Vorbis

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This article is about the audio compression codec. For the Discworld character, see Discworld characters#Vorbis.
Vorbis <tr><td style="text-align: center;" colspan="2">Image:Fish xiph org.png</td></tr> <tr><th style="white-space: nowrap;">File extension:</th><td>.ogg</td></tr><tr><th style="white-space: nowrap;">MIME type:</th><td>application/ogg</td></tr><tr><th style="white-space: nowrap;">Developed by:</th><td>Xiph.Org Foundation</td></tr><tr><th style="white-space: nowrap;">Type of format:</th><td>Audio codec</td></tr><tr><th style="white-space: nowrap;">Contained by:</th><td>Ogg</td></tr><tr><th style="white-space: nowrap;">Standard(s):</th><td>Specification</td></tr>

Vorbis is an open source, lossy audio codec project headed by the Xiph.Org Foundation and, for all purposes, intended to replace MP3. It is most commonly used in conjunction with the Ogg container and is then called Ogg Vorbis.

Vorbis development began following a September 1998 letter from Fraunhofer Gesellschaft announcing plans to charge licensing fees for the MP3 audio format. Soon after, founder Christopher "Monty" Montgomery commenced work on the project and was assisted by a growing number of other developers. They continued refining the source code until a stable version 1.0 of the codec was released on July 19, 2002.

The latest official version is 1.1.2 released on 2005-11-28, but there are some fine-tuned forks available such as aoTuV. Source code (called libvorbis) for the Xiph.Org release is available from the official xiph.org download page.

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[edit] Usage

Image:Somevorbisapps.png
Vorbis applications OggdropXPd, a drag-and-drop encoder, and Amarok, an audio player. Vorbis' support of Unicode is also evident.

The Ogg Vorbis format has proven popular among supporters of free software. They argue that its higher fidelity and completely free nature make it an excellent replacement for patented and restricted formats like MP3. However, MP3 has been widely used since the mid-1990s and as of 2006, is still the most popular standard in the consumer electronics industry. Of the consumer products supporting lossy compressed digital audio, virtually all support playback of MP3 audio while relatively few support alternative formats like Ogg Vorbis.

In the commercial sector, Vorbis support is on the rise. Many mainstream video game titles store game audio as Vorbis. Popular software players support Ogg Vorbis playback either natively or through an external plugin. A number of Web sites use it, such as Jamendo and Mindawn, as well as several national radio stations like Radio France, CBC Radio and Virgin Radio. Since it is not encumbered by patents and has been found to offer high audio fidelity, Vorbis is the preferred audio format on many wikis, including Wikipedia.

[edit] Codec comparisons

For many applications, Vorbis has clear advantages over other lossy audio codecs in that it is patent-free and has open-source implementations and therefore is free to use, implement, or modify as one sees fit, yet produces smaller files than most other codecs at equivalent or higher quality.

Listening tests have attempted to find the best quality lossy audio codecs at certain bitrates. Some conclusions made by recent listening tests:

  • At 128kb/s the most recent large-scale test shows a four-way tie between aoTuV Vorbis, LAME-encoded MP3, WMA Pro, and QuickTime AAC, with each codec essentially transparent (sounds identical to the original music file).
  • At mid to low bitrates, private tests (80 kb/s, 96 kb/s) shows that aoTuV Vorbis has a better quality than other lossy audio codecs (AAC (HE and LC), MP3, MPC, WMA).
  • At high bitrates (more than 128 kb/s), most people do not hear significant differences. However, trained listeners can often hear significant differences between codecs at identical bitrates, and aoTuV Vorbis performs very well, i.e. better than other formats such as AAC, MP3, and MPC (see this 180 kb/s test on classical music).

Many of these results, however, are difficult to keep up to date due to the ever-evolving nature of the codecs.

[edit] 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 his/her 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 judgement of the quality. Otherwise, the encoded version is considered to be transparent. Below are links to several listening test results.

[edit] Techical Details

Vorbis nominal bitrate at quality levels for 44.1 kHz stereo input (effective bitrate may vary)
Quality Nominal Bitrate
Official Xiph.Org Vorbis aoTuV beta 3 and later
-q-2 not available 32 kb/s
-q-1 45 kb/s 48 kb/s
-q0 64 kb/s
-q1 80 kb/s
-q2 96 kb/s
-q3 112 kb/s
-q4 128 kb/s
-q5 160 kb/s
-q6 192 kb/s
-q7 224 kb/s
-q8 256 kb/s
-q9 320 kb/s
-q10 500 kb/s

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 settings run from -1 to 10 (-2 to 10 for aoTuV tunings) and are an arbitrary metric — files encoded at -q5, for example, 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 bitrates mentioned above are only approximate; Vorbis is inherently variable-bitrate (VBR), so bitrate may vary considerably from sample to sample.

Vorbis uses the modified discrete cosine transform (MDCT) 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), which many people find more pleasant than the metallic warbling in the MP3 format . The sound of the compression at low bitrates can be perhaps described as reverberations in an amphithreatre or a room.

