release/src/router/libvorbis/doc/a1-encapsulation-ogg.tex

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