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+<title>Ogg Documentation</title>
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+<div id="xiphlogo">
+ <a href="http://www.xiph.org/"><img src="fish_xiph_org.png" alt="Fish Logo and Xiph.org"/></a>
+</div>
+
+<h1>Page Multiplexing and Ordering in a Physical Ogg Stream</h1>
+
+<p>The low-level mechanisms of an Ogg stream (as described in the Ogg
+Bitstream Overview) provide means for mixing multiple logical streams
+and media types into a single linear-chronological stream. This
+document specifies the high-level arrangement and use of page
+structure to multiplex multiple streams of mixed media type within a
+physical Ogg stream.</p>
+
+<h2>Design Elements</h2>
+
+<p>The design and arrangement of the Ogg container format is governed by
+several high-level design decisions that form the reasoning behind
+specific low-level design decisions.</p>
+
+<h3>Linear media</h3>
+
+<p>The Ogg bitstream is intended to encapsulate chronological,
+time-linear mixed media into a single delivery stream or file. The
+design is such that an application can always encode and/or decode a
+full-featured bitstream in one pass with no seeking and minimal
+buffering. Seeking to provide optimized encoding (such as two-pass
+encoding) or interactive decoding (such as scrubbing or instant
+replay) is not disallowed or discouraged, however no bitstream feature
+must require nonlinear operation on the bitstream.</p>
+
+<h3>Multiplexing</h3>
+
+<p>Ogg bitstreams multiplex multiple logical streams into a single
+physical stream at the page level. Each page contains an abstract
+time stamp (the Granule Position) that represents an absolute time
+landmark within the stream. After the pages representing stream
+headers (all logical stream headers occur at the beginning of a
+physical bitstream section before any logical stream data), logical
+stream data pages are arranged in a physical bitstream in strict
+non-decreasing order by chronological absolute time as
+specified by the granule position.</p>
+
+<p>The only exception to arranging pages in strictly ascending time order
+by granule position is those pages that do not set the granule
+position value. This is a special case when exceptionally large
+packets span multiple pages; the specifics of handling this special
+case are described later under 'Continuous and Discontinuous
+Streams'.</p>
+
+<h3>Seeking</h3>
+
+<p>Ogg is designed to use an interpolated bisection search to
+implement exact positional seeking. Interpolated bisection search is
+a spec-mandated mechanism.</p>
+
+<p><i>An index may improve objective performance, but it seldom
+improves subjective performance outside of a few high-latency use
+cases and adds no additional functionality as bisection search
+delivers the same functionality for both one- and two-pass stream
+types. For these reasons, use of indexes is discouraged, except in
+cases where an index provides demonstrable and noticable performance
+improvement.</i></p>
+
+<p>Seek operations are by absolute time; a direct bisection search must
+find the exact time position requested. Information in the Ogg
+bitstream is arranged such that all information to be presented for
+playback from the desired seek point will occur at or after the
+desired seek point. Seek operations are neither 'fuzzy' nor
+heuristic.</p>
+
+<p><i>Although key frame handling in video appears to be an exception to
+"all needed playback information lies ahead of a given seek",
+key frames can still be handled directly within this indexless
+framework. Seeking to a key frame in video (as well as seeking in other
+media types with analogous restraints) is handled as two seeks; first
+a seek to the desired time which extracts state information that
+decodes to the time of the last key frame, followed by a second seek
+directly to the key frame. The location of the previous key frame is
+embedded as state information in the granulepos; this mechanism is
+described in more detail later.</i></p>
+
+<h3>Continuous and Discontinuous Streams</h3>
+
+<p>Logical streams within a physical Ogg stream belong to one of two
+categories, "Continuous" streams and "Discontinuous" streams.
