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