From: Monty Date: Sun, 14 Jul 2002 08:49:24 +0000 (+0000) Subject: The Vorbis->Ogg encapsulation spec X-Git-Tag: v1.3.3~709 X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=1e87d71a65821d4f4b8828369bf4c2932d07081c;p=platform%2Fupstream%2Flibvorbis.git The Vorbis->Ogg encapsulation spec svn path=/trunk/vorbis/; revision=3631 --- diff --git a/doc/vorbis-ogg.html b/doc/vorbis-ogg.html new file mode 100644 index 0000000..3c28a18 --- /dev/null +++ b/doc/vorbis-ogg.html @@ -0,0 +1,164 @@ +xiph.org: Ogg Vorbis documentation + +

+ +

+Ogg Vorbis I format specification: embedding Vorbis into an Ogg stream +

+ +Last update to this document: July 14, 2002
+ +

Overview

+ +This document describes using Ogg logical and physical transport +streams to encapsulate Vorbis compressed audio packet data into file +form.

+ +_Ogg Vorbis I format specification: +high-level description_ provides an overview of the construction +of Vorbis audio packets.

The _Ogg +bitstream overview_ and Ogg logical +bitstream and framing spec_ provide detailed descriptions of Ogg +transport streams. This specification document assumes a working +knowledge of the concepts covered in these named backround +documents. Please read them first.

+ + +

Restrictions

+ +The Ogg/Vorbis I specification currently dictates that Ogg/Vorbis +streams use Ogg transport streams in degenerate, unmultiplexed +form only. That is: + +

A meta-headerless Ogg file encapsulates the Vorbis I packets +
The Ogg stream may be chained, i.e. contain multiple, contigous logical streams (links). +
The Ogg stream must be unmultiplexed (only one stream, a Vorbis audio stream, per link) +

+ +This is not to say that it is not currently possible to multiplex +Vorbis with other media types into a multi-stream Ogg file. At the +time this document was written, Ogg was becoming a popular container +for low-bitrate movies consisting of DiVX video and Vorbis audio. +However, a 'Vorbis I audio file' is taken to imply Vorbis audio +existing alone within a degenerate Ogg stream. A compliant 'Vorbis +audio player' is not required to implement Ogg support beyond the +specific support of Vorbis within a degenrate ogg stream (naturally, +application authors are encouraged to support full multiplexed Ogg +handling). +

+ +

MIME type

+ +The correct MIME type of any Ogg file is application/x-ogg. +However, if a file is a Vorbis I audio file (which implies a +degenerate Ogg stream including only unmultiplexed Vorbis audio), the +mime type audio/x-vorbis is also allowed. + +

Encapsulation

+ +Ogg encapsulation of a Vorbis packet stream is straightforward.

+ +

The first Vorbis packet [the indentification header], which +uniquely identifies a stream as Vorbis audio, is placed alone in the +first page of the logical Ogg stream. This results in a first Ogg +page of exactly 58 bytes at the very beginning of the logical stream.
+ +
This first page is marked 'beginning of stream' in the page flags.
+ +
The second and third vorbis packets [comment and setup +headers] may span one or more pages beginning on the second page of +the logical stream. However many pages they span, the third header +packet finishes the page on which it ends. The next [first audio] packet +must begin on a fresh page.
+ +
The granule position of these first pages containing only headers is +zero.
+ +
The first audio packet of the logical stream begins a fresh Ogg page.
+ +
Packets are placed into ogg pages in order until the end of stream.
+ +
The last page is marked 'end of stream' in the page flags.
+ +
Vorbis packets may span page boundaries.
+ +
The granule position of pages contining Vorbis audio is in units +of PCM audio samples (per channel; a stereo stream's granule position +does no increment at twice the speed of a mono stream).
+ +
The granule position of a page represents the end PCM sample +position of the last packet completed on that page. A page +that is entirely spanned by a single packet (that completes on a +subsequent page) has no granule position, and the granule position is +set to '-1'.
+ +
The granule (PCM) position of the first page need not indicate + that the stream started at position zero. Although the granule + position belongs to the last completed packet on the page and a + valid granule position must be positive, by + inference it may indicate that the PCM position of the beginning + of audio is positive or negative.
+ +
- A positive starting value simply indicates that this stream begins at + some positive time offset, potentially within a larger + program. This is a common case when connecting to the middle + of broadcast stream.
- A negative value indicates that + output samples preceeding time zero should be discarded during + decoding; this technique is used to allow sample-granularity + editing of the stream start time of already-encoded Vorbis + streams. The number of samples to be discarded must not exceed + the overlap-add span of the first two audio packets.
  +
+ In both of these cases in which the initial audio PCM starting + offset is nonzero, the second finished audio packet must flush the + page on which it appears and the third packet begin a fresh page. + This allows the decoder to always be able to perform PCM position + adjustments before needing to return any PCM data from synthesis, + resulting in correct positioning information without any aditional + seeking logic.
+ + (Note however that failure to do so should, at worst, cause a + decoder implementation to return incorrect positioning information + for seeking operations at the very beginning of the stream.)
+ +
A granule position on the final page in a stream that indicates +less audio data than the final packet would normally return is used to +end the stream on other than even frame boundaries. The difference +between the actual available data returned and the declared amount +indicates how many trailing samples to discard from the decoding +process.
+

+ +

+ + + +Ogg is a Xiph.org Foundation effort +to protect essential tenets of Internet multimedia from corporate +hostage-taking; Open Source is the net's greatest tool to keep +everyone honest. See About +the Xiph.org Foundation for details. +

+ +Ogg Vorbis is the first Ogg audio CODEC. Anyone may freely use and +distribute the Ogg and Vorbis specification, whether in a private, +public or corporate capacity. However, the Xiph.org Foundation and +the Ogg project (xiph.org) reserve the right to set the Ogg Vorbis +specification and certify specification compliance.

+ +Xiph.org's Vorbis software CODEC implementation is distributed under a +BSD-like license. This does not restrict third parties from +distributing independent implementations of Vorbis software under +other licenses.

+ + +