| Opus | |
|---|---|
 | |
| Filename extension | .opus[1] |
| Internet media type | audio/ogg[2]audio/opus (RTP)[3] |
| Developed by | IETFcodec working group |
| Initial release | September 11, 2012; 13 years ago (2012-09-11) |
| Type of format | Lossyaudio |
| Contained by | Ogg,Matroska,WebM,MPEG-TS,ISOBMFF,CAF |
| Extended from | SILK,CELT |
| Standard | RFC 6716 |
| Open format? | Yes |
| Free format? | Yes |
| Website | opus-codec |
| libopus | |
|---|---|
 Screenshot of opusinfo showing information of an .opus file | |
| Developer | Xiph.Org Foundation |
| Initial release | August 26, 2012 (2012-08-26) |
| Stable release | 1.5.2 / April 12, 2024; 19 months ago (2024-04-12) |
| Repository | gitlab |
| Written in | C89 |
| Platform | Cross-platform |
| Type | Audio codec,reference implementation |
| License | 3-clause BSD license (with patent license) |
| Website | Opus codec downloads |
Opus is afree and open sourcelossyaudio coding format developed by theXiph.Org Foundation and standardized by theInternet Engineering Task Force, designed for efficientlow-latency encoding of bothspeech and general audio. Due to its lower latency relative to other standard codecs, Opus finds specific use cases in real-time interactive communication for low-end embedded processors.[4][5] Opus replaces bothVorbis andSpeex for new applications.
Opus combines the speech-orientedLPC-basedSILK algorithm and the lower-latencyMDCT-basedCELT algorithm, switching between or combining them as needed.[4] Bitrate, audio bandwidth, complexity, and algorithm choice can be adjusted for each individual frame. Opus has low algorithmic delay (26.5 ms by default)[6]: 2 ideal for use as part of a real-time communication link,networked music performances, and livelip sync; by trading off quality or bitrate, the delay can be further reduced down to 5 ms. Its delay thus is significantly lower compared to competing codecs, which require well over 100 ms. Opus remains competitive with these formats in terms of quality per bitrate.[7]
As anopen format standardized through RFC 6716, areference implementation calledlibopus is available under theNew BSD License. The reference has bothfixed-point andfloating-point optimizations for low- and high-end devices, withSIMD optimizations on platforms that support them. All knownsoftware patents that cover Opus are licensed underroyalty-free terms.[8] Opus is widely used as avoice over IP (VoIP) codec in applications such asDiscord,[9]WhatsApp,[10][11][12] and thePlayStation 4.[13] Listening tests have ranked it higher-quality than other standard audio formats at any given bitrate untiltransparency is reached, includingMP3,AAC, andHE-AAC.[14][15]
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Opus supportsconstant andvariable bitrate encoding from 6 kbit/s to 510 kbit/s (or up to 256 kbit/s per channel for multi-channel tracks), frame sizes from 2.5 ms to 60 ms, and fivesampling rates from 8 kHz (with 4 kHz bandwidth) to 48 kHz (with 20 kHz bandwidth, the humanhearing range). An Opus stream can support up to 255audio channels, and it allowschannel coupling between channels in groups of two using mid-side coding.[6]: 1
Opus has very shortlatency (26.5 ms using the libopus default of 20 ms frames and default application setting), which makes it suitable forreal-time applications such astelephony,voice over IP andvideoconferencing;[5] research byXiph led to theCELT codec currently adopted by Opus, which preserved quality while maintaining low delay. In any Opus stream, the bitrate, bandwidth, and delay can be continually varied without introducing any distortion or discontinuity; mixing packets from different streams will cause a smooth non-intrusive change, rather than the distortion common in other codecs. Unlike Vorbis, Opus does not require largecodebooks for each individual file, making it relatively more efficient and resilient for short clips of audio.
The Opus format is based on a combination of the full-bandwidthCELT format and the speech-orientedSILK format, both heavily modified: CELT is based on themodified discrete cosine transform (MDCT) that most music codecs use, usingCELP techniques in the frequency domain for better prediction, while SILK useslinear predictive coding (LPC) and an optional long-term prediction filter to model speech. In Opus, both were modified to support more frame sizes, as well as further algorithmic improvements and integration, such as using CELT'srange encoder for both types. To minimize overhead at low bitrates, if latency is not as pressing, SILK has support for packing multiple 20 ms frames together, sharing context and headers; SILK also allows Low Bitrate Redundancy (LBRR) frames, allowing low-quality packet loss recovery. CELT includes both spectral replication and noise generation, similar to AAC's SBR and PNS, and can further save bits by filtering out all harmonics of tonal sounds entirely, then replicating them in the decoder.[16] Better tone detection is an ongoing project to improve quality.
