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Vorbis
副檔名
.ogg .oga
網路媒體型式
audio/ogg, audio/vorbis, audio/vorbis-config
開發者Xiph.Org 基金會
首次發佈2000年5月8日 (2000-05-08) [1][2]
最新版本
Vorbis I
2010年2月3日 (2010-02-03)[3]
格式類型音頻壓縮格式
使用容器Ogg, Matroska, WebM
標準Specification
libvorbis
開發者Xiph.Org 基金會
首次發佈2002年7月19日 (2002-07-19)
目前版本1.3.2[4]（2010年11月1日，​14年前​（2010-11-01））
程式語言C
作業系統跨平台
類型編解碼器, 參考實現
許可協定BSD-style license[5][6]
網站Xiph.org downloads

Vorbis 是一種有損音頻壓縮格式，由Xiph.Org 基金會領導並開放原始碼。Vorbis通常以Ogg作為容器格式[7]，所以常合稱為Ogg Vorbis，通常所稱「Ogg音頻格式」即指Vorbis格式。

Vorbis is a continuation of audio compression development started in 1993 by Chris Montgomery.[8][9] Intensive development began following a September 1998 letter from the Fraunhofer Society announcing plans to charge licensing fees for the MP3 audio format.[10][11] Vorbis project started as part of the Xiphophorus company's Ogg project (also known as OggSquish multimedia project).[12][13] 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[1][2][14] and a stable version (1.0) of the reference software was released on July 19, 2002.[15][16][17]

libvorbis是Xiph.Org基金會維護的參考實現，目前的最新官方版本為1.3.2。[4]此外還有一些非官方的實現，其中最知名的是aoTuV。尤其在低位元速率上，它能提供更好的音質。這些改善每過一段時間都會被加入到官方的代碼庫中。

名稱

"Vorbis" is named after a Discworld character, Exquisitor Vorbis in Small Gods by Terry Pratchett.[18] The Ogg format, however, is not named for Nanny Ogg, another Discworld character; the name is in fact derived from ogging, jargon that arose in the computer game Netrek.[9]

用途

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.

Vorbis範例

Vorbis has different uses for consumer products. Many video game titles such as 18 Wheels of Steel, Halo, Unreal Tournament 2004, Unreal Tournament 3, Mafia: The City of Lost Heaven, Jets 'n' guns, Grand Theft Auto: San Andreas, Crimsonland, Devil May Cry 3, Live For Speed 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, including Wikipedia, use it. Others include Jamendo and Mindawn, as well as several national radio stations[19] like JazzRadio, Absolute Radio, NPR, Radio New Zealand[20] and Deutschlandradio.[21] The Spotify audio streaming service uses Vorbis for its audio streams.[22]

Vorbis格式流行於自由軟件的支持者之中[23]。他們認為，Vorbis因其更高的保真度以及不受專利限制的特點能夠成為MP3等受專利限制的格式的很好的替代品。 Vorbis在消費類產品中也有很多應用。許多電子遊戲以Vorbis格式儲存遊戲音頻。流行的播放器大多支援播放Vorbis格式的檔案。越來越多的網站，包括維基百科[19] [24] [25]使用Vorbis格式儲存音頻。

但總體上講，Vorbis格式的普及程度仍然不高。

Quality: Codec comparisons

通常，相比於其他較古老的編碼格式，Vorbis能夠在更小的檔案中提供更高的音質。而且它可以同其他使用MDCT的編碼格式如AAC和WMA相抗衡。[26][27]

聆聽測試是用來尋找在特定位元速率下最好的有損音頻編碼的方法。以下是一些通過聆聽測試得出的結論：

  • 低位元速率（小於等於64kbit/s）：最近一次測試（2007年）顯示，在48kbit/s下，aoTuV的音質比WMA和LC-AAC更好，與WMA Professional持平，比HE-AAC略差。[28]
  • 中位元速率（64kbit/s到128kbit/s）：在2005年，一個私人的測試表明，在80kbit/s和96kbit/s下aoTuV比其他失真壓縮格式（LC-AAC，HE-AAC，MP3，MPC，WMA）音質更好。[29][30]
  • 高位元速率（大於128kbit/s）：多數人聽不出有任何顯著的差別。但是，經過特別訓練的人士能夠辨識出特定位元速率的音頻，此時aoTuV的表現仍比LC-AAC，MP3和MPC好。[31]

由於音頻編碼器不斷更新，以上結果可能會過時。

聆聽測試

參見：Codec listening test § Results

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, 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.[29]
  • 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.[30]
  • 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.[31]
  • 2011, April comparison by Hydrogenaudio - LC-AAC vs Vorbis vs HE-AAC vs Opus at 48 kbit/s. Vorbis was on average between LC-AAC and HE-AAC, while the upcoming Opus (by Xiph, previously known as CELT) was best.[32]

Audio quality: 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 (the sound of castanets is commonly cited as causing this effect).

