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