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.
The book · Volume 4 of 7
Audio for Video: A Complete Guide to Sound in Video Products: From PCM and Codecs to Loudness, Accessibility, and Real-Time Voice Processing
CELT is the part of Opus that handles music and ultra-low delay, and knowing when Opus switches to it explains many quality surprises. Volume 4 covers CELT in detail within audio codecs: Opus and its SILK and CELT modes, frame sizes and delay, bitrates for voice and music, and real-time voice use. Written by Nikolay Sapunov, CEO at Fora Soft.
Tuning Opus and CELT for music or low-latency audio in your app?
Describe your audio content and latency target, and we will choose Opus mode, frame size and bitrate and verify quality on your material. 250+ video products shipped since 2005.
