a3062c7040
This keeps the header description from being rendered blue. svn path=/trunk/ogg/; revision=18422
430 lines
14 KiB
HTML
430 lines
14 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 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 bitstream framing</h1>
|
|
|
|
<h2>Ogg bitstreams</h2>
|
|
|
|
<p>The Ogg transport bitstream is designed to provide framing, error
|
|
protection and seeking structure for higher-level codec streams that
|
|
consist of raw, unencapsulated data packets, such as the Vorbis audio
|
|
codec or Theora video codec.</p>
|
|
|
|
<h2>Application example: Vorbis</h2>
|
|
|
|
<p>Vorbis encodes short-time blocks of PCM data into raw packets of
|
|
bit-packed data. These 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 uses 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>Design constraints for Ogg bitstreams</h2>
|
|
|
|
<ol>
|
|
<li>True streaming; we must not need to seek to build a 100%
|
|
complete bitstream.</li>
|
|
<li>Use no more than approximately 1-2% of bitstream bandwidth for
|
|
packet boundary marking, high-level framing, sync and seeking.</li>
|
|
<li>Specification of absolute position within the original sample
|
|
stream.</li>
|
|
<li>Simple mechanism to ease limited editing, such as a simplified
|
|
concatenation mechanism.</li>
|
|
<li>Detection of corruption, recapture after error and direct, random
|
|
access to data at arbitrary positions in the bitstream.</li>
|
|
</ol>
|
|
|
|
<h2>Logical and Physical Bitstreams</h2>
|
|
|
|
<p>A <em>logical</em> Ogg bitstream is a contiguous stream of
|
|
sequential pages belonging only to the logical bitstream. A
|
|
<em>physical</em> Ogg bitstream is constructed from one or more
|
|
than one logical Ogg bitstream (the simplest physical bitstream
|
|
is simply a single logical bitstream). We describe below the exact
|
|
formatting of an Ogg logical bitstream. Combining logical
|
|
bitstreams into more complex physical bitstreams is described in the
|
|
<a href="oggstream.html">Ogg bitstream overview</a>. The exact
|
|
mapping of raw Vorbis packets into a valid Ogg Vorbis physical
|
|
bitstream is described in the Vorbis I Specification.</p>
|
|
|
|
<h2>Bitstream structure</h2>
|
|
|
|
<p>An Ogg stream is structured by dividing incoming packets into
|
|
segments of up to 255 bytes and then wrapping a group of contiguous
|
|
packet segments into a variable length page preceded by a page
|
|
header. Both the header size and page size are variable; the page
|
|
header contains sizing information and checksum data to determine
|
|
header/page size and data integrity.</p>
|
|
|
|
<p>The bitstream is captured (or recaptured) by looking for the beginning
|
|
of a page, specifically the capture pattern. Once the capture pattern
|
|
is found, the decoder verifies page sync and integrity by computing
|
|
and comparing the checksum. At that point, the decoder can extract the
|
|
packets themselves.</p>
|
|
|
|
<h3>Packet segmentation</h3>
|
|
|
|
<p>Packets are logically divided into multiple segments before encoding
|
|
into a page. Note that the segmentation and fragmentation process is a
|
|
logical one; it's used to compute page header values and the original
|
|
page data need not be disturbed, even when a packet spans page
|
|
boundaries.</p>
|
|
|
|
<p>The raw packet is logically divided into [n] 255 byte segments and a
|
|
last fractional segment of < 255 bytes. A packet size may well
|
|
consist only of the trailing fractional segment, and a fractional
|
|
segment may be zero length. These values, called "lacing values" are
|
|
then saved and placed into the header segment table.</p>
|
|
|
|
<p>An example should make the basic concept clear:</p>
|
|
|
|
<pre>
|
|
<tt>
|
|
raw packet:
|
|
___________________________________________
|
|
|______________packet data__________________| 753 bytes
|
|
|
|
lacing values for page header segment table: 255,255,243
|
|
</tt>
|
|
</pre>
|
|
|
|
<p>We simply add the lacing values for the total size; the last lacing
|
|
value for a packet is always the value that is less than 255. Note
|
|
that this encoding both avoids imposing a maximum packet size as well
|
|
as imposing minimum overhead on small packets (as opposed to, eg,
|
|
simply using two bytes at the head of every packet and having a max
|
|
packet size of 32k. Small packets (<255, the typical case) are
|
|
penalized with twice the segmentation overhead). Using the lacing
|
|
values as suggested, small packets see the minimum possible
|
|
byte-aligned overhead (1 byte) and large packets, over 512 bytes or
|
|
so, see a fairly constant ~.5% overhead on encoding space.</p>
|
|
|
|
<p>Note that a lacing value of 255 implies that a second lacing value
|
|
follows in the packet, and a value of < 255 marks the end of the
|
|
packet after that many additional bytes. A packet of 255 bytes (or a
|
|
multiple of 255 bytes) is terminated by a lacing value of 0:</p>
|
|
|
|
<pre><tt>
|
|
raw packet:
|
|
_______________________________
|
|
|________packet data____________| 255 bytes
|
|
|
|
lacing values: 255, 0
|
|
</tt></pre>
|
|
|
|
<p>Note also that a 'nil' (zero length) packet is not an error; it
|
|
consists of nothing more than a lacing value of zero in the header.</p>
|
|
|
|
<h3>Packets spanning pages</h3>
|
|
|
|
<p>Packets are not restricted to beginning and ending within a page,
|
|
although individual segments are, by definition, required to do so.