The format has three different modes: speech, hybrid, and CELT. When compressing speech, SILK is used for audio frequencies up to 8 kHz. If wider bandwidth is desired, a hybrid mode uses CELT to encode the frequency range above 8 kHz. The third mode is pure-CELT, designed for general audio. SILK is inherently VBR and cannot hit a bitrate target, while CELT can always be encoded to any specific number of bytes, enabling hybrid and CELT mode when CBR is required.
SILK supports frame sizes of 10, 20, 40 and 60 ms. CELT supports frame sizes of 2.5, 5, 10 and 20 ms. Thus, hybrid mode only supports frame sizes of 10 and 20 ms; frames shorter than 10 ms will always use CELT mode. A typical Opus packet contains a single frame, but packets of up to 120 ms are produced by combining multiple frames per packet. Opus can transparently switch between modes, frame sizes, bandwidths, and channel counts on a per-packet basis, although specific applications may choose to limit this.
The reference implementation is written inC and compiles on hardware architectures with or without afloating-point unit, although floating-point is currently required for audio bandwidth detection (dynamic switching between SILK, CELT, and hybrid encoding) and most speed optimizations.
Opus packets are not self-delimiting, but are designed to be used inside acontainer of some sort which supplies the decoder with each packet's length. Opus was originally specified for encapsulation inOgg containers, specified asaudio/ogg; codecs=opus, and for Ogg Opus files the.opus filename extension is recommended.[2] Opus streams are also supported inMatroska,[17]WebM,[18]MPEG-TS,[19] andMP4.[20]
Alternatively, each Opus packet may be wrapped in anetwork packet which supplies the packet length. Opus packets may be sent over an ordered datagram protocol such asRTP.[21]
An optional self-delimited packet format is defined in an appendix to the specification.[22] This uses one or two additional bytes per packet to encode the packet length, allowing packets to be concatenated without encapsulation.
Opus allows the following bandwidths during encoding. Opus compression does not depend on the input sample rate; timestamps are measured in 48 kHz units even if the full bandwidth is not used. Likewise, the output sample rate may be freely chosen. For example, audio can be input at 16 kHz yet be set to encode only narrowband audio.[23]
| Abbreviation | Audio bandwidth | Effective sample rate |
|---|---|---|
| NB (narrowband) | 4 kHz | 8 kHz |
| MB (medium-band)[note 1] | 6 kHz | 12 kHz |
| WB (wideband) | 8 kHz | 16 kHz |
| SWB (super-wideband) | 12 kHz | 24 kHz |
| FB (fullband)[note 2] | 20 kHz | 48 kHz |
Opus was proposed for the standardization of a new audio format at the IETF, which was eventually accepted and granted by thecodecworking group. It is based on two initially separate standard proposals from the Xiph.Org Foundation and Skype Technologies S.A. (nowMicrosoft). Its main developers are Jean-Marc Valin (Xiph.Org, Octasic,Mozilla Corporation, Amazon), Koen Vos (Skype), and Timothy B. Terriberry (Xiph.Org, Mozilla Corporation, Amazon). Among others, Juin-Hwey (Raymond) Chen (Broadcom), Gregory Maxwell (Xiph.Org,Wikimedia), andChris Montgomery (Xiph.Org) were also involved.
The development of the CELT part of the format originated from discussions on a successor forVorbis under the working nameGhost. As a newer speech codec from the Xiph.Org Foundation, Opus replaces Xiph's older speech codecSpeex, an earlier project of Jean-Marc Valin. CELT has been worked on since November 2007.
The SILK part has been under development at Skype since January 2007 as the successor of theirSVOPC, an internal project to make the company independent from third-party codecs likeiSAC andiLBC and respective license payments.
In March 2009, Skype suggested the development and standardization of a wideband audio format within the IETF. Nearly a year passed with much debate on the formation of an appropriateworking group.[24] Representatives of several companies which were taking part in the standardization of patent-encumbered competing format, includingPolycom andEricsson—the creators and licensors ofG.719—as well asFrance Télécom,Huawei and theOrange Labs (department of France Télécom), which were involved in the creation ofG.718, stated objections against the start of the standardization process for a royalty-free format. (Some of the opponents would later claim patent rights that Xiph dismissed; see below.)[25] The working group finally formed in February 2010, and even the corresponding Study Group 16 from the ITU-T pledged to support its work.
