worst-case and 2 WMOPS average compared with the G.722 decoder without PLC, the G.722 PLC algorithm described in this appendix provides significantly better speech quality than the G.722 PLC specified in G.722 Appendix IV, which provides an alternative quality-complexity trade-off.