|
|
Packets are not restricted to a maximum size, although excessively
|
|
large packets in the data stream are discouraged.</p>
|
|
|
|
<p>After segmenting a packet, the encoder may decide not to place all the
|
|
resulting segments into the current page; to do so, the encoder places
|
|
the lacing values of the segments it wishes to belong to the current
|
|
page into the current segment table, then finishes the page. The next
|
|
page is begun with the first value in the segment table belonging to
|
|
the next packet segment, thus continuing the packet (data in the
|
|
packet body must also correspond properly to the lacing values in the
|
|
spanned pages. The segment data in the first packet corresponding to
|
|
the lacing values of the first page belong in that page; packet
|
|
segments listed in the segment table of the following page must begin
|
|
the page body of the subsequent page).</p>
|
|
|
|
<p>The last mechanic to spanning a page boundary is to set the header
|
|
flag in the new page to indicate that the first lacing value in the
|
|
segment table continues rather than begins a packet; a header flag of
|
|
0x01 is set to indicate a continued packet. Although mandatory, it
|
|
is not actually algorithmically necessary; one could inspect the
|
|
preceding segment table to determine if the packet is new or
|
|
continued. Adding the information to the packet_header flag allows a
|
|
simpler design (with no overhead) that needs only inspect the current
|
|
page header after frame capture. This also allows faster error
|
|
recovery in the event that the packet originates in a corrupt
|
|
preceding page, implying that the previous page's segment table
|
|
cannot be trusted.</p>
|
|
|
|
<p>Note that a packet can span an arbitrary number of pages; the above
|
|
spanning process is repeated for each spanned page boundary. Also a
|
|
'zero termination' on a packet size that is an even multiple of 255
|
|
must appear even if the lacing value appears in the next page as a
|
|
zero-length continuation of the current packet. The header flag
|
|
should be set to 0x01 to indicate that the packet spanned, even though
|
|
the span is a nil case as far as data is concerned.</p>
|
|
|
|
<p>The encoding looks odd, but is properly optimized for speed and the
|
|
expected case of the majority of packets being between 50 and 200
|
|
bytes (note that it is designed such that packets of wildly different
|
|
sizes can be handled within the model; placing packet size
|
|
restrictions on the encoder would have only slightly simplified design
|
|
in page generation and increased overall encoder complexity).</p>
|
|
|
|
<p>The main point behind tracking individual packets (and packet
|
|
segments) is to allow more flexible encoding tricks that requiring
|
|
explicit knowledge of packet size. An example is simple bandwidth
|
|
limiting, implemented by simply truncating packets in the nominal case
|
|
if the packet is arranged so that the least sensitive portion of the
|
|
data comes last.</p>
|
|
|
|
<a name="page_header"></a>
|
|
<h3>Page header</h3>
|
|
|
|
<p>The headering mechanism is designed to avoid copying and re-assembly
|
|
of the packet data (ie, making the packet segmentation process a
|
|
logical one); the header can be generated directly from incoming
|
|
packet data. The encoder buffers packet data until it finishes a
|
|
complete page at which point it writes the header followed by the
|
|
buffered packet segments.</p>
|
|
|
|
<h4>capture_pattern</h4>
|
|
|
|
<p>A header begins with a capture pattern that simplifies identifying
|
|
pages; once the decoder has found the capture pattern it can do a more
|
|
intensive job of verifying that it has in fact found a page boundary
|
|
(as opposed to an inadvertent coincidence in the byte stream).</p>
|
|
|
|
<pre><tt>
|
|
byte value
|
|
|
|
0 0x4f 'O'
|
|
1 0x67 'g'
|
|
2 0x67 'g'
|
|
3 0x53 'S'
|
|
</tt></pre>
|
|
|
|
<h4>stream_structure_version</h4>
|
|
|
|
<p>The capture pattern is followed by the stream structure revision:</p>
|
|
|
|
<pre><tt>
|
|
byte value
|
|
|
|
4 0x00
|
|
</tt></pre>
|
|
|
|
<h4>header_type_flag</h4>
|
|
|
|
<p>The header type flag identifies this page's context in the bitstream:</p>
|
|
|
|
<pre><tt>
|
|
byte value
|
|
|
|
5 bitflags: 0x01: unset = fresh packet
|
|
set = continued packet
|
|
0x02: unset = not first page of logical bitstream
|
|
set = first page of logical bitstream (bos)
|
|
0x04: unset = not last page of logical bitstream
|
|
set = last page of logical bitstream (eos)
|
|
</tt></pre>
|
|
|
|
<h4>absolute granule position</h4>
|
|
|
|
<p>(This is packed in the same way the rest of Ogg data is packed; LSb
|
|
of LSB first. Note that the 'position' data specifies a 'sample'
|
|
number (eg, in a CD quality sample is four octets, 16 bits for left
|
|
and 16 bits for right; in video it would likely be the frame number.