In July 2010, a prototype of a hybrid format was presented that combined the two proposed format candidates SILK and CELT. In September 2010, Opus was submitted to the IETF as proposal for standardization. For a short time the format went under the name ofHarmony before it got its present name in October 2010.[26] At the beginning of February 2011, thebitstream format was tentatively frozen, subject to last changes.[27] Near the end of July 2011, Jean-Marc Valin was hired by theMozilla Corporation to continue working on Opus.[28]
In November 2011, the working group issued the last call for changes on the bitstream format. The bitstream has been frozen since January 8, 2012.[29] On July 2, 2012, Opus was approved by theIETF for standardization.[30] The reference software entered release candidate state on August 8, 2012.[31] The final specification was released as RFC 6716 on September 10, 2012.[32][33] and versions 1.0 and 1.0.1 of thereference implementation libopus were released the day after.
On July 11, 2013, libopus 1.0.3 brought bug fixes and a newsurround sound API that improves channel allocation and quality, especially forLFE.[34]
On December 5, 2013, libopus 1.1 was released,[34] incorporating overall speed improvements and significant encoder quality improvements: Tonality estimation boosts bitrate and quality for previouslyproblematic samples, like harpsichords; automated speech/music detection improves quality in mixed audio;mid-side stereo reduces the bitrate needs of many songs; band precision boosting for improved transients; and DC rejection below 3 Hz. Two newVBR modes were added: unconstrained for more consistent quality, and temporal VBR that boosts louder frames and generally improves quality.
libopus 1.1.1 was released on November 26, 2015, and 1.1.2 on January 12, 2016, both adding speed optimizations and bug fixes. July 15, 2016 saw the release of version 1.1.3 and includes bug fixes, optimizations, documentation updates and experimentalAmbisonics work.
libopus 1.2 Beta was released on May 24, 2017. libopus 1.2 was released on June 20, 2017.[35] Improvements brought in 1.2 allow it to create fullband music at bitrates as low as 32 kbit/s, and wideband speech at just 12 kbit/s.[36]
libopus 1.2 includes optional support for the decoder specification changes made in drafts of RFC 8251, improving the quality of output from such low-rate streams.[37]
libopus 1.3 was released on October 18, 2018.[38] The Opus 1.3 major release again brings quality improvements, new features, and bug fixes.[39] Changes since 1.2.x include:
Notable bug fixes include:
libopus 1.3.1 was released on April 12, 2019.[40] This Opus 1.3.1 minor release fixes an issue with the analysis on files with digital silence (all zeros), especially onx87 builds (mostly affects 32-bit builds). It also includes two new features:
libopus 1.4 was released on April 20, 2023.[41] Opus 1.4 brings the following improvements and fixes:
libopus 1.5 was released on March 4, 2024.[44] It added the following backward-compatible improvements:
The codec is under active development.[45] The current focus is on adding adeep learning based redundancyencoder that enhances packet loss concealment by embedding one second of recovery data in each encoded packet. The deep redundancy (DRED) algorithm was developed by among others Jean-Marc Valin, Ahmed Mustafa, Jan Büthe, Timothy Terriberry,Chris Montgomery, Michael Klingbeil, andParis Smaragdis fromAmazon Web Services[46] with sponsorship to open source the algorithm and subsequently extend theIETF standard from Sid Rao.[47] This encoder is a backwards compatible change to the codec enabling customers to easily upgrade applications to take advantage of this machine learning capability. Adraft RFC is underway to standardize the new capability.[48] This RFC is one of the first attempts to standardize a deep learning algorithm in the IETF.
