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EsearchTsearchssearchc Onmybaby.com n Onmybaby.com rsearchl e Babies tsearchr Recipes ssearchl Onmybaby.com wa Recipes g Recipes rsearcht Babies m Recipes csearchd Babies lsearchysearch t May asearchlw searchn Recipes y Onmybaby.com ay Recipes co Onmybaby.com oRc Maysearchpsearchssearcharch May Recipes Rsearchcsearchp Recipes s a Recipes M Recipes ysearche Onmybaby.com eacsearchr Babies es May abissearchn Babies abis yasearhsearchser Onmybaby.com h May msearchrh Recipes Onmybaby.com asearchies e M Recipes y Babies May Osearchmy May ab Onmybaby.com .searcho Babies Onmybaby.com ysearchi Recipes alsearchy Babies 3t Onmybaby.com Recipes s Onmybaby.com bsearcht May i osearchfgur Onmybaby.com blsearch May o Onmybaby.com be Onmybaby.com osearch Babies searchs Jpflashers a Recipes May hsearch psearchi May eof Onmybaby.com m Recipes r Babies searchi May rtsearch searchosearchr Recipes ah May a s Recipes m Babies l Recipes radsearchosearchq Babies a Onmybaby.com i Recipes ysearch[2] 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.[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).[1][3]

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-AACv1, thanks to the band folding.[4][5] In comparative double-blind listening tests it proved to be noticeably superior to HE-AACv1 at ~64 kBit/s.[6]

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.[7]

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).


block diagram of the codec

CELT is a transform codec based on the modified discrete cosine transform (MDCT) and concepts from CELP (with a code book for excitation, but in the frequency domain).

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. For a further reduction of the algorithmic delay to the expense of a minor sacrifice in audio quality, the by nature 50% of overlap between the blocks is practically cut down to half by silencing the signal during one eight at both ends of a block, respectively.[2]

The coefficients are grouped to resemble the critical bands of the human auditory system. The entire amount of energy of each group is analysed and the values quantised for data reduction and compressed through prediction by only transmitting the difference to the predicted values (delta encoding).

The (unquantised) band energy values are removed from the raw DCT coefficients (normalisation). The coefficients of the resulting residual signal (so-called 鈥渂and shape鈥) are coded by Pyramid Vector Quantisation (PVQ, a spherical vector quantisation).[8] This encoding leads to code words of fixed (predictable) length, which in turn enables for robustness against bit errors and leaves no need for entropy encoding.[5] Finally, all output of the encoder are coded to one bitstream by a range encoder.[9] In connection with the PVQ, CELT uses a technique known as band folding, which is said to deliver a similar effect to the spectral band replication (SBR) by reusing coefficients of lower bands for higher ones, while at the same time it has much less implications on the algorithmic delay and computational complexity than the SBR. This works against 鈥渂irdie鈥 artifacts by preserving more richness in the appropriate frequency bands.

The decoder unpacks the individual components from the range coded bitstream, multiplies the band energy to the band shape coefficients and transforms them back (via iMDCT) to PCM data. The individual blocks are rejoined using weighted overlap-add (WOLA). Many parameters are not explicitly coded, but instead reconstructed by using the same functions as the encoder.

For the channel coupling CELT may use M/S stereo or intensity stereo. Blocks can be described independent from adjacent frames (Intra-frame); for example to enable a decoder to jump into a running stream. With transform codecs so-called pre-echo artifacts can get audible, because the quantisation error of sharp, energy-heavy sounds (transients) can spread over the entire DCT block and the transient doesn't mask them backward in time as well as forward. With CELT each block can be further divided to thwart such artifacts.


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 鈥淰orbis II鈥). This discussion together with Vorbis creator Christopher Montgomery led to Jean-Marc Valin鈥瞫 interest in a particularly low-latency codec. Valin has worked on CELT since 2007.[5] In December 2007, the first draft version of libcelt was published as version 0.0.1, initially named 鈥淐ode-Excited Lapped Transform鈥.[10]