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 技術細節.

與大多數現代格式一樣，Vorbis最常被提及的問題是預回聲，即在真實的聲音發出之前的().

當編碼器在規定的位元速率下無法編碼出

技術細節

在給定44.1kHz立體聲輸入下各質素等級的標稱位元速率。（實際位元速率可能會有所不同）
質素 標稱位元速率（Nominal Bitrate）
官方編碼器 aoTuV
-q-2 不可用 32 kbit/s
-q-1 45 kbit/s 48 kbit/s
-q0 64 kbit/s
-q1 80 kbit/s
-q2 96 kbit/s
-q3 112 kbit/s
-q4 128 kbit/s
-q5 160 kbit/s
-q6 192 kbit/s
-q7 224 kbit/s
-q8 256 kbit/s
-q9 320 kbit/s
-q10 500 kbit/s

Vorbis可以使用從8kHz到192kHz的取樣率，它亦支援多種聲道格式（單聲道、立體聲、5.1聲道，最多255個獨立聲道）。[33] Vorbis使用可變位元速率（VBR），所以位元速率可能隨着採樣點變化而有顯著變化。 在給定44.1kHz取樣率（標準CD音頻取樣率）下，編碼器可以輸出平均位元速率為45至500kbit/s（aoTuV為32至500kbit/s）的檔案，這取決於給定的質素選項。在Xiph函式庫中，質素選項的取值範圍為-0.1到1.0，aoTuV則是-0.2到1.0。編碼器的前端通常將質素選項乘以10後對映到整數上，即-1到10或-2到10。所有的編碼器在給定的質素選項下輸出的音質應相同，但新版本的編碼器可能用更低的位元速率實現了這種音質，故表格中數據僅供參考。

Vorbis的目標在於比MP3更高效，它能夠在更低的位元速率在實現透明編碼。

演算法概述

Vorbis I是一種以改進的離散餘弦變換（MDCT）為基礎的向前相容的編碼演算法。[34] 它通過MDCT實現音頻數據從時域到頻域的變換。形成的頻域數據被分成本底噪聲和殘差頻譜兩部分，之後可以選擇是否使用聲道耦合技術進一步降低冗餘度，然後再對殘差訊號有效地以熵編碼以及向量量化的方法量化表示。最後將各部分數據組織成Vorbis格式所要求的形式。

解碼過程與編碼正好相反。編碼器利用碼本將殘差頻譜重建出頻域數據，再進行逆向MDCT處理恢復出原始的音頻數據。相較於編碼，解碼的實現更簡單，有利於硬件實現。

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.[35]

位元速率削減

Vorbis格式支援位元速率削減，即能夠在不重新編碼的情況下削減檔案的位元速率。但目前僅有一些實驗性的實現，[36][37]且處理過的音頻音質幾乎「不可聽」，[36]尚不如先解碼後再編碼的音質好。

進行位元速率削減後質素降低如此嚴重的原因是，Vorbis格式並不指定編碼訊號的相對重要性，此外現在還沒有按照重要性排列數據的編碼器。因此，程式不能十分容易地確定數據的重要性。

容器格式

參見：容器格式

除Ogg之外，Vorbis數據流可以被封裝進其他容器格式之中。[38]常用的有Matroska、WebM（一個基於Matroska子集的容器格式）。[39]Vorbis數據流也可以被封裝進RTP負載格式（RTP payload format）中進行傳輸。[40]

元數據

主條目：Vorbis註釋

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 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 positive integer that can be expressed in 32 bits). This vector is stored in the second header packet that begins a Vorbis bitstream.[41]

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 ReplayGain information.

許可

Vorbis標準的定義內容屬於公有領域。[5]但出於對標準自身的考慮，Xiph.Org 基金會保留對Vorbis標準更改及認證的權利。libvorbis函式庫遵循3句版BSD許可證，而相關的工具則遵循GNU通用公共許可協定。函式庫最初是以GNU寬通用公共許可證發佈的，但由於理查德·斯托曼的支援，函式庫最終選擇了BSD許可證。[42]Xiph.Org 基金會聲稱，Vorbis格式，就像他們發佈的其他標準一樣，完全沒有許可和專利的問題。雖然Xiph.Org 基金會如此聲明，但仍有外界人士（尤其是競爭對手格式的工程師）對Vorbis是不受專利限制的格式一事表示懷疑。[43]

支援

硬件支援

軟件支援

大多數平台都有支援Vorbis格式的軟件。如Windows下流行的媒體播放器Winamp、foobar2000、千千靜聽、The KMPlayer、暴風影音等。Windows Media Player 並不能原生支援Vorbis格式，但Xiph.Org官方提供一個DirectShow filter，[44]Windows Media Player和其他媒體播放器可以通過DirectShow來支援Vorbis。蘋果的iTunes也不原生支援Vorbis，但Xiph.Org提供一個Quicktime外掛程式，[45]這樣無論在Windows下還是Mac OS下都可以使用iTunes和Quicktime播放Vorbis音頻。

