VoIP Voice and Fax Signal Processing

5.6 PLC FOR LOW-BIT-RATE Codecs

In the previous sections, PLC is given for the G.711 and G.726 codecs that makes use of previous speech to extrapolate the lost packets. Waveform-based codecs store a large amount of previous samples. As such, previous parameters are not important with waveform codecs. In low-bit-rate codecs of G.729, G.723.1, and Internet low bit rate codec (iLBC), coded parameters of LPC, pitch, and excitation are used. The decoder keeps preserving the parameters during good frames, and these parameters from good frames are reused for predicting the lost speech. During long erasures, both waveform and low-bit-rate codecs (LBCs) reduce their amplitude and make it merge with the silence level. The transmitter–receiver-based approaches are common in all codecs. High-level applications outside the speech codec manage operations of FEC and redundancy. Even after applying FEC and redundancy, some loss of packet could occur based on the impediment conditions. In such situations, a decoder-based PLC has to continue improving on packet losses.

In summary, waveform codecs preserve previous speech, and low-bit-rate codecs preserve previous parameters like LPC, pitch, and excitation. PLC algorithms for low-bit-rate codecs are embedded with a decoder of the main codec. A user may not make any changes to the PLC operation in low-bit-rate codecs and may modify certain application-level conditions outside the decoder to decide on packet loss conditions. The test vectors used with ...