Many users feel that Vorbis reaches transparency (sound quality that is indistinguishable from the original source recording) at a quality setting of -q5, approximately 160 kbit/s.[citation needed] For comparison, it is commonly felt that MP3 reaches transparency at around 192 kbit/s, resulting in larger file sizes for the same sound quality.

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. The most consistently cited problem with Vorbis is pre-echo, a faint copy of a sharp attack that occurs just before the actual sound (the sound of castanets is commonly cited as causing this effect). Most of the tuned versions of Vorbis attempt to fix this 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.

The Vorbis format supports bitrate peeling for reducing the bitrate of already encoded files, and an experimental implemention is available.<ref>Template:Cite web</ref> However, encoding at the lower bitrate will result in better audio quality than this experimental bitrate peeler.

Vorbis streams can be encapsulated in other media container formats besides Ogg. A commonly used alternative is Matroska.

[edit] Metadata

Vorbis metadata, called comments, support metadata 'tags' similar to those implemented in the ID3 standard for MP3. The metadata is stored in a vector of eight-bit-clean 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 integer that can be expressed in 32 bits). This vector is stored in the second header packet that begins a Vorbis bitstream.<ref>Template:Cite web</ref>

The strings are assumed to be encoded as UTF-8. Music tags are typically implemented as strings of the form "[TAG]=[VALUE]", for instance, "ARTIST=The John Smith Band". Since there is no strict field definition as in 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 discnumber and Replay Gain information.

[edit] Licensing

Knowledge of Vorbis's specifications is in the public domain. Concerning the specification itself, Xiph.Org reserves the right to set the Vorbis specification and certify compliance. Its libraries are released under a BSD-style license and its tools are released under the GPL (GNU General Public License). The libraries were originally released under the GNU Lesser General Public Licence, but a BSD licence was later chosen with the endorsement of Richard Stallman.<ref>Template:Cite web</ref> The Xiph.Org Foundation states that Vorbis, like all its developments, is completely free from the licensing or patent issues raised by other proprietary formats such as MP3. Although Xiph.Org states it has conducted a patent search that supports its claims, outside parties (notably engineers working on rival formats) have expressed doubt that Vorbis is free of patented technology.<ref>Template:Cite web</ref>

Xiph.Org maintains that it was privately issued a legal opinion subject to attorney/client privilege. It has not released an official statement on the patent status of Vorbis, pointing out that such a statement is technically impossible due to the number and scope of patents in existence and the questionable validity of many of them. Such issues cannot be resolved outside of a court of law. Some Vorbis proponents have derided the uncertainty concerning the patent status as "FUD": misinformation spread by large companies with a vested interest.

Ogg Vorbis is supported by several large digital audio player manufacturers such as Samsung, Rio, Neuros Technology, Cowon, and iRiver. Many feel that the growing support for the Vorbis codec within the industry supports their interpretation of its patent status, as multinational corporations are unlikely to distribute software with questionable legal status. The same could be said about its growing popularity in other commercial enterprises like mainstream computer games.

[edit] Hardware and software support

[edit] Hardware

Tremor, a version of the Vorbis decoder which uses fixed-point arithmetic (rather than floating point), was made available to the public on September 2, 2002 (also under a BSD-style license). Tremor, or platform specific versions based on it, is more suited to implementation on the limited facilities available in commercial portable players. A number of versions that make adjustments for specific platforms and include customized optimizations for given embedded microprocessors have been produced. Several hardware manufacturers have expressed an intention to produce Vorbis-compliant devices, and new Vorbis devices seem to be appearing at a steady rate, especially in South Korea, although availability may differ from country to country.

The VorbisHardware node at the xiph.org wiki has an up-to-date list of Vorbis-supporting hardware, such as portables, PDAs, and microchips. Most digital audio players supported by Rockbox, an open-source firmware project, are capable of decoding Vorbis.

[edit] Software

Software supporting Vorbis exists for many platforms. Although Apple iTunes does not natively support Vorbis, Xiph.Org provides a QuickTime component which can be used in iTunes and QuickTime on both Microsoft Windows and Mac OS. On Windows, DirectShow filters exist to decode Vorbis in multimedia players like Windows Media Player and others which support DirectShow. Vorbis is well-supported on the Linux platform in programs like XMMS, xine, and many more.

More information about Vorbis-supporting software can be found at the VorbisSoftwarePlayers node at the xiph.org wiki. Users can test these programs using the list of Vorbis audio streams available at the Vorbis streams page on the same wiki.

[edit] Etymology

  • "Ogg" is derived from ogging, jargon that arose in the computer game, Netrek. Originally meaning a kamikaze attack and later, more generally, to do something forcefully possibly without consideration of the drain on future resources.
  • "Vorbis" is named after a Discworld character, Exquisitor Vorbis in Small Gods.

[edit] Notes and references

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[edit] See also

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[edit] External links


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