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<h1>Ogg logical and physical bitstream overview</h1>
<h2>Ogg bitstreams</h2>
<p>Ogg codecs use octet vectors of raw, compressed data
(<em>packets</em>). These compressed packets do not have any
high-level structure or boundary information; strung together, they
appear to be streams of random bytes with no landmarks.</p>
<p>Raw packets may be used directly by transport mechanisms that provide
their own framing and packet-separation mechanisms (such as UDP
datagrams). For stream based storage (such as files) and transport
(such as TCP streams or pipes), Vorbis and other future Ogg codecs use
the Ogg bitstream format to provide framing/sync, sync recapture
after error, landmarks during seeking, and enough information to
properly separate data back into packets at the original packet
boundaries without relying on decoding to find packet boundaries.</p>
<h2>Logical and physical bitstreams</h2>
<p>Raw packets are grouped and encoded into contiguous pages of
structured bitstream data called <em>logical bitstreams</em>. A
logical bitstream consists of pages, in order, belonging to a single
codec instance. Each page is a self contained entity (although it is
possible that a packet may be split and encoded across one or more
pages); that is, the page decode mechanism is designed to recognize,
verify and handle single pages at a time from the overall bitstream.</p>
<p>Multiple logical bitstreams can be combined (with restrictions) into a
single <em>physical bitstream</em>. A physical bitstream consists of
multiple logical bitstreams multiplexed at the page level and may
include a 'meta-header' at the beginning of the multiplexed logical
stream that serves as identification magic. Whole pages are taken in
order from multiple logical bitstreams and combined into a single
physical stream of pages. The decoder reconstructs the original
logical bitstreams from the physical bitstream by taking the pages in
order from the physical bitstream and redirecting them into the
appropriate logical decoding entity. The simplest physical bitstream
is a single, unmultiplexed logical bitstream with no meta-header; this
is referred to as a 'degenerate stream'.</p>
<p><a href="framing.html">Ogg Logical Bitstream Framing</a> discusses
the page format of an Ogg bitstream, the packet coding process
and logical bitstreams in detail. The remainder of this document
specifies requirements for constructing finished, physical Ogg
bitstreams.</p>
<h2>Mapping Restrictions</h2>
<p>Logical bitstreams may not be mapped/multiplexed into physical
bitstreams without restriction. Here we discuss design restrictions
on Ogg physical bitstreams in general, mostly to introduce
design rationale. Each 'media' format defines its own (generally more
restrictive) mapping. An 'Ogg Vorbis Audio Bitstream', for example, has a
specific physical bitstream structure.
An 'Ogg A/V' bitstream (not currently specified) will also mandate a
specific, restricted physical bitstream format.</p>
<h3>additional end-to-end structure</h3>
<p>The <a href="framing.html">framing specification</a> defines
'beginning of stream' and 'end of stream' page markers via a header
flag (it is possible for a stream to consist of a single page). A
stream always consists of an integer number of pages, an easy
requirement given the variable size nature of pages.</p>
<p>In addition to the header flag marking the first and last pages of a
logical bitstream, the first page of an Ogg bitstream obeys
additional restrictions. Each individual media mapping specifies its
own implementation details regarding these restrictions.</p>
<p>The first page of a logical Ogg bitstream consists of a single,
small 'initial header' packet that includes sufficient information to
identify the exact CODEC type and media requirements of the logical
bitstream. The intent of this restriction is to simplify identifying
the bitstream type and content; for a given media type (or across all
Ogg media types) we can know that we only need a small, fixed
amount of data to uniquely identify the bitstream type.</p>
<p>As an example, Ogg Vorbis places the name and revision of the Vorbis
CODEC, the audio rate and the audio quality into this initial header,
thus simplifying vastly the certain identification of an Ogg Vorbis
audio bitstream.</p>
<h3>sequential multiplexing (chaining)</h3>
<p>The simplest form of logical bitstream multiplexing is concatenation
(<em>chaining</em>). Complete logical bitstreams are strung
one-after-another in order. The bitstreams do not overlap; the final
page of a given logical bitstream is immediately followed by the
initial page of the next. Chaining is the only logical->physical
mapping allowed by Ogg Vorbis.</p>
<p>Each chained logical bitstream must have a unique serial number within
the scope of the physical bitstream.</p>
<h3>concurrent multiplexing (grouping)</h3>
<p>Logical bitstreams may also be multiplexed 'in parallel'
(<em>grouped</em>). An example of grouping would be to allow
streaming of separate audio and video streams, using different codecs
and different logical bitstreams, in the same physical bitstream.
Whole pages from multiple logical bitstreams are mixed together.</p>
<p>The initial pages of each logical bitstream must appear first; the
media mapping specifies the order of the initial pages. For example,
Ogg A/V will eventually specify an Ogg video bitstream with
audio. The mapping may specify that the physical bitstream must begin
with the initial page of a logical video bitstream, followed by the
initial page of an audio stream. Unlike initial pages, terminal pages
for the logical bitstreams need not all occur contiguously (although a
specific media mapping may require this; it is not mandated by the
generic Ogg stream spec). Terminal pages may be 'nil' pages,
that is, pages containing no content but simply a page header with
position information and the 'last page of bitstream' flag set in the
page header.</p>
<p>Each grouped bitstream must have a unique serial number within the
scope of the physical bitstream.</p>
<h3>sequential and concurrent multiplexing</h3>
<p>Groups of concurrently multiplexed bitstreams may be chained
consecutively. Such a physical bitstream obeys all the rules of both
grouped and chained multiplexed streams; the groups, when unchained ,
must stand on their own as a valid concurrently multiplexed
bitstream.</p>
<h3>multiplexing example</h3>
<p>Below, we present an example of a grouped and chained bitstream:</p>
<p><img src="stream.png" alt="stream"/></p>
<p>In this example, we see pages from five total logical bitstreams
multiplexed into a physical bitstream. Note the following
characteristics:</p>
<ol>
<li>Grouped bitstreams begin together; all of the initial pages
must appear before any data pages. When concurrently multiplexed
groups are chained, the new group does not begin until all the
bitstreams in the previous group have terminated.</li>
<li>The pages of concurrently multiplexed bitstreams need not conform
to a regular order; the only requirement is that page <tt>n</tt> of a
logical bitstream follow page <tt>n-1</tt> in the physical bitstream.
There are no restrictions on intervening pages belonging to other
logical bitstreams. (Tying page appearance to bitrate demands is one
logical strategy, ie, the page appears at the chronological point
where decode requires more information).</li>
</ol>
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