Opus performs relatively well at both low and highbitrates.[7]
In listening tests around 64 kbit/s, Opus shows superior quality compared toHE-AAC codecs, which were previously dominant due to their use of the patentedspectral band replication (SBR) technology.[49][14] In listening tests around 96 kbit/s, Opus shows slightly superior quality compared toAAC and significantly better quality compared toVorbis andMP3.[15][50]
Opus has very low algorithmic delay,[4] a necessity for use as part of a low-audio-latency communication link, which can permit natural conversation,networked music performances, orlip sync at live events. Total algorithmic delay for an audio format is the sum of delays that must be incurred in the encoder and the decoder of a live audio stream regardless of processing speed and transmission speed, such as buffering audio samples into blocks or frames, allowing forwindow overlap and possibly allowing fornoise-shaping look-ahead in a decoder and any other forms of look-ahead, or for an MP3 encoder, the use ofbit reservoir.[51]
Total one-way latency below 150 ms is the preferred target of mostVoIP systems[52] to enable natural conversation with turn-taking little affected by delay. Musicians typically feel in-time with up to around 30 ms audio latency,[53] roughly in accord with the fusion time of theHaas effect, though matching playback delay of each user's own instrument to the round-trip latency can also help.[54]It is suggested for lip sync that around 45–100 ms audio latency may be acceptable.[55]
Opus permits trading-off reduced quality or increased bitrate to achieve an even smaller algorithmic delay (5.0 ms minimum).[56] While the reference implementation's default Opus frame is 20.0 ms long, the SILK layer requires a further 5.0 ms lookahead plus 1.5 ms for resampling, giving a default delay of 26.5 ms. When the CELT layer is active, it requires 2.5 ms lookahead forwindow overlap to which a matching delay of 4.0 ms is added by default to synchronize with the SILK layer. If the encoder is instantiated in the specialrestricted low delay mode, the 4.0 ms matching delay is removed and the SILK layer is disabled, permitting the minimal algorithmic delay of 5.0 ms.[6]: 2
The format and algorithms are openly documented and thereference implementation is published asfree software. Xiph's reference implementation is calledlibopus and a package calledopus-tools provides command-line encoder and decoder utilities. It is published under the terms of aBSD-like license. It is written inC and can be compiled for hardware architectures with or without afloating-point unit. The accompanying diagnostic toolopusinfo reports detailed technical information about Opus files, including information on the standard compliance of the bitstream format. It is based onogginfo from thevorbis-tools and therefore — unlike the encoder and decoder — is available under the terms of version 2 of theGPL.
RFC 6716 contains a complete source code for an older version of the reference implementation written in C. RFC8251 contains errata. Libopus is the more up-to-date but non-normative branch of the reference implementation.
TheFFmpeg project[57] has encoder and decoder implementations not derived from the reference library. The documentation describes it as CELT-only and poorer-quality than the reference.[58]
The libopus reference library has been ported to bothC# andJava as part of a project called Concentus. These ports sacrifice performance for the sake of being easily integrated into cross-platform applications.[59]
Digital Radio Mondiale – a digital radio format for AM frequencies – can broadcast and receive Opus audio (albeit not recognised in official standard) using Dreamsoftware-defined radio.
TheWikimedia Foundation sponsored a free and open source onlineJavaScript Opus encoder for browsers supporting the requiredHTML5 features.[60]
A list of radio stations that stream using Opus audio codec can be found on theXiph.Org FoundationIcecast directory.[61]
In late 2014 and 2015, Google's video platformYouTube started offering Opus audio along withVP9 video in theWebM file format, throughDASH streaming.[62] In July 2024, YouTube rolled out an additional high quality audio format option, 256 kbit/s Opus, toYouTube Music Premium subscribers.[63]
Since 2016,WhatsApp has been using Opus as its audio file format.[64]
Signal switched fromSpeex to Opus audio codec for better audio quality in the beginning of 2017.
In 2018,SoundCloud switched from MP3 to Opus, reducing half of its required bandwidth for music streaming.[65]
In January 2021,Vimeo introduced Opus to its video platform.[66]
In 2021, the Danish journalism websiteZetland switched from MP3 to Opus for its articles' audio recordings, which attained a 35 percent reduction in bandwidth and reduced climate footprint.[67][68]
Most end-user software relies onmultimedia frameworks provided by theoperating system. Native Opus codec support is implemented in most major multimedia frameworks forUnix-like operating systems, includingGStreamer,FFmpeg, andLibav libraries.[69][70][71]
TheWebM container.webm is mostly used ononline video platforms (e.g.YouTube), and is usually treated as a video file by operating systems & media players. Even if a WebM file contains only Opus audio and no video, some music players do not recognize WebM files as audio files and do not support reading of file metadata.[72]
TheOgg container.opus is preferred for audio-only files, and most media players have support for audio file metadata tagged in theVorbis comment format.
