Vorbis
From MediaCoderWiki
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. 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 began 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.
Contents |
Name
"Vorbis" is named after a Discworld character, Exquisitor Vorbis in Small Gods, by Terry Pratchett. Coincidentally, Nanny Ogg is another Discworld character, a witch who appears in several books including Witches Abroad, though the Ogg format was not named after her. "Ogg" is in fact derived from ogging, jargon that arose in the computer game Netrek.
Usage
The Amarok software playing a Vorbis-encoded song. Also demonstrates Unicode support.
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 like MP3. However, MP3 has been widely used since the late-1990s and as of 2009, continues to remain popular in the consumer electronics industry.
Vorbis has different uses for consumer products. Many video game titles such as 18 Wheels of Steel, Halo, Unreal Tournament, Mafia: The City of Lost Heaven, Grand Theft Auto: San Andreas, Crimsonland, Devil May Cry 3 and Guitar Hero: On Tour store in-game audio as Vorbis. Popular software players support 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 CBC Radio, JazzRadio, Virgin Radio and Radio New Zealand.
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.
Listening tests have attempted to find the best quality lossy audio codecs at certain bitrates. Some conclusions made by recent listening tests:
- Low bitrate (less than 64 kbit/s): the most recent public multiformat test at 48 kbit/s shows that aoTuV Vorbis has 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 at 80 kbit/s and 96 kbit/s shows that aoTuV Vorbis has a better quality than other lossy audio codecs (LC-AAC, HE-AAC, MP3, MPC, WMA).
- Mid bitrate (128kbit/s): most recent public multiformat test at 128 kbit/s 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).
- High bitrates (more 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 performs better than LC-AAC, MP3, and MPC.
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.
- 2005-2006 Public group test of Lame MP3, Vorbis AoTuV, iTunes AAC, Nero AAC, and WMA Pro at ~135 kbit/s nominal. Results suggest that further group testing at this bitrate is unnecessary because all codecs are statistically tied near transparency.
- 2005, July comparison - AAC vs MP3 vs Vorbis vs WMA at 80 kbit/s. States that 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 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, 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.
Audio quality: Characteristic artifacts
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). 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; Vorbis's behavior is due to the noise floor approach to encoding; see technical details.