Linux平台下的媒體播放器如XMMS、Amarok等，以及跨平台的媒體播放器如VLC media player、mplayer對Vorbis均有良好的支援。Audacity等跨平台的音頻編輯軟件以及FFmpeg、GStreamer和Helix DNA等多媒體框架均能良好支援Vorbis。Xiph.Org 官方維基和vorbis.org網站提供一份支援Vorbis格式軟件的列表。[46][47]關於軟件支援的更多資訊請參見媒體播放器列表。

As originally recommended by HTML 5, these web browsers natively support Vorbis audio (without a plug-in) using the <audio> element: Mozilla Firefox 3.5 (and later versions),[48][49] Google Chrome (as of version 3.0.182.2),[50] SeaMonkey (as of version 2.0).[51] Opera 9.5 experimental video builds released in 2007 and 2008 have only <video> support and play back Vorbis audio included in Ogg video files.[52][53] Opera 10.5 browser has support for Vorbis audio, WAVE PCM audio and Theora video.[54][55]

其他實現

Xiph.Org 基金會的參考實現是免費且開源的，它在BSD許可證下發佈。但是它並沒有限制其他開發者開發相容的編碼器和解碼器。

aoTuV

aoTuV（Aoyumi Tuned Vorbis）是一個名叫蒼弓的日本人開發的改進版編碼器。它在低位元速率下擁有更好的音質。目前aoTuV的最新版本為Beta 6.03，是libvorbis 1.3.2的改進版。[56]

Tremor

Tremor是Vorbis解碼器的定點運算實現，它可以在一些不能使用浮點運算的平台上使用。[57]

FFmpeg

主條目：Libav

libavcodec 函式庫包含一個以LGPL授權的Vorbis解碼器，其解碼速度比參考實現略快。[58]

stb_vorbis

stb_vorbis是Vorbis解碼器的C語言實現，它屬於公有領域。它缺乏一些參考實現具備的小功能（特別是定位），但它聲稱要與格式規範完全相容。[59]

