libogg/doc/ogg-multiplex.html

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<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 a bisection search to implement exact
positional seeking rather than building an index; an index requires
two-pass encoding and as such is not acceptable given the requirement
for full-featured linear encoding.</p>

<p><i>Even making an index optional then requires an
application to support multiple methods (bisection search for a
one-pass stream, indexing for a two-pass stream), which adds no
additional functionality as bisection search delivers the same
functionality for both stream types.</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>

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