Google added native support for Opus audio playback inAndroid 5.0 "Lollipop".[73] However, it was limited to Opus audio encapsulated inMatroska andWebM containers, such as.mkv,.mka and.webm files.[74][75]Android 7.0 "Nougat" introduced support for Opus audio encapsulated inOgg containers.[76]Android 10 finally added native support for.opusextensions.[77]
Due to the addition ofWebRTC support in Apple'sWebKit rendering engine,macOS High Sierra andiOS 11 were released with native playback support for Opus audio encapsulated inCore Audio Format containers.[78]macOS Sonoma added support for mono and stereo Opus audio encapsulated in MPEG-4 and WebM containers inSafari.[79][80] Safari 18.4 added support for Opus and Vorbis codecs with Ogg containers when usingmacOS Sequoia 15.4 or iOS 18.4 and later.[81][82]
OnWindows 10, version1607, Microsoft provided native support for Opus audio encapsulated inMatroska andWebM containers.[83] On version1709, support for Opus audio encapsulated inOgg containers was made available through a pre-installed add-on called Web Media Extensions.[84] On Windows 10 version1903, native support for the.opus extension was added.[77] OnWindows 8.1 and older, third-party decoders, such asLAV Filters, are available to provide support for the format.[85] A January 2024 update for Windows 10 & 11 caused File Explorer to freeze up when renaming or deleting Ogg files (including .oga, .ogg, .ogm, .ogv, .ogx, .opus), due to bugs involving theMF Media Source Pack Property Handler shell extension by Microsoft and theWeb Media Extensions package in Windows, which were not patched before the Windows Updates were rolled out.[86]
iOS 17 includes support for natively encoding and decoding the Opus codec through the operating system'sAudioToolbox framework. Playback ofOgg files in the.opus container is supported through theFiles application. Opus is additionally supported within theCore Audio Format.caf container, theQuickTime.mov container, and theISOBMFF orMP4 file format.mp4 container.[note 3] As the AudioToolbox framework is shared withmacOS, similar support should be present in both operating systems as they further develop.[88] In 2024,Apple Podcasts does not support Opus.[89]
| Windows | macOS | Linux | Android | iOS | |
|---|---|---|---|---|---|
| Codec support | Yes Partial: Win 10 v1607, 1709, 1809 Full: Win 10 v1903 | Yes Partial: macOS High Sierra (10.13) Full: macOS Sonoma (14.0) | Yes | Yes Partial: Android 5, 6, 7 Full: Android 10 | Yes Partial: iOS 11 Full: iOS 17 |
| Container support | On Windows 10 Anniversary Update (1607): WebM (.webm is not recognised; requires pseudo extension) Matroska (.mka, .mkv) On Windows 10 Fall Creators Update (1709) with Web Media Extensions add-on: On Windows 10 October 2018 Update (1809): On Windows 10 May 2019 Update (1903): | OnmacOS High Sierra: Core Audio Format (.caf) OnmacOS Monterey: OnmacOS Sonoma: OnmacOS Sequoia: | Ogg (.opus) WebM (.webm) Matroska (.mka, .mkv) MPEG-TS (.ts) | OnAndroid 5: WebM (.webm) Matroska (.mka, .mkv) OnAndroid 7: OnAndroid 10: | OniOS 17 and later: Supports playback throughFiles; supportsOgg as .opus; supported inMP4, standalone[note 3] or in combination with any officially supported video codec; standalone and expanded but unofficially supported combinations inMOV; standalone support inCAF OniOS 18.4 and later: |
| Notes | On Windows 10 &Windows 11: OnWindows 10: OnWindows 8.1 and older: | In Safari 15, Apple added support for Opus audio in WebM containers. Experimental support is only present in macOS.[94] | — | – Up till Android 9, the filename extension .opus was not recognised (substitute with a pseudo file extension such as .ogg or .m4a)[76] | – When originally introduced in iOS 11, only constant bitrate Opus audio in a CAF container was supported. – In Safari 15, Apple added support for Opus audio in WebM containers. However, it is non-functional in iOS 15, as of September 2021.[95] – In iOS 17, native support for playing variable bitrate multichannel .opus files throughQuick Look in the Files app was implemented; the codec is additionally natively supported if encapsulated in ISOBMFF/MP4, CAF, or MOV. – As of iOS 17, WebM support is present in Safari, but not Files. – Opus can additionally be utilized within MP4 in combination with any video codec supported by the device. |
While support in multimedia frameworks automatically enables Opus support in software which is built on top of such frameworks, several applications developers made additional efforts for supporting the Opus audio format in their software. Such support was added toAIMP,[96]Amarok,[97] cmus,Music Player Daemon,foobar2000,[98]Mpxplay,MusicBee,[99]SMplayer,VLC media player,[100]Winamp[101] andXMPlay audio players;Icecast,[102]Airtime (software)[103] audio streaming software; andAsunder audio CD ripper,CDBurnerXP CD burner, FFmpeg, Libav andMediaCoder media encoding tools. Streaming Icecast radio trials are live since September 2012 and January 2013.[104][105]SteamOS uses Opus or Vorbis for streaming audio.[106]
