Opus: An audio codec

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TheNut 179 Aug 04, 2012 at 02:42 audio sound

I was reading up on the upcoming Firefox 15 and discovered it’s incorporating an audio codec called Opus. Apparently this has been in the works since 2007 and claims highest quality per bitrate in comparison with other popular codecs such as mp3, ogg, aac, and speex. What really fascinates me is this codec compresses all types of audio, from speech over IP to music and sound files in one elegant library. I’m use to shuffling between speex, ogg/mp3, and flac for compressing audio based on their strengths. It almost sounds (no pun) too good to be true to have a universal codec handle every possible scenario. I’m quite curious to hear how much of an improvement it has over speex, which already sounds crystal clear at 32kbps when I use it to VOIP with others. According to their chart, it should sound almost studio like at that bitrate :o

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alphadog 101 Aug 04, 2012 at 14:22

I’ve heard quality is really good, but is in active development to catch the plethora of edge cases that would come up with such an broadly-intended audience.

As I understand it, Skype has been using Opus for a long time. (The speech encoding part is from Skype people.)

Definitely one to keep an eye on.

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Euph0ria 101 Aug 09, 2012 at 23:25

Skype has been using CELT for a long time, which is part of the encoding technology that goes into Opus. Opus works alot like HE-AAC in a way, whereas HE-AAC encodes the high and lows in the frequency domains. Opus does somethign similar where audio is split into two bands. The lowpass uses a tweaked speex and the high uses a tweaked CELT and they use one, or the other or both to optimize the encoding based on the complexity of the audio. And since both were built from the ground up for low latency and relatively high quality at extremely low bitrates (<16k-32k), you get all the advantages of an encoder that adapts to the complexity of whatever it’s encoding, with superior quality, and ultra low latency. Giving a generous 64Kbps is enough to make HE-AAC sound pretty good (and that includes some preservation of phase info) early tests of Opus at the same bitrate are said to yeild even higher quality than the best HE-AAC encoders (Apple/Nero), though I don’t know how well (or if at all) it preserves channel phase data yet. On the down side, It’s not as flexible as HE-AAC at bitrates lower than 64k because that is the trigger bitrate for the full stereo CELT Codec to kick in. Any lower and you get monaural audio. So HE-AACv2 @48k or lower still has it’s place for fair quality audio. But it would not compete for something such as a PA system, or a telephone call, or anything that doesn’t need to be stereo music.
It’s already an elegant codec, and I look forward to seeing it in use in HTML5. Gotta praise Firefox for jumping in and adding support for it right away.
I expect to see this as THE codec for video/tele conferencing and streaming music, especially in mobile devices.