We’d like to announce that the Opus codec is now ready for testing. The bit-stream is now is a “pseudo-freeze”, which means that unless a problem is found during testing/review, there are no longer any changes planned. The only exception to this are the SILK-mode FEC and the stereo SILK mode, which should be landing in the next few days. Considering that these are not critical features, we felt like the testing phase could already begin.
Please recall that OPUS is the new codec arising from the combination of CELT and Skype’s SILK. It’s multiple operating modes accommodate many different applications, from extremely low-latency high-quality links between production studios, to voice applications on very low bit-rate channels. OPUS brings us the current state-of-the-art in audio codec technology in a royalty-free, open source form.
The samples and explanations provided are first rate. They clearly illustrate the merit in ultra-low-latency for some applications, as well as exemplify how CELT currently fares against other common codecs at a variety of bit-rates and with various types of source material.
As someone who’s passionately involved spreading the gospel of HDVoice I’ve been following the mailing list of the IETF CODEC Working Group. They’ve been working towards a new IETF RFC on a brand new wideband codec standard.
Starting with four submissions from Broadvoice, SpiritDSP, Skype and Xiph.org I think that they have made startlingly good progress over the past year. The group has actually arrived at a solution that provides for a codec that is a hybrid of SILK and CELT. It was recently announced on the mailing list that this new hybrid codec is to be known as “Opus.”
At IETF78 in Nagasaki, Japan the working group met to further their efforts. There’s a good recording of the session that, amongst other things, gives considerable detail about the hybrid nature of Opus.
There has been close co-operation between developers at Skype and Jean-Marc Valin of Xiph.org, such that they already have sample code running and have conducted some structured listening tests. The results of the listening tests are reported to be excellent.
It’s very interesting how the hybrid codec works. It can leverage both CELT and SILK principles acting on different frequency bands to generate the most optimal audio for a given bit-rate. It supports a wide range of sample rates from 8 KHz (PSTN narrowband) to 48 KHz (production quality audio) and bit-rates from 8 kbps to 128+ kbps per audio channel.
Opus may have a huge role to play in our transition away from the legacy PSTN. It’s very encouraging to hear that the CODEC Working Group is progressing so quickly.