Packet loss concealment (PLC)

PLC in general

Packet loss concealment (PLC) is a technique to mask the effects of packet loss in VoIP communications. Because the voice signal is sent as UDP packets on a VoIP network, they may travel different routes to get to destination. At the receiver a packet might arrive very late, corrupted or simply might not arrive. One of the cases in which the last situation could happen is where a packet is rejected by a server which has a full buffer and cannot accept any more data. In a VoIP connection, error-control techniques such as ARQ are not feasible and the receiver should be able to cope with packet loss.

PLC techniques:

1. Zero insertion: the lost speech frames are replaced with zero
2. Waveform substitution: the missing gap is reconstructed by repeating a portion of already received speech. The simplest form of this would be to repeat the last received frame. Other techniques account for fundamental frequency, gap duration etc. Waveform substitution methods are popular because of their simplicity to understand and implement.
3. Model-based methods: an increasing number of algorithms that take advantage of speech models of interpolating and extrapolating speech gaps are being introduced and developed.

PLC in AlphaCom

AlphaCom XE servers are using technique 2: Waveform substitution. The missing gap is reconstructed by repeating the last received frame.