CELT is the music-oriented, low-latency half of the Opus codec, built on the MDCT and designed to deliver high quality at very short frame sizes — down to 2.5 ms, far quicker than typical music codecs. Opus uses CELT alone for music and ultra-low-delay applications, and blends it with the SILK speech layer in a hybrid mode for general full-band content. Where SILK models the voice, CELT codes the spectrum directly and excels at the transients and tonal richness of music. Together the two layers are what let a single codec, Opus, span a phone call and a stereo music stream without ever changing codecs.