|
|
It is up to the specific codec in use to define the semantic meaning
|
|
of the granule position value). The position specified is the total
|
|
samples encoded after including all packets finished on this page
|
|
(packets begun on this page but continuing on to the next page do not
|
|
count). The rationale here is that the position specified in the
|
|
frame header of the last page tells how long the data coded by the
|
|
bitstream is. A truncated stream will still return the proper number
|
|
of samples that can be decoded fully.</p>
|
|
|
|
<p>A special value of '-1' (in two's complement) indicates that no packets
|
|
finish on this page.</p>
|
|
|
|
<pre><tt>
|
|
byte value
|
|
|
|
6 0xXX LSB
|
|
7 0xXX
|
|
8 0xXX
|
|
9 0xXX
|
|
10 0xXX
|
|
11 0xXX
|
|
12 0xXX
|
|
13 0xXX MSB
|
|
</tt></pre>
|
|
|
|
<h4>stream serial number</h4>
|
|
|
|
<p>Ogg allows for separate logical bitstreams to be mixed at page
|
|
granularity in a physical bitstream. The most common case would be
|
|
sequential arrangement, but it is possible to interleave pages for
|
|
two separate bitstreams to be decoded concurrently. The serial
|
|
number is the means by which pages physical pages are associated with
|
|
a particular logical stream. Each logical stream must have a unique
|
|
serial number within a physical stream:</p>
|
|
|
|
<pre><tt>
|
|
byte value
|
|
|
|
14 0xXX LSB
|
|
15 0xXX
|
|
16 0xXX
|
|
17 0xXX MSB
|
|
</tt></pre>
|
|
|
|
<h4>page sequence no</h4>
|
|
|
|
<p>Page counter; lets us know if a page is lost (useful where packets
|
|
span page boundaries).</p>
|
|
|
|
<pre><tt>
|
|
byte value
|
|
|
|
18 0xXX LSB
|
|
19 0xXX
|
|
20 0xXX
|
|
21 0xXX MSB
|
|
</tt></pre>
|
|
|
|
<h4>page checksum</h4>
|
|
|
|
<p>32 bit CRC value (direct algorithm, initial val and final XOR = 0,
|
|
generator polynomial=0x04c11db7). The value is computed over the
|
|
entire header (with the CRC field in the header set to zero) and then
|
|
continued over the page. The CRC field is then filled with the
|
|
computed value.</p>
|
|
|
|
<p>(A thorough discussion of CRC algorithms can be found in <a
|
|
href="http://www.ross.net/crc/download/crc_v3.txt">"A
|
|
Painless Guide to CRC Error Detection Algorithms"</a> by Ross
|
|
Williams <a href="mailto:ross@ross.net">ross@ross.net</a>.)</p>
|
|
|
|
<pre><tt>
|
|
byte value
|
|
|
|
22 0xXX LSB
|
|
23 0xXX
|
|
24 0xXX
|
|
25 0xXX MSB
|
|
</tt></pre>
|
|
|
|
<h4>page_segments</h4>
|
|
|
|
<p>The number of segment entries to appear in the segment table. The
|
|
maximum number of 255 segments (255 bytes each) sets the maximum
|
|
possible physical page size at 65307 bytes or just under 64kB (thus
|
|
we know that a header corrupted so as destroy sizing/alignment
|
|
information will not cause a runaway bitstream. We'll read in the
|
|
page according to the corrupted size information that's guaranteed to
|
|
be a reasonable size regardless, notice the checksum mismatch, drop
|
|
sync and then look for recapture).</p>
|
|
|
|
<pre><tt>
|
|
byte value
|
|
|
|
26 0x00-0xff (0-255)
|
|
</tt></pre>
|
|
|
|
<h4>segment_table (containing packet lacing values)</h4>
|
|
|
|
<p>The lacing values for each packet segment physically appearing in
|
|
this page are listed in contiguous order.</p>
|
|
|
|
<pre><tt>
|
|
byte value
|
|
|
|
27 0x00-0xff (0-255)
|
|
[...]
|
|
n 0x00-0xff (0-255, n=page_segments+26)
|
|
</tt></pre>
|
|
|
|
<p>Total page size is calculated directly from the known header size and
|
|
lacing values in the segment table. Packet data segments follow
|
|
immediately after the header.</p>
|
|
|
|
<p>Page headers typically impose a flat .25-.5% space overhead assuming
|
|
nominal ~8k page sizes. The segmentation table needed for exact
|
|
packet recovery in the streaming layer adds approximately .5-1%
|
|
nominal assuming expected encoder behavior in the 44.1kHz, 128kbps
|
|
stereo encodings.</p>
|
|
|
|
<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>
|