Ogg

From Academic Kids

Ogg is a patent-free, fully open multimedia bitstream container format designed for efficient streaming and file compression (storage).

The name "Ogg" hence refers to the file format which includes a number of separate independent open source codecs for both audio and video. Files ending in the .ogg extension may be of any Ogg media filetype, and because their use is free, Ogg's various codecs have been incorporated into a number of different free and commercial media players. The term "Ogg" sometimes incorrectly refers to the Vorbis audio codec. Other prominent components of Ogg are its video codec Theora, and the human speech audio compression format, Speex.

Contents

The Ogg file format

The Ogg bitstream format, spearheaded by the Xiph.org Foundation, has been created as the framework of a larger initiative aimed at developing a set of components for the coding and decoding of multimedia content which are both freely available and freely re-implementable in software.

The format consists of chunks of data each called an Ogg Page. Each page begins with the "OggS" string which can be used to identify the file as Ogg.

A serial number and page number in the page header identifies each page as part of a series of pages which make up a bitstream. Multiple bitstreams may be muxed in the file where pages from each bitstream ordered by the seek time of the contained data. Bitstreams may also be appended to existing files, a process known as chaining, to cause the bitstreams to be decoded in sequence.

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Ogg_stream.png
Ogg Stream Example

A BSD-licensed library, called libogg, is available to encode and decode data from Ogg streams. Independent Ogg implementations are used in several projects such as RealPlayer and a set of DirectShow filters.

It is often assumed that the name "Ogg" comes from the character of Nanny Ogg in Terry Pratchett's Discworld novels. Rather, it is 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. At its inception the Ogg project was thought to be somewhat ambitious given the power of the PC hardware of the time.

The Ogg bitstream is defined in RFC 3533 and its MIME media type (application/ogg) in RFC 3534. Its current version, released on September 23, 2004, is 1.1.2 (http://lists.xiph.org/pipermail/vorbis-dev/2004-September/017693.html).

Ogg codecs

  • Audio codecs
    • lossy
      • Speex: handles voice data at low bitrates (~8-32 kbit/s/channel)
      • Vorbis: handles general audio data at mid- to high-level bitrates (~16-256 kbit/s/channel)
    • lossless
      • FLAC: handles archival and high fidelity audio data
  • Text codec
    • Writ: a text codec designed to embed subtitles or captions

Proprietary alternatives

Various components of the project are intended to stand as alternatives to proprietary codecs such as:

External links

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