參見

參考連結

  1. ^ 1.0 1.1 Christopher Montgomery. vorbis-dev - Merge done. 2000-05-08 [2009-09-03]. 
  2. ^ 2.0 2.1 Xiph.Org Foundation. xiph.org Ogg Vorbis. [2009-09-11]. 
  3. ^ Xiph.Org Foundation. Vorbis I specification. Xiph.Org Foundation. 2010-02-03 [2010-03-28]. 
  4. ^ 4.0 4.1 Xiph.Org. libOgg 1.2.1 and libVorbis 1.3.2 released. 2010-03-26 [2010-11-11]. 
  5. ^ 5.0 5.1 Vorbis.com FAQ. Xiph.Org Foundation. [2009-08-29]. 
  6. ^ Sample Xiph.Org Variant of the BSD License. Xiph.Org Foundation. [2009-08-29]. 
  7. ^ Xiph.org wiki, MIME Types and File Extensions. [2007-10-13]. 
  8. ^ Advogado. Interview: Christopher Montgomery of Xiphophorus. Advogado. 2000-04-04 [2009-09-02]. 
  9. ^ 9.0 9.1 Xiph.Org Foundation. xiph.org: naming. 
  10. ^ Xiph.org. About Xiph.org. [2009-08-31]. 
  11. ^ Michael Robertson. Fraunhofer Lowers Patent Boom On MP3 Software Developers. 1998-09-11 [2009-08-31]. （原始內容存檔於2000-08-16）. 
  12. ^ Brian Zisk. vorbis - Dvorak Interviews Monty. 2000-04-19 [2008-09-04]. 
  13. ^ Xiphophorus company. The Ogg project homepage. 2000-05-20 [2008-09-02]. （原始內容存檔於2000-05-20）. 
  14. ^ Xiphophorus company. The Ogg Vorbis CODEC project. 2000-05-11 [2009-09-03]. （原始內容存檔於2000-06-10）. 
  15. ^ AfterDawn. OGG Vorbis 1.0 officially released. 2002-07-19 [2009-06-04]. 
  16. ^ CNET News. Ogg Vorbis official release is here. 2002-07-19 [2009-06-04]. 
  17. ^ Montgomery, Chris. Vorbis 1.0 released. vorbis (郵寄清單). 2002-07-19 [2009-06-04]. 
  18. ^ Xiph.Org Foundation. xiph.org: naming. 
  19. ^ 19.0 19.1 Free Software Foundation. Ogg Sites. Free Software Foundation. [2009-10-02]. 
  20. ^ Radio New Zealand Oggcasts. [2009-01-14]. 
  21. ^ Deutschlandradio im Internet mit OGG-Vorbis-Stream. [2009-10-08]. 
  22. ^ Spotify FAQ. [2009-03-16]. 
  23. ^ Recordings about our Philosophy - GNU Project - Free Software Foundation (FSF). [2008-01-12]. （原始內容存檔於2007-11-26）. 
  24. ^ Wikipedia:有聲維基百科專題, Retrieved 2011
  25. ^ Wikimedia Commons:First steps/Quality and description. [2009-10-02]. 
  26. ^ MedO. Ogg Vorbis - Better Than Mp3?. 
  27. ^ Nanook. Ogg Vorbis - An Alternative to MP3. 
  28. ^ Results of Public, Multiformat Listening Test @ 48 kbps (November 2006). [2008-01-12]. （原始內容存檔於2007-09-28）. 
  29. ^ 29.0 29.1 80 kbps personal listening test (summer 2005) - Hydrogenaudio Forums. [2008-01-12]. 
  30. ^ 30.0 30.1 MP3, WMA, AAC, OGG qualité à 96 kbps (évaluation) - Traitement Audio - Video & Son - FORUM HardWare.fr. [2008-01-12] （French）. 
  31. ^ 31.0 31.1 MPC vs VORBIS vs MP3 vs AAC at 180 kbit/s - Hydrogenaudio Forums. [2008-01-12]. 
  32. ^ 64kbit/sec stereo multiformat listening test. [2011-05-10]. 
  33. ^ Xiph.org Foundation. Vorbis I specification - Application. Xiph.org Foundation. 2009-06-02 [2009-09-11]. 
  34. ^ Xiph.Org Foundation. Vorbis I specification - 1.1.2 Classification. Xiph.Org Foundation. 2009-06-02 [2009-09-22]. 
  35. ^ Recommended Encoder Versions and Settings - Hydrogenaudio Forums. [2008-01-13]. 
  36. ^ 36.0 36.1 Experimental Ogg vorbis Bitrate Peeler, Bitrate reduction of ogg vorbis
  37. ^ VINJEY. Vorbis & Downloads. [2006-11-13]. 
  38. ^ The Community. Oggless proposal. 
  39. ^ Xiph.Org. Xiph.Org announces support for the WebM open media project. 2010-05-19 [2010-05-20]. 
  40. ^ RFC 5215 - RTP Payload Format for Vorbis Encoded Audio. 2008-08 [2010-06-16]. 
  41. ^ Xiph.Org Foundation. Vorbis Comment Field Documentation. [2007-03-14]. 
  42. ^ Jack Moffitt. [icecast-dev] Xiph.org announces Vorbis Beta 4 and the Xiph.org Foundation. 2001-02-26 [2006-11-13]. 
  43. ^ Vorbis development, status & patent issues. 2003 [2006-11-13]. 
  44. ^ illiminable Ogg Directshow Filters for Speex, Vorbis, Theora and FLAC. [2008-06-24]. 
  45. ^ Xiph.Org: QuickTime Components. [2008-01-13]. 
  46. ^ VorbisSoftwarePlayers - XiphWiki. Xiph.Org Foundation. [2008-01-13]. 
  47. ^ Xiph.Org Foundation. Vorbis.com: 3rd party software. [2009-10-12]. 
  48. ^ MozillaWiki, Firefox3.5/Features, MozillaWiki, 2009-03-18 [2009-10-11] 
  49. ^ Mozilla Corporation, Mozilla Firefox 3.5 Release Notes, Mozilla Corporation, 2009-06-30 [2009-10-11] 
  50. ^ Google Chrome to support HTML 5 video, SoftSailor, 2009-05-28 [2009-10-11] 
  51. ^ Robert Kaiser, What's New in SeaMonkey 2.0 Beta 2, seamonkey-project.org, 2009-09-16 [2009-10-11] 
  52. ^ Experimental Opera-video build with native Ogg Theora support, Opera, 2007-04-25 [2009-10-11] 
  53. ^ A call for video on the web - Opera <video> release on Labs, Opera, 2007-11-07 [2009-10-11] 
  54. ^ Philip Jägenstedt. (re-)Introducing <video> - Official blog for Core developers at Opera. Opera. 2009-12-31 [2010-01-02]. 
  55. ^ Arjan van Leeuwen. Happy New Year! - Official blog for Core developers at Opera. Opera. 2009-12-31 [2010-01-02]. 
  56. ^ aoTuV aoTuV官方網站
  57. ^ Tremor - XiphWiki. 
  58. ^ mailing list thread comparing Vorbis decoding performance. [2007-08-17]. 
  59. ^ stb_vorbis首頁. public domain Ogg Vorbis decoder. [2009-08-11]. 

外部連結

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