+Although these are discussed in more detail later, the distinction is
+important to a high-level understanding of how to buffer an Ogg
+stream.</p>
+
+<p>A stream that provides a gapless, time-continuous media type with a
+fine-grained timebase is considered to be 'Continuous'. A continuous
+stream should never be starved of data. Clear examples of continuous
+data types include broadcast audio and video.</p>
+
+<p>A stream that delivers data in a potentially irregular pattern or with
+widely spaced timing gaps is considered to be 'Discontinuous'. A
+discontinuous stream may be best thought of as data representing
+scattered events; although they happen in order, they are typically
+unconnected data often located far apart. One possible example of a
+discontinuous stream types would be captioning. Although it's
+possible to design captions as a continuous stream type, it's most
+natural to think of captions as widely spaced pieces of text with
+little happening between.</p>
+
+<p>The fundamental design distinction between continuous and
+discontinuous streams concerns buffering.</p>
+
+<h3>Buffering</h3>
+
+<p>Because a continuous stream is, by definition, gapless, Ogg buffering
+is based on the simple premise of never allowing any active continuous
+stream to starve for data during decode; buffering proceeds ahead
+until all continuous streams in a physical stream have data ready to
+decode on demand.</p>
+
+<p>Discontinuous stream data may occur on a fairly regular basis, but the
+timing of, for example, a specific caption is impossible to predict
+with certainty in most captioning systems. Thus the buffering system
+should take discontinuous data 'as it comes' rather than working ahead
+(for a potentially unbounded period) to look for future discontinuous
+data. As such, discontinuous streams are ignored when managing
+buffering; their pages simply 'fall out' of the stream when continuous
+streams are handled properly.</p>
+
+<p>Buffering requirements need not be explicitly declared or managed for
+the encoded stream; the decoder simply reads as much data as is
+necessary to keep all continuous stream types gapless (also ensuring
+discontinuous data arrives in time) and no more, resulting in optimum
+implicit buffer usage for a given stream. Because all pages of all
+data types are stamped with absolute timing information within the
+stream, inter-stream synchronization timing is always explicitly
+maintained without the need for explicitly declared buffer-ahead
+hinting.</p>
+
+<p>Further details, mechanisms and reasons for the differing arrangement
+and behavior of continuous and discontinuous streams is discussed
+later.</p>
+
+<h3>Whole-stream navigation</h3>
+
+<p>Ogg is designed so that the simplest navigation operations treat the
+physical Ogg stream as a whole summary of its streams, rather than
+navigating each interleaved stream as a separate entity.</p>
+
+<p>First Example: seeking to a desired time position in a multiplexed (or
+unmultiplexed) Ogg stream can be accomplished through a bisection
+search on time position of all pages in the stream (as encoded in the
+granule position). More powerful searches (such as a key frame-aware
+seek within video) are also possible with additional search
+complexity, but similar computational complexity.</p>
+
+<p>Second Example: A bitstream section may consist of three multiplexed
+streams of differing lengths. The result of multiplexing these
+streams should be thought of as a single mixed stream with a length
+equal to the longest of the three component streams. Although it is
+also possible to think of the multiplexed results as three concurrent
+streams of different lengths and it is possible to recover the three
+original streams, it will also become obvious that once multiplexed,
+it isn't possible to find the internal lengths of the component
+streams without a linear search of the whole bitstream section.
