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Constrained Energy Lapped Transform (CELT) is an open, royalty-free lossy audio compression format and a free software codec with especially low algorithmic delay for use in low-latency audio communication. The algorithms are openly documented and may be used free of software patent restrictions. 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 is suitable for both speech and music. It borrows ideas from the CELP algorithm, but avoids some of its limitations by operating in the frequency domain exclusively.〔 The original stand-alone CELT has been merged into Opus. Therefore CELT as stand-alone format is now abandoned and obsolete. Development is going on only for its hybridised form as a layer of Opus, integrated with SILK. This article covers the historic, stand-alone format, for the integrated form and its evolution since the integration into Opus see the article on Opus. == Properties == CELT's central feature is low algorithmic delay. It allows for latencies of typically 3 to 9 ms but is configurable to below 2 ms at the price of more bitrate to reach a similar audio quality.〔 CELT supports mono and stereo audio and is applicable to both speech and music. It can use a sampling rate from 32 kHz to 48 kHz and above and an adaptive bitrate from 24 kbit/s to 128 kbit/s per channel and above.〔 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).〔〔 Like Vorbis, CELT is a fullband (entire human hearing range) general-purpose codec, i.e. not specialized for special types of audio signals and therefore different from its sibling project Speex. The format enables for transparent results at high bitrates, as well as very decent quality at lower bitrates. 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-AAC It has a comparably low computational complexity that resembles that of the low-delay variant of AAC (AAC-LD) and stays significantly below the complexity of Vorbis.〔 It enables for constant and variable bitrate. If the signal disappears into the noise floor in speech pauses and similar cases, the transmission can be limited to signal the output of comfort noise to the decoder. Most settings of the naturally streaming-enabled format can be changed on the fly without interrupting transmission. The format is robust to transmission errors. Loss of whole packets as well as bit errors can be masked with a steady degradation of audio quality (packet loss concealment, PLC). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「CELT」の詳細全文を読む スポンサード リンク
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