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.
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.
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.
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.
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'.
Ogg is designed to use an interpolated bisection search to implement exact positional seeking. Interpolated bisection search is a spec-mandated mechanism.
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.
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.
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.
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.
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.
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.
The fundamental design distinction between continuous and discontinuous streams concerns buffering.
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.
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.
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.
Further details, mechanisms and reasons for the differing arrangement and behavior of continuous and discontinuous streams is discussed later.
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.
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.
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.
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.
The granule position is governed by the following rules:
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.
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).
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.
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 [granule_position] * [samples_per_frame] / [samples_per_second].
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 [granule_position] / [samples_per_second].
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:
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.
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.
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.
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.
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.
A granule position represents the instantaneous time location between two pages. 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.
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).
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.
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.
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.
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.
flushes around key frames? RFC suggestion: repaginating or building a stream this way is nice but not required