CELT - History

History

First work on plans and drafts for a Vorbis successor was done in 2005 at Xiph.org as part of the Ghost project (initially talked about as “Vorbis II”). This discussion together with Vorbis creator Christopher Montgomery led to Jean-Marc Valin′s interest in a particularly low-latency codec. Valin has worked on CELT since 2007. In December 2007, the first draft version of libcelt was published as version 0.0.1, initially named “Code-Excited Lapped Transform”. CELT was established as an IETF technology in July 2009 under the "ietfcodec" working group. In May 2009, a draft of RTP payload format for the CELT Codec was published.

In version 0.9, the pitch prediction operating in the frequency domain used until then was replaced by a less complex solution with a pre- and postfilter pair in time domain, which was contributed by Raymond Chen of Broadcom.

With CELT 0.11 from February 4, 2011 the format was tentatively frozen (“soft freeze”) – reserving the possibility of unexpectedly necessary last changes.

Shortly after the advent of the CELT/SILK hybrid codec Opus (formerly known as Harmony), the development of CELT as a separate project was halted, instead living on the basis of Opus, which aims to treat the lower part of the spectral range in the time domain with linear prediction (SILK) and the higher part in the frequency domain with the MDCT. The draft for Opus has been registered at the IETF since September 2010.