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