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1% -*- mode: latex; TeX-master: "Vorbis_I_spec"; -*-
2%!TEX root = Vorbis_I_spec.tex
3\section{Embedding Vorbis into an Ogg stream} \label{vorbis:over:ogg}
4
5\subsection{Overview}
6
7This document describes using Ogg logical and physical transport
8streams to encapsulate Vorbis compressed audio packet data into file
9form.
10
11The \xref{vorbis:spec:intro} provides an overview of the construction
12of Vorbis audio packets.
13
14The \href{oggstream.html}{Ogg
15bitstream overview} and \href{framing.html}{Ogg logical
16bitstream and framing spec} provide detailed descriptions of Ogg
17transport streams. This specification document assumes a working
18knowledge of the concepts covered in these named backround
19documents. Please read them first.
20
21\subsubsection{Restrictions}
22
23The Ogg/Vorbis I specification currently dictates that Ogg/Vorbis
24streams use Ogg transport streams in degenerate, unmultiplexed
25form only. That is:
26
27\begin{itemize}
28 \item
29 A meta-headerless Ogg file encapsulates the Vorbis I packets
30
31 \item
32 The Ogg stream may be chained, i.e., contain multiple, contigous logical streams (links).
33
34 \item
35 The Ogg stream must be unmultiplexed (only one stream, a Vorbis audio stream, per link)
36
37\end{itemize}
38
39
40This is not to say that it is not currently possible to multiplex
41Vorbis with other media types into a multi-stream Ogg file. At the
42time this document was written, Ogg was becoming a popular container
43for low-bitrate movies consisting of DivX video and Vorbis audio.
44However, a 'Vorbis I audio file' is taken to imply Vorbis audio
45existing alone within a degenerate Ogg stream. A compliant 'Vorbis
46audio player' is not required to implement Ogg support beyond the
47specific support of Vorbis within a degenrate Ogg stream (naturally,
48application authors are encouraged to support full multiplexed Ogg
49handling).
50
51
52
53
54\subsubsection{MIME type}
55
56The MIME type of Ogg files depend on the context. Specifically, complex
57multimedia and applications should use \literal{application/ogg},
58while visual media should use \literal{video/ogg}, and audio
59\literal{audio/ogg}. Vorbis data encapsulated in Ogg may appear
60in any of those types. RTP encapsulated Vorbis should use
61\literal{audio/vorbis} + \literal{audio/vorbis-config}.
62
63
64\subsection{Encapsulation}
65
66Ogg encapsulation of a Vorbis packet stream is straightforward.
67
68\begin{itemize}
69
70\item
71 The first Vorbis packet (the identification header), which
72 uniquely identifies a stream as Vorbis audio, is placed alone in the
73 first page of the logical Ogg stream. This results in a first Ogg
74 page of exactly 58 bytes at the very beginning of the logical stream.
75
76
77\item
78 This first page is marked 'beginning of stream' in the page flags.
79
80
81\item
82 The second and third vorbis packets (comment and setup
83 headers) may span one or more pages beginning on the second page of
84 the logical stream. However many pages they span, the third header
85 packet finishes the page on which it ends. The next (first audio) packet
86 must begin on a fresh page.
87
88
89\item
90 The granule position of these first pages containing only headers is zero.
91
92
93\item
94 The first audio packet of the logical stream begins a fresh Ogg page.
95
96
97\item
98 Packets are placed into ogg pages in order until the end of stream.
99
100
101\item
102 The last page is marked 'end of stream' in the page flags.
103
104
105\item
106 Vorbis packets may span page boundaries.
107
108
109\item
110 The granule position of pages containing Vorbis audio is in units
111 of PCM audio samples (per channel; a stereo stream's granule position
112 does not increment at twice the speed of a mono stream).
113
114
115\item
116 The granule position of a page represents the end PCM sample
117 position of the last packet \emph{completed} on that
118 page. The 'last PCM sample' is the last complete sample returned by
119 decode, not an internal sample awaiting lapping with a
120 subsequent block. A page that is entirely spanned by a single
121 packet (that completes on a subsequent page) has no granule
122 position, and the granule position is set to '-1'.
123
124
125 Note that the last decoded (fully lapped) PCM sample from a packet
126 is not necessarily the middle sample from that block. If, eg, the
127 current Vorbis packet encodes a "long block" and the next Vorbis
128 packet encodes a "short block", the last decodable sample from the
129 current packet be at position (3*long\_block\_length/4) -
130 (short\_block\_length/4).
131
132
133\item
134 The granule (PCM) position of the first page need not indicate
135 that the stream started at position zero. Although the granule
136 position belongs to the last completed packet on the page and a
137 valid granule position must be positive, by
138 inference it may indicate that the PCM position of the beginning
139 of audio is positive or negative.
140
141
142 \begin{itemize}
143 \item
144 A positive starting value simply indicates that this stream begins at
145 some positive time offset, potentially within a larger
146 program. This is a common case when connecting to the middle
147 of broadcast stream.
148
149 \item
150 A negative value indicates that
151 output samples preceeding time zero should be discarded during
152 decoding; this technique is used to allow sample-granularity
153 editing of the stream start time of already-encoded Vorbis
154 streams. The number of samples to be discarded must not exceed
155 the overlap-add span of the first two audio packets.
156
157 \end{itemize}
158
159
160 In both of these cases in which the initial audio PCM starting
161 offset is nonzero, the second finished audio packet must flush the
162 page on which it appears and the third packet begin a fresh page.
163 This allows the decoder to always be able to perform PCM position
164 adjustments before needing to return any PCM data from synthesis,
165 resulting in correct positioning information without any aditional
166 seeking logic.
167
168
169 \begin{note}
170 Failure to do so should, at worst, cause a
171 decoder implementation to return incorrect positioning information
172 for seeking operations at the very beginning of the stream.
173 \end{note}
174
175
176\item
177 A granule position on the final page in a stream that indicates
178 less audio data than the final packet would normally return is used to
179 end the stream on other than even frame boundaries. The difference
180 between the actual available data returned and the declared amount
181 indicates how many trailing samples to discard from the decoding
182 process.
183
184\end{itemize}
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