Ogg

Ogg
Filename extension	.ogg
Internet media type	application/ogg
Developed by	Xiph.org Foundation
Type of format	Container format
Container for	Vorbis, Theora, Speex, etc.

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

The name "Ogg" refers to the file format which includes a number of separate independent open source codecs for audio, video and text (e.g. subtitles). Files ending in the .ogg extension may be of any Ogg media filetype, and because the format is free, Ogg's various codecs have been incorporated into a number of different free and commercial media players as well as portable media players from different manufacturers.

The term "ogg" often refers to the audio file format Ogg Vorbis, that is, Vorbis-encoded audio in an Ogg container. Other prominent components of Ogg are its video codec Theora, and the human speech audio compression format, Speex.

The current version of Xiph's free reference implementation, released on November 27, 2005, is libogg 1.1.3 [1].

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 to identify the file as Ogg format.

A serial number and page number in the page header identifies each page as part of a series of pages making up a bitstream. Multiple bitstreams may be multiplexed in the file where pages from each bitstream are 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.

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.

Page Structure

The following is the field layout of an ogg page header:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1| Byte
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| capture_pattern: Magic number for page start "OggS"           | 0-3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version       | header_type   | granule_position              | 4-7
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               | 8-11
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               | bitstream_serial_number       | 12-15
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               | page_sequence_number          | 16-19
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               | CRC_checksum                  | 20-23
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               |page_segments  | segment_table | 24-27
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ...                                                           | 28-
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Capture Pattern - 32 bits

The capture pattern or sync code is a magic number used to ensure synchronisation when parsing ogg files. Every page starts with the four byte sequence 'OggS'. This assists in resynchronising a parser in cases where data has been lost or is corrupted, and is a sanity check before commencing parsing the page structure.

Version - 8 bits

This field indicates the version of the ogg bitstream format, to allow for future expansion. It is currently mandated to be 0.

Header Type - 8 bits

This is an 8 bit field of flags, which indicates the type of page which follows. The rightmost or least significant bit is considered bit 0, with value 0x01, the next least significant digit is bit 1, with value 0x02. The third is bit 2, with value 0x04, and so on.

Bit 0 (0x01) - Continuation flag: This flag when set indicates that the first packet on this page is a continuation of the previous packet in this logical bitstream.
Bit 1 (0x02) - BOS flag: This flag when set indicates that this page is the first page in this logical bitstream (Beginning Of Stream). This flag is required to be set on the first page of every logical bitstream, and may not be set on any other page.
Bit 2 (0x04) - EOS flag: This flag when set indicates that this page is the last page in this logical bitstream (End Of Stream). This flag is required to be set on the final page of every logical bitstream, and may not be set on any other page.

Granule Position - 64 bits

A granule position is the time marker in ogg files. It is an abstract value, whose meaning is determined by the codec. It may for example be a count of the number of samples, the number of frames or a more complex scheme.

Bitstream Serial Number - 32 bits

This field is a serial number which identifies a page as belonging to a particular logical bitstream. Each logical bitstream in a file has a unique value, and this field allows implementations to deliver the pages to the appropriate decoder. In a typical vorbis+theora file, one stream will be the audio (vorbis), and the other will be the video (theora)

Page Sequence Number - 32 bits

This field is a monotonically increasing field for each logical bitstream. The first page is 0, the second 1, etc. This allows implementations to detect when data has been lost.

Checksum - 32 bits

This field provides a checksum of the data in the entire page, performed with the checksum field set to 0. This allows verification that the data has not been corrupted since it was created. Pages which fail the checksum should be discarded.

Page Segments - 8 bits

This field indicates the number of segments that exist in this page. It also indicates how many bytes are in the segment table which follows this field. There can be a maximum of 255 segments in any one page.

Segment Table

The segment table is an 8 bit vector of values indicating the length of each segment within the page body. The number of segments is determined from the preceding Page Segments field. Each segment is between 0 and 255 bytes in length.

The segments provide a way to group segments into packets, which are meaningful units of data for the decoder. When the segments length is indicated to be 255, this indicates that the following segment is to be concatanated to this one and is part of the same packet. When the segments length is 0-254, this indicates that this segment is the final segment in this packet. Where a packets length is a multiple of 255, the final segment will be of length 0.

Where the final packet continues on the next page, the final segment value will be 255, and the continuation flag will be set on the following page to indicate that the start of the new page is a continuation of last page.

History

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

In May 2003, two Internet RFCs were published relating to the format. The Ogg bitstream was defined in RFC 3533 (which is classified as `informative') and its Internet content type (application/ogg) in RFC 3534 (which is, as of 2006, a proposed standard protocol).

Although Ogg hasn't reached anywhere near the ubiquity of the MPEG standards (e.g.: MP3), as of 2006, it is commonly used to encode free content (such as free music, multimedia on Wikimedia projects and Creative Commons files) and has started to be supported by a significant minority of digital audio players. The more popular Ogg Vorbis codec has built-in support on many software players and extensions are available for nearly all the rest. Other codecs are less well supported although extensions are often available.

Ogg codecs

Ogg is only a container format. The actual music or video encoded by a codec will be stored inside an Ogg container. Ogg containers may contain multiple codecs, for example, an audio video file may contain data encoded by both an audio codec and a video codec.

Being a Container format, ogg can embed third-party codecs (such as DivX, Dirac, XviD, mp3 and others) but usually Ogg is used with the following:

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 variable bitrates (~16-500 kbit/s/channel)
- lossless
  - FLAC: handles archival and high fidelity audio data

Text codec
- Writ: a text codec designed to embed subtitles or captions

Video codecs
- Theora: based upon On2's VP3, it is targeted at competing with MPEG-4 video (i.e. DivX and XviD), RealVideo, or Windows Media Video.
- Tarkin: an experimental codec utilizing 3D wavelet transforms. It has been put on hold, with Theora becoming the main focus for video encoding.
- Dirac: an experimental codec developed by the BBC as the basis of a new codec for the transmission of video over the Internet. The Schrödinger project aims to provide portable libraries, written in C, that implement the Dirac codec. It also allows to embed Dirac inside the Ogg container format.