WikiPortallus: Vorbis.html

Vorbis is a free and open source, lossy audio codec project headed by the Xiph.Org Foundation and intended to serve as a replacement for MP3.² It is most commonly used in conjunction with the Ogg container and is therefore 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 1.2.0 released on July 25, 2007, but there are some fine-tuned forks, most notably aoTuV, that offer better audio quality, particularly at low bitrates. Work is in progress to merge back those improvements. Source code (called libvorbis) for the Xiph.Org Foundation release is available from the official download page.³

Name

Usage

The Vorbis format has proven popular among supporters of free software.⁵ They argue that its higher fidelity and completely free nature, unencumbered by patents, make it a well-suited replacement for patented and restricted formats like MP3. However, MP3 has been widely used since the late-1990s and as of 2008, continues to remain popular in the consumer electronics industry.

Quality: Codec comparisons

For many applications, Vorbis has clear advantages over other lossy audio codecs in that it is patent-free and has free and 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.⁶⁷

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

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

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.

Audio quality: Characteristic artifacts

The most consistently cited problem with Vorbis is pre-echo,^{citation needed} 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). This also occurs in similar codecs based on transforming to the frequency domain, including MP3 and AAC.

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. This is considered more pleasant than the metallic warbling that MP3 exhibits in the same situation;^{citation needed} Vorbis's behavior is due to the noise floor approach to encoding; see technical details.

Technical details

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). 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 bitrates mentioned above are only approximate; Vorbis is inherently variable-bitrate (VBR), so bitrate may vary considerably from sample to sample.

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

Outline of codec

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 artifacts at low bitrates can be perhaps described as reverberations in an amphithreatre 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. 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.¹³

Bitrate peeling

The Vorbis format supports bitrate peeling for reducing the bitrate of already encoded files without re-encoding, and several experimental implementations exist.¹⁴¹⁵ However, the quality is "unusable",¹⁴ and markedly inferior to decoding and re-encoding.

The reason that naïve peeling programs degrade quality so severely is that the format does not specify the relative importance of signal data, nor do existing encoders order the data in order of importance, hence peelers cannot easily determine if data is important or not.

Container formats

Vorbis streams can be encapsulated in other media container formats besides Ogg.¹⁶ A commonly used alternative is Matroska.

Metadata

Vorbis metadata, called Vorbis 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.¹⁷

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". The tags 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 discnumber and Replay Gain information.

Licensing

Knowledge of Vorbis's 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.¹⁸ 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 the Xiph.Org Foundation 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.¹⁹

The Xiph.Org Foundation 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 "Fear, uncertainty and doubt" (also known as "FUD") : misinformation spread by large companies with a vested interest.

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.

Support

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).²⁰ 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^{citation needed} an intention to produce Vorbis-compliant devices, and new Vorbis devices seem to be appearing at a steady rate.

Apple's iPod does not natively support Vorbis but through the use of Rockbox, an open-source firmware project, is capable of decoding Vorbis files. The Xiph.Org Foundation wiki has an up-to-date list of Vorbis-supporting hardware, such as portables, PDAs, and microchips.²⁴ Also see Internet radio device for an overview.

Software

Software supporting Vorbis exists for many platforms. Although Apple iTunes does not natively support Vorbis, the Xiph.Org Foundation provides a QuickTime component²⁵ which can be used in iTunes and QuickTime on both Microsoft Windows and Mac OS. Windows Media Player also does not natively support Vorbis; however, DirectShow filters exist²⁶ to decode Vorbis in Windows Media Player and other Windows multimedia players that 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 Xiph.Org Foundation wiki.²⁷ Users can test these programs using the list of Vorbis audio streams available on the same wiki.²⁸ Winamp can also play ".ogg" vorbis files, as can the open-source VLC media player and Foobar2000.

Implementations

Because of Vorbis' free and open nature, developers are free to write compliant encoders and decoders.

Official

The Xiph.Org Foundation's reference implementation is free and open source, and licensed under the BSD License and the GNU Lesser General Public License.

aoTuV

Modified versions of the reference encoder are also available: aoTuV is a version that has received quality tweaks since the release of libVorbis 1.0; previous changes from aoTuV (Beta2) have been merged back into the Xiph.Org Foundation's libVorbis 1.1. Lancer is a modified version of aoTuV focusing on encoding speed. Current version: aoTuV Beta5.5 (2008/03/31)

Tremor

Tremor is an implementation of a Vorbis decoder using fixed-point arithmetic. It is intended for environments when a floating-point arithmetic unit is unavailable. The Xiph.Org Foundation has expressed interest in modifying Tremor into a floating-point version, which would replace the current floating-point reference decoder.²⁹

FFmpeg

The libavcodec library contains an LGPL-licensed Vorbis decoder that is faster than the reference implementation. ³⁰

stb_vorbis

stb_vorbis is a C implementation of an Ogg Vorbis decoder placed in the public domain. It lacks some minor features present in the reference implementation (notably seeking), but is still fully compliant with the format specification.

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