vorbis/doc/xml/a1-encapsulation_ogg.xml

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<appendix id="vorbis-over-ogg">
<appendixinfo>
 <releaseinfo>
  $Id$
 </releaseinfo>
</appendixinfo>
<title>Embedding Vorbis into an Ogg stream</title>

<section>
<title>Overview</title>

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

<para>
The <xref linkend="vorbis-spec-intro"/> provides an overview of the construction
of Vorbis audio packets.</para>

<para>
The <ulink url="oggstream.html">Ogg
bitstream overview</ulink> and <ulink url="framing.html">Ogg logical
bitstream and framing spec</ulink> 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.</para>

<section><title>Restrictions</title>

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

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

<para>
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).
</para>

</section>

<section><title>MIME type</title>

<para>
The MIME type of Ogg files depend on the context.  Specifically, complex
multimedia and applications should use <literal>application/ogg</literal>,
while visual media should use <literal>video/ogg</literal>, and audio
<literal>audio/ogg</literal>.  Vorbis data encapsulated in Ogg may appear
in any of those types.  RTP encapsulated Vorbis should use
<literal>audio/vorbis</literal> + <literal>audio/vorbis-config</literal>.
</para>

</section>

</section>

<section>
<title>Encapsulation</title>

<para>
Ogg encapsulation of a Vorbis packet stream is straightforward.</para>

<itemizedlist>

<listitem><simpara>
  The first Vorbis packet (the identification 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.
</simpara></listitem>

<listitem><simpara>
  This first page is marked 'beginning of stream' in the page flags.
</simpara></listitem>

<listitem><simpara>
  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.
</simpara></listitem>

<listitem><simpara>
  The granule position of these first pages containing only headers is zero.
</simpara></listitem>

<listitem><simpara>
  The first audio packet of the logical stream begins a fresh Ogg page.
</simpara></listitem>

<listitem><simpara>
  Packets are placed into ogg pages in order until the end of stream.
</simpara></listitem>

<listitem><simpara>
  The last page is marked 'end of stream' in the page flags.
</simpara></listitem>

<listitem><simpara>
  Vorbis packets may span page boundaries.
</simpara></listitem>

<listitem><simpara>
  The granule position of pages containing Vorbis audio is in units
  of PCM audio samples (per channel; a stereo stream's granule position
  does not increment at twice the speed of a mono stream).
</simpara></listitem>

<listitem><simpara>
  The granule position of a page represents the end PCM sample
  position of the last packet <emphasis>completed</emphasis> on that
  page.  The 'last PCM sample' is the last complete sample returned by
  decode, not an internal sample awaiting lapping with a
  subsequent block.  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'.  </simpara>

<simpara>
  Note that the last decoded (fully lapped) PCM sample from a packet
  is not necessarily the middle sample from that block. If, eg, the
  current Vorbis packet encodes a "long block" and the next Vorbis
  packet encodes a "short block", the last decodable sample from the
  current packet be at position (3*long_block_length/4) -
  (short_block_length/4).
</simpara>

</listitem>

<listitem>
  <simpara>
    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.
  </simpara>
    
  <itemizedlist>
    <listitem><simpara>
        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.
    </simpara></listitem>
    <listitem><simpara>
        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.
    </simpara></listitem>
  </itemizedlist>
  
  <simpara>
    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.
  </simpara>
  
  <note><simpara>
    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.
  </simpara></note>
</listitem>
  
<listitem><simpara>
  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.
 </simpara></listitem>
</itemizedlist>

</section>

</appendix>

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