Opus support is mandatory forWebRTC implementations.[107] Opus is supported inMozilla Firefox,[108]Chromium andGoogle Chrome,[109]Blink-basedOpera,[110][111] as well as all browsers forUnix-like systems relying onGStreamer for multimedia formats support. AlthoughInternet Explorer will not provide Opus playback natively, support for the format is built into theEdge browser, along withVP9, for fullWebM support.[112][113] Safari supports Opus as of iOS 11 and macOS High Sierra.[114]
Due to its abilities, Opus gained early interest fromvoice over IP (VoIP) software vendors. SeveralSIP clients, includingAcrobits Softphone, CSipSimple (via additional plug-in),Empathy (via GStreamer),Jitsi,[115]Tuenti,[116]Line2 (currently only oniOS),[117]Linphone,[118]Phoner andPhonerLite,[119]SFLphone,[120]Telephone,Mumble,Discord[9] andTeamSpeak 3 voice chat software also support Opus.[121][122][123] TrueConf supports Opus in its VoIP products.[124]Asterisk lacked builtin Opus support for legal reasons,[125] but a third-party patch was available for download[126] and official support via abinary blob was added in September 2016.[127]Tox P2P videoconferencing software uses Opus exclusively.[128] Classified-ads distributed messaging app sends raw opus frames inside TLS socket in its VoIP implementation.[129]
Opus is widely used as thevoice codec inWhatsApp,[10][12][11] which has over 1.5 billion users worldwide.[130] WhatsApp uses Opus at 8–16kHzsampling rates,[10][11] with theReal-time Transport Protocol (RTP).[12] ThePlayStation 4 video game console also uses the CELT/Opus codec for itsPlayStation Network system party chat.[13] TheSource Engine uses Opus for itsvoice chat system.[131] It is also used in the Zoom videoconferencing app.[132]
Since version 3.13,Rockbox enables Opus playback on supportedportable media players, including some products from theiPod series byApple, devices made byiriver,Archos andSandisk, and onAndroid devices using "Rockbox as an Application".[133][134] All recentGrandstreamIP phones support Opus audio both for encoding and decoding. OBihai OBi1062, OBi1032 and OBi1022IP phones all support Opus. Recent BlueSound wireless speakers support Opus playback.[135] Devices running Hiby OS, like the Hiby R3, are capable of decoding Opus files natively.
Many broadcast IP codecs include Opus such as those manufactured byComrex,GatesAir and Tieline.[136]
TheSonyPlayStation 5 supports capturing 1080p and 2160p footage using VP9 video and Opus audio in a WebM container.[137][138]
Android 13 supports Opus as a bluetooth headphone (A2DP) codec, motivated by its low latency, which is important for head-tracked spatial audio. Google's Pixel Buds Pro supports this A2DP codec.[139]
As an open standard, the algorithms are openly documented, and areference implementation (including thesource code) is published.Broadcom and the Xiph.Org Foundation ownsoftware patents on some of the CELT algorithms, andSkype Technologies/Microsoft own some on the SILK algorithms; each offers a royalty-free perpetual license for use with Opus, reserving only the right to make use of their patents to defend against infringement suits of third parties.Qualcomm,Huawei,France Telecom, andEricsson have claimed that their patents may apply, which Xiph's legal counsel denies, and none have pursued any legal action.[8][140] The Opus license automatically and retroactively terminates for any entity that attempts to file a patent suit.
.m4a extension,[87] relegating.mp4 to video, so playback will present a blank video track even if none are present within the container. Re-naming a.mp4 file to.m4a will work as expected, however..opus filename extension isn't recognized by Android's MediaScanner service andUniversal Windows Platform apps. A pseudo extension of another audio format (such as.ogg or.m4a) is required to detect and playback files.[76][90]CELT's look-ahead is 2.5 ms, while SILK's look-ahead is 5 ms, plus 1.5 ms for the resampling (including both encoder and decoder resampling). For this reason, the CELT path in the encoder adds a 4 ms delay. However, an application can restrict the encoder to CELT and omit that delay. This reduces the total look-ahead to 2.5 ms.
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This article contains quotations from the Opus Codec website, which is available under theCreative Commons Attribution 3.0 (CC BY 3.0) license.