"Thanks to a cooperative effort with Polycom,
FreeSWITCH can use Polycom's Siren codecs for wideband and
super-wideband audio. The software also adds a free open source
codec using Constrained Energy Lapped Transform (CELT) technology
that should provide sample rates of 44.1 to 48 KHz with minimal
latency, especially suited for speech and music."