mirror of
https://github.com/cookiengineer/audacity
synced 2025-11-04 16:14:00 +01:00
235 lines
8.4 KiB
HTML
235 lines
8.4 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
|
|
<html>
|
|
<head>
|
|
|
|
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-15"/>
|
|
<title>Ogg Vorbis Documentation</title>
|
|
|
|
<style type="text/css">
|
|
body {
|
|
margin: 0 18px 0 18px;
|
|
padding-bottom: 30px;
|
|
font-family: Verdana, Arial, Helvetica, sans-serif;
|
|
color: #333333;
|
|
font-size: .8em;
|
|
}
|
|
|
|
a {
|
|
color: #3366cc;
|
|
}
|
|
|
|
img {
|
|
border: 0;
|
|
}
|
|
|
|
#xiphlogo {
|
|
margin: 30px 0 16px 0;
|
|
}
|
|
|
|
#content p {
|
|
line-height: 1.4;
|
|
}
|
|
|
|
h1, h1 a, h2, h2 a, h3, h3 a {
|
|
font-weight: bold;
|
|
color: #ff9900;
|
|
margin: 1.3em 0 8px 0;
|
|
}
|
|
|
|
h1 {
|
|
font-size: 1.3em;
|
|
}
|
|
|
|
h2 {
|
|
font-size: 1.2em;
|
|
}
|
|
|
|
h3 {
|
|
font-size: 1.1em;
|
|
}
|
|
|
|
li {
|
|
line-height: 1.4;
|
|
}
|
|
|
|
#copyright {
|
|
margin-top: 30px;
|
|
line-height: 1.5em;
|
|
text-align: center;
|
|
font-size: .8em;
|
|
color: #888888;
|
|
clear: both;
|
|
}
|
|
</style>
|
|
|
|
</head>
|
|
|
|
<body>
|
|
|
|
<div id="xiphlogo">
|
|
<a href="http://www.xiph.org/"><img src="fish_xiph_org.png" alt="Fish Logo and Xiph.Org"/></a>
|
|
</div>
|
|
|
|
<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>
|
|
|
|
<div id="copyright">
|
|
The Xiph Fish Logo is a
|
|
trademark (™) of Xiph.Org.<br/>
|
|
|
|
These pages © 1994 - 2005 Xiph.Org. All rights reserved.
|
|
</div>
|
|
|
|
</body>
|
|
</html>
|