+However, it is possible to find the length of the whole bitstream
+section easily (in near-constant time per section) just as it is for a
+single-media unmultiplexed stream.</p>
+
+<h2>Granule Position</h2>
+
+<h3>Description</h3>
+
+<p>The Granule Position is a signed 64 bit field appearing in the header
+of every Ogg page. Although the granule position represents absolute
+time within a logical stream, its value does not necessarily directly
+encode a simple timestamp. It may represent frames elapsed (as in
+Vorbis), a simple timestamp, or a more complex bit-division encoding
+(such as in Theora). The exact encoding of the granule position is up
+to a specific codec.</p>
+
+<p>The granule position is governed by the following rules:</p>
+
+<ul>
+
+<li>Granule Position must always increase forward or remain equal from
+page to page, be unset, or be zero for a header page. The absolute
+time to which any correct sequence of granule position maps must
+similarly always increase forward or remain equal. <i>(A codec may
+make use of data, such as a control sequence, that only affects codec
+working state without producing data and thus advancing granule
+position and time. Although the packet sequence number increases in
+this case, the granule position, and thus the time position, do
+not.)</i></li>
+
+<li>Granule position may only be unset if there no packet defining a
+time boundary on the page (that is, if no packet in a continuous
+stream ends on the page, or no packet in a discontinuous stream begins
+on the page. This will be discussed in more detail under Continuous
+and Discontinuous streams).</li>
+
+<li>A codec must be able to translate a given granule position value
+to a unique, deterministic absolute time value through direct
+calculation. A codec is not required to be able to translate an
+absolute time value into a unique granule position value.</li>
+
+<li>Codecs shall choose a granule position definition that allows that
+codec means to seek as directly as possible to an immediately
+decodable point, such as the bit-divided granule position encoding of
+Theora allows the codec to seek efficiently to key frame without using
+an index. That is, additional information other than absolute time
+may be encoded into a granule position value so long as the granule
+position obeys the above points.</li>
+
+</ul>
+
+<h4>Example: timestamp</h4>
+
+<p>In general, a codec/stream type should choose the simplest granule
+position encoding that addresses its requirements. The examples here
+are by no means exhaustive of the possibilities within Ogg.</p>
+
+<p>A simple granule position could encode a timestamp directly. For
+example, a granule position that encoded milliseconds from beginning
+of stream would allow a logical stream length of over 100,000,000,000
+days before beginning a new logical stream (to avoid the granule
+position wrapping).</p>
+
+<h4>Example: framestamp</h4>
+
+<p>A simple millisecond timestamp granule encoding might suit many stream
+types, but a millisecond resolution is inappropriate to, eg, most
+audio encodings where exact single-sample resolution is generally a
+requirement. A millisecond is both too large a granule and often does
+not represent an integer number of samples.</p>
+
+<p>In the event that audio frames are always encoded as the same number of
+samples, the granule position could simply be a linear count of frames
+since beginning of stream. This has the advantages of being exact and
+efficient. Position in time would simply be <tt>[granule_position] *
+[samples_per_frame] / [samples_per_second]</tt>.</p>
+
+<h4>Example: samplestamp (Vorbis)</h4>
+
+<p>Frame counting is insufficient in codecs such as Vorbis where an audio
+frame [packet] encodes a variable number of samples. In Vorbis's
+case, the granule position is a count of the number of raw samples
+from the beginning of stream; the absolute time of
+a granule position is <tt>[granule_position] /
+[samples_per_second]</tt>.</p>
+
+<h4>Example: bit-divided framestamp (Theora)</h4>
+
+<p>Some video codecs may be able to use the simple framestamp scheme for
+granule position. However, most modern video codecs introduce at
+least the following complications:</p>
+
+<ul>
+
+<li>video frames are relatively far apart compared to audio samples;
+for this reason, the point at which a video frame changes to the next
+frame is usually a strictly defined offset within the frame 'period'.
