CELT

Development of the format was maintained by the Xiph.Org Foundation (as part of the Ogg codec family) and later coordinated by the Opus working group of the Internet Engineering Task Force (IETF).

CELT was meant to bridge the gap between Vorbis and Speex for applications where both high quality audio and low delay are desired.

It borrows ideas from the CELP algorithm, but avoids some of its limitations by operating in the frequency domain exclusively.

[2] There are no known intellectual property issues pertaining to the CELT algorithm, and its reference implementation is published under a permissive open-source license (the 2-clause BSD).

All in all, the compression capabilities are said to be significantly superior to those of MP3, and as another useful feature for realtime applications like telephony, CELT's audio quality at lower bitrates are even on par with HE-AACv1, thanks to the band folding.

The initial PCM-coded signal is handled in relatively small, overlapping blocks for the MDCT (window function) and transformed to frequency coefficients.

Choosing an especially short block size on the one hand enables for a low latency, but also leads to poor frequency resolution that has to be compensated.

Blocks can be described independent from adjacent frames (Intra-frame); for example to enable a decoder to jump into a running stream.

[5] 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,[17] 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.

Despite the format not being finally frozen, it was being used in many VoIP applications such as Ekiga[18] and FreeSWITCH,[19] which switched to CELT upon entering soft-freeze in January 2009, as well as Mumble, TeamSpeak and other[20] software.