SILK is the speech-optimized half of the Opus codec, contributed by Skype, where it originally powered Skype calls. It uses linear-predictive coding (LPC), the classic approach for voice, modelling the vocal tract to represent speech compactly and efficiently at low bitrates from narrowband to wideband. Opus automatically engages SILK (or a hybrid of SILK and CELT) when the content is speech and the bitrate is modest, which is most of the time on a voice call. Its presence is a big reason Opus sounds so good for conversation at rates where music codecs would struggle, while the CELT layer takes over for music and very low latency.