+That is, video at 50fps could just as easily define frame transitions
+&lt;.015, .035, .055...&gt; as at &lt;.00, .02, .04...&gt;.</li>
+
+<li>frame rates often include drop-frames, leap-frames or other
+rational-but-non-integer timings.</li>
+
+<li>Decode must begin at a 'key frame' or 'I frame'. Keyframes usually
+occur relatively seldom.</li>
+
+</ul>
+
+<p>The first two points can be handled straightforwardly via the fact
+that the codec has complete control mapping granule position to
+absolute time; non-integer frame rates and offsets can be set in the
+codec's initial header, and the rest is just arithmetic.</p>
+
+<p>The third point appears trickier at first glance, but it too can be
+handled through the granule position mapping mechanism. Here we
+arrange the granule position in such a way that granule positions of
+key frames are easy to find. Divide the granule position into two
+fields; the most-significant bits are an absolute frame counter, but
+it's only updated at each key frame. The least significant bits encode
+the number of frames since the last key frame. In this way, each
+granule position both encodes the absolute time of the current frame
+as well as the absolute time of the last key frame.</p>
+
+<p>Seeking to a most recent preceding key frame is then accomplished by
+first seeking to the original desired point, inspecting the granulepos
+of the resulting video page, extracting from that granulepos the
+absolute time of the desired key frame, and then seeking directly to
+that key frame's page. Of course, it's still possible for an
+application to ignore key frames and use a simpler seeking algorithm
+(decode would be unable to present decoded video until the next
+key frame). Surprisingly many player applications do choose the
+simpler approach.</p>
+
+<h3>granule position, packets and pages</h3>
+
+<p>Although each packet of data in a logical stream theoretically has a
+specific granule position, only one granule position is encoded
+per page. It is possible to encode a logical stream such that each
+page contains only a single packet (so that granule positions are
+preserved for each packet), however a one-to-one packet/page mapping
+is not intended to be the general case.</p>
+
+<p>Because Ogg functions at the page, not packet, level, this
+once-per-page time information provides Ogg with the finest-grained
+time information is can use. Ogg passes this granule positioning data
+to the codec (along with the packets extracted from a page); it is the
+responsibility of codecs to track timing information at granularities
+finer than a single page.</p>
+
+<h3>start-time and end-time positioning</h3>
+
+<p>A granule position represents the <em>instantaneous time location
+between two pages</em>. However, continuous streams and discontinuous
+streams differ on whether the granulepos represents the end-time of
+the data on a page or the start-time. Continuous streams are
+'end-time' encoded; the granulepos represents the point in time
+immediately after the last data decoded from a page. Discontinuous
+streams are 'start-time' encoded; the granulepos represents the point
+in time of the first data decoded from the page.</p>
+
+<p>An Ogg stream type is declared continuous or discontinuous by its
+codec. A given codec may support both continuous and discontinuous
+operation so long as any given logical stream is continuous or
+discontinuous for its entirety and the codec is able to ascertain (and
+inform the Ogg layer) as to which after decoding the initial stream
+header. The majority of codecs will always be continuous (such as
+Vorbis) or discontinuous (such as Writ).</p>
+
+<p>Start- and end-time encoding do not affect multiplexing sort-order;
+pages are still sorted by the absolute time a given granulepos maps to
+regardless of whether that granulepos represents start- or
+end-time.</p>
+
+<h2>Multiplex/Demultiplex Division of Labor</h2>
+
+<p>The Ogg multiplex/demultiplex layer provides mechanisms for encoding
+raw packets into Ogg pages, decoding Ogg pages back into the original
+codec packets, determining the logical structure of an Ogg stream, and
+navigating through and synchronizing with an Ogg stream at a desired
+stream location. Strict multiplex/demultiplex operations are entirely
+in the Ogg domain and require no intervention from codecs.</p>
+
+<p>Implementation of more complex operations does require codec
+knowledge, however. Unlike other framing systems, Ogg maintains
+strict separation between framing and the framed bitstream data; Ogg
+does not replicate codec-specific information in the page/framing
+data, nor does Ogg blur the line between framing and stream
+data/metadata. Because Ogg is fully data-agnostic toward the data it
+frames, operations which require specifics of bitstream data (such as
+'seek to key frame') also require interaction with the codec layer
+(because, in this example, the Ogg layer is not aware of the concept
+of key frames). This is different from systems that blur the
+separation between framing and stream data in order to simplify the
+separation of code. The Ogg system purposely keeps the distinction in
+data simple so that later codec innovations are not constrained by
+framing design.</p>
+
+<p>For this reason, however, complex seeking operations require
+interaction with the codecs in order to decode the granule position of
+a given stream type back to absolute time or in order to find
+'decodable points' such as key frames in video.</p>
+
+<h2>Unsorted Discussion Points</h2>
+
+<p>flushes around key frames? RFC suggestion: repaginating or building a
+stream this way is nice but not required</p>
+
+<h2>Appendix A: multiplexing examples</h2>
+
+<div id="copyright">
+ The Xiph Fish Logo is a
+ trademark (&trade;) of Xiph.Org.<br/>
+
+ These pages &copy; 1994 - 2005 Xiph.Org. All rights reserved.
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