draft-ietf-codec-oggopus.xml 66.2 KB
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<?xml version="1.0" encoding="utf-8"?>
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<!DOCTYPE rfc SYSTEM 'rfc2629.dtd' [
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<!ENTITY rfc2119 PUBLIC '' 'http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml'>
<!ENTITY rfc3533 PUBLIC '' 'http://xml.resource.org/public/rfc/bibxml/reference.RFC.3533.xml'>
<!ENTITY rfc3629 PUBLIC '' 'http://xml.resource.org/public/rfc/bibxml/reference.RFC.3629.xml'>
<!ENTITY rfc4732 PUBLIC '' 'http://xml.resource.org/public/rfc/bibxml/reference.RFC.4732.xml'>
<!ENTITY rfc5334 PUBLIC '' 'http://xml.resource.org/public/rfc/bibxml/reference.RFC.5334.xml'>
<!ENTITY rfc6381 PUBLIC '' 'http://xml.resource.org/public/rfc/bibxml/reference.RFC.6381.xml'>
<!ENTITY rfc6716 PUBLIC '' 'http://xml.resource.org/public/rfc/bibxml/reference.RFC.6716.xml'>
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<!ENTITY rfc6982 PUBLIC '' 'http://xml.resource.org/public/rfc/bibxml/reference.RFC.6982.xml'>
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]>
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<?rfc toc="yes" symrefs="yes" ?>

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<rfc ipr="trust200902" category="std" docName="draft-ietf-codec-oggopus-04">
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<front>
<title abbrev="Ogg Opus">Ogg Encapsulation for the Opus Audio Codec</title>
<author initials="T.B." surname="Terriberry" fullname="Timothy B. Terriberry">
<organization>Mozilla Corporation</organization>
<address>
<postal>
<street>650 Castro Street</street>
<city>Mountain View</city>
<region>CA</region>
<code>94041</code>
<country>USA</country>
</postal>
<phone>+1 650 903-0800</phone>
<email>tterribe@xiph.org</email>
</address>
</author>

<author initials="R." surname="Lee" fullname="Ron Lee">
<organization>Voicetronix</organization>
<address>
<postal>
<street>246 Pulteney Street, Level 1</street>
<city>Adelaide</city>
<region>SA</region>
<code>5000</code>
<country>Australia</country>
</postal>
<phone>+61 8 8232 9112</phone>
<email>ron@debian.org</email>
</address>
</author>

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<author initials="R." surname="Giles" fullname="Ralph Giles">
<organization>Mozilla Corporation</organization>
<address>
<postal>
<street>163 West Hastings Street</street>
<city>Vancouver</city>
<region>BC</region>
<code>V6B 1H5</code>
<country>Canada</country>
</postal>
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<phone>+1 778 785 1540</phone>
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<email>giles@xiph.org</email>
</address>
</author>

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<date day="9" month="August" year="2014"/>
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<area>RAI</area>
<workgroup>codec</workgroup>

<abstract>
<t>
This document defines the Ogg encapsulation for the Opus interactive speech and
 audio codec.
This allows data encoded in the Opus format to be stored in an Ogg logical
 bitstream.
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Ogg encapsulation provides Opus with a long-term storage format supporting
 all of the essential features, including metadata, fast and accurate seeking,
 corruption detection, recapture after errors, low overhead, and the ability to
 multiplex Opus with other codecs (including video) with minimal buffering.
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It also provides a live streamable format, capable of delivery over a reliable
 stream-oriented transport, without requiring all the data, or even the total
 length of the data, up-front, in a form that is identical to the on-disk
 storage format.
</t>
</abstract>
</front>

<middle>
<section anchor="intro" title="Introduction">
<t>
The IETF Opus codec is a low-latency audio codec optimized for both voice and
 general-purpose audio.
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See <xref target="RFC6716"/> for technical details.
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This document defines the encapsulation of Opus in a continuous, logical Ogg
 bitstream&nbsp;<xref target="RFC3533"/>.
</t>
<t>
Ogg bitstreams are made up of a series of 'pages', each of which contains data
 from one or more 'packets'.
Pages are the fundamental unit of multiplexing in an Ogg stream.
Each page is associated with a particular logical stream and contains a capture
 pattern and checksum, flags to mark the beginning and end of the logical
 stream, and a 'granule position' that represents an absolute position in the
 stream, to aid seeking.
A single page can contain up to 65,025 octets of packet data from up to 255
 different packets.
Packets may be split arbitrarily across pages, and continued from one page to
 the next (allowing packets much larger than would fit on a single page).
Each page contains 'lacing values' that indicate how the data is partitioned
 into packets, allowing a demuxer to recover the packet boundaries without
 examining the encoded data.
A packet is said to 'complete' on a page when the page contains the final
 lacing value corresponding to that packet.
</t>
<t>
This encapsulation defines the required contents of the packet data, including
 the necessary headers, the organization of those packets into a logical
 stream, and the interpretation of the codec-specific granule position field.
It does not attempt to describe or specify the existing Ogg container format.
Readers unfamiliar with the basic concepts mentioned above are encouraged to
 review the details in <xref target="RFC3533"/>.
</t>

</section>

<section anchor="terminology" title="Terminology">
<t>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
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 "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in <xref target="RFC2119"/>.
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</t>

<t>
Implementations that fail to satisfy one or more "MUST" requirements are
 considered non-compliant.
Implementations that satisfy all "MUST" requirements, but fail to satisfy one
 or more "SHOULD" requirements are said to be "conditionally compliant".
All other implementations are "unconditionally compliant".
</t>

</section>

<section anchor="packet_organization" title="Packet Organization">
<t>
An Opus stream is organized as follows.
</t>
<t>
There are two mandatory header packets.
The granule position of the pages on which these packets complete MUST be zero.
</t>
<t>
The first packet in the logical Ogg bitstream MUST contain the identification
 (ID) header, which uniquely identifies a stream as Opus audio.
The format of this header is defined in <xref target="id_header"/>.
It MUST be placed alone (without any other packet data) on the first page of
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 the logical Ogg bitstream, and must complete on that page.
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This page MUST have its 'beginning of stream' flag set.
</t>
<t>
The second packet in the logical Ogg bitstream MUST contain the comment header,
 which contains user-supplied metadata.
The format of this header is defined in <xref target="comment_header"/>.
It MAY span one or more pages, beginning on the second page of the logical
 stream.
However many pