Libvorbis

Vorbis is a free and open-source software project headed by the Xiph.Org Foundation. The project produces an audio coding format and software reference encoder/decoder ( codec) for lossy audio compression. Vorbis is most commonly used in conjunction with the Ogg container format and it is therefore often referred to as Ogg Vorbis.

Vorbis is a continuation of audio compression development started in 1993 by Chris Montgomery. Intensive development began following a September 1998 letter from the Fraunhofer Society announcing plans to charge licensing fees for the MP3 audio format. The Vorbis project started as part of the Xiphophorus company's Ogg project (also known as OggSquish multimedia project). Chris Montgomery began work on the project and was assisted by a growing number of other developers. They continued refining the source code until the Vorbis file format was frozen for 1.0 in May 2000. Originally licensed as LGPL, in 2001 the Vorbis license was changed to the BSD license to encourage adoption, with the endorsement of Richard Stallman. A stable version (1.0) of the reference software was released on July 19, 2002.

The Xiph.Org Foundation maintains a reference implementation, libvorbis. There are also some fine-tuned forks, most notably aoTuV, that offer better audio quality, particularly at low bitrates. aoTuV improvements were intended to be merged back into the reference codebase periodically, but that only happened once in libvorbis 1.1, due to the lack of upstream resources for reviewing further patches.

Since February 2013, Xiph.Org has stated that the use of Vorbis should be deprecated in favor of the Opus codec, which is also a Xiph.Org Foundation project and also free and open-source. Compared to Vorbis, Opus can simultaneously achieve higher compression efficiency—per both Xiph.Org itself and third-party listening tests—and lower encode/decode latency (in most cases, low enough for real-time applications such as internet telephony or live singing, rarely possible with Vorbis).

Name

Vorbis is named after a '' Discworld'' character Exquisitor Vorbis in ''Small Gods'' by Sir Terry Pratchett. The Ogg format, however, is ''not'' named after Nanny Ogg, another Discworld character; the name is in fact derived from ''ogging'', jargon that arose in the computer game Netrek.

Usage

Vorbis faces competition from other audio formats, such as MP3. Though Vorbis is technically superior (addressing many of the limitations inherent to the MP3 design), MP3 has a far higher public profile. Because Vorbis does not have financial support from large organisations, support for the format is not as widespread, though programs such as Audacity can convert to more popular formats, and support in games has gradually improved.

The Vorbis format has proven popular among supporters of free software. They argue that its higher fidelity and completely free nature, unencumbered by patents, make it a well-suited replacement for patented and restricted formats.

Vorbis has different uses for consumer products. Many video games store in-game audio as Vorbis, including '' Amnesia: The Dark Descent'', '' Grand Theft Auto: San Andreas'', '' Halo: Combat Evolved'', ''Minecraft'', and ''World of Warcraft'', among others. Popular software players support Vorbis playback either natively or through an external plugin. A number of websites, including Wikipedia, use it. Others include Jamendo and Mindawn, as well as several national radio stations like JazzRadio, Absolute Radio, NPR, Radio New Zealand and Deutschlandradio. The Spotify audio streaming service has used Vorbis in a basket of codecs for its audio streams, but now has also been using AAC profiles. Also, the French music site Qobuz offers its customers the possibility to download their purchased songs in Vorbis format, as does the American music site Bandcamp.

Quality

Listening tests conducted through 2014 showed Vorbis performed significantly better than many other lossy audio formats in that it produced smaller files at equivalent or higher quality while retaining computational complexity comparable to other MDCT formats such as AAC and Windows Media Audio.

Listening tests have attempted to find the best-quality lossy audio codecs at particular bitrates. Some conclusions made by listening tests:
* Low bitrate (less than 64 kbit/s): the most recent (2007) public multiformat test at 48 kbit/s showed that aoTuV Vorbis had a better quality than WMA and LC-AAC, the same quality as WMA Professional, and a lower quality than HE-AAC.
* Mid to low bitrates (less than 128 kbit/s down to 64 kbit/s): private tests in 2005 at 80 kbit/s and 96 kbit/s showed that aoTuV Vorbis had a better quality than other lossy audio formats (LC-AAC, HE-AAC, MP3, MPC, WMA).
* High bitrates (greater than 128 kbit/s): most people do not hear significant differences. However, trained listeners can often hear significant differences between codecs at identical bitrates, and aoTuV Vorbis performed better than LC-AAC, MP3, and MPC.

Due to the ever-evolving nature of audio codecs, the results of many of these tests have become outdated.

Listening tests

Listening tests are normally carried out as ABX tests, i.e., the listener has to identify an unknown sample X as being A or B, with A (the original) and B (the encoded version) available for reference. The outcome of a test must be statistically significant. This setup ensures that the listener is not biased by their expectations and that the outcome is very unlikely to be the result of chance. If sample X can be identified reliably, the listener can assign a score as a subjective judgment of the quality. Otherwise, the encoded version is considered to be ''transparent''. Below are links to several listening test results.
; 2005, July comparison: AAC vs MP3 vs Vorbis vs WMA at 80 kbit/s. States that Vorbis aoTuV beta 4 is the best encoder for either classical or various music in this bitrate, and that its quality is comparable to the LAME ABR MP3 at 128 kbit/s.
; 2005, August comparison: AAC vs MP3 vs Vorbis vs WMA at 96 kbit/s. States that Vorbis aoTuV beta 4 and AAC are tied as the best encoders for classical music in this bitrate, while aoTuV beta 4 is the best encoder for pop music, even better than LAME at 128 kbit/s.
; 2005, August comparison: MPC vs Vorbis vs MP3 vs AAC at 180 kbit/s. An audiophile listening test, which states that, for classical music, Vorbis aoTuV beta 4 has 93% percent probability of being the best encoder, tied with MPC. MPC is tied with both Vorbis, in the first place, and LAME in the second.
; 2011, April comparison by Hydrogenaudio: Vorbis vs HE-AAC vs Opus at 64 kbit/s. Vorbis was on average between the LC-AAC low anchor and Nero HE-AAC, while the upcoming Opus (by Xiph) was best.

Characteristic artifacts

As with most modern formats, the most consistently cited problem with Vorbis is pre-echo, a faint copy of a sharp attack that occurs just before the actual sound (for example, when reproducing the sound of castanets).

When the bitrate is too low to encode the audio without perceptible loss, Vorbis exhibits an analog noise-like failure mode, which can be described as reverberations in a room or amphitheater. Vorbis's behavior is due to the noise floor approach to encoding; see .

Technical details

Vorbis is intended for sample rates from 8 kHz telephony to 192 kHz digital masters and a range of channel representations (monaural, polyphonic, stereo, quadraphonic, 5.1, ambisonic, or up to 255 discrete channels). Given 44.1 kHz (standard CD audio sampling frequency) stereo input, the encoder will produce output from roughly 45 to 500 kbit/s (32 to 500 kbit/s for aoTuV tunings) depending on the specified quality setting. Quality setting goes from −0.1 to 1.0 for the Xiph library and −0.2 to 1.0 for aoTuV. Encoding front-ends map these values to an integer-based quality setting that goes from −1 to 10 for the Xiph library and −2 to 10 for aoTuV. Files encoded with a given quality setting should have the same quality of sound in all versions of the encoder, but newer versions should be able to achieve that quality with a lower bitrate. The bit rates mentioned above are only approximate; Vorbis is inherently variable-bitrate (VBR), so bitrate may vary considerably from sample to sample. (It is a free-form variable-bitrate codec and packets have no minimum size, maximum size, or fixed/expected size.)

Vorbis aims to be more efficient than MP3, with '' data compression transparency'' being available at lower bitrates.

Outline of coder algorithm

Vorbis I is a forward-adaptive monolithic transform codec based on the modified discrete cosine transform (MDCT). Vorbis uses the modified discrete cosine transform for converting sound data from the time domain to the frequency domain. The resulting frequency-domain data is broken into noise floor and residue components, and then quantized and entropy coded using a codebook-based vector quantization algorithm. The decompression algorithm reverses these stages. The noise-floor approach gives Vorbis its characteristic analog noise-like failure mode when the bitrate is too low to encode the audio without perceptible loss. The sound of compression artifacts at low bitrates can be perhaps described as reverberations in an amphitheater or a room.

Tuned versions

Various tuned versions of the encoder (Garf, aoTuV or MegaMix) attempt to provide better sound at a specified quality setting, usually by dealing with certain problematic waveforms by temporarily increasing the bitrate. Most of the tuned versions of Vorbis attempt to correct the pre-echo problem and to increase the sound quality of lower quality settings (-q-2 through -q4). Some tuning suggestions created by the Vorbis user community (especially the aoTuV beta 2 tunings) have been incorporated into the 1.1.0 release.

Bitrate peeling

The Vorbis ''format'' supports bitrate peeling for reducing the bitrate of already encoded files without re-encoding, and several experimental implementations exist.Experimental Ogg vorbis Bitrate Peeler, Bitrate reduction of ogg vorbis
/ref> However, the quality is "unusable" and markedly inferior to decoding and re-encoding.

The reason that peeling programs degrade quality so severely is that the format does not specify the relative importance of signal data, and existing encoders do not order the data in order of importance. Hence peelers cannot easily determine whether data is important or not.

Container formats

Vorbis streams can be encapsulated in other media container formats besides Ogg. A commonly used alternative is Matroska. It is also used in WebM, a container format based on a subset of Matroska. Vorbis streams can also be encapsulated in an RTP payload format.

Metadata

Vorbis metadata, called Vorbis comments, supports metadata tags similar to those implemented in the ID3 standard for MP3. The metadata is stored in a vector of byte strings of arbitrary length and size. The size of the vector and the size of each string in bytes is limited to 2³² − 1 (about 4.3 billion, or any positive integer that can be expressed in 32 bits). This vector is stored in the second header packet that begins a Vorbis bitstream.

The strings are assumed to be encoded as UTF-8. Music tags are typically implemented as strings of the form " AG ALUE, for instance, "ARTIST=The John Smith Band". The tag names are case-insensitive, thus typing "ARTIST=The John Smith Band" would be the same as "artist=The John Smith Band". Like the current version of ID3, users and encoding software are free to use whichever tags are appropriate for the content. For example, an encoder could use localized tag labels, live music tracks might contain a "Venue=" tag or files could have multiple genre definitions. Most applications also support common de facto standards such as disc number and ReplayGain information.

Licensing

Knowledge of Vorbis' specifications is in the public domain. Concerning the specification itself, the Xiph.Org Foundation reserves the right to set the Vorbis specification and certify compliance. Its libraries are released under the revised 3-clause BSD license and its tools are released under the GNU General Public License. The libraries were originally released under the GNU Lesser General Public Licence, but a BSD license was later chosen with the endorsement of Richard Stallman. The Xiph.Org Foundation states that Vorbis, like all its developments, is completely free from the licensing or patent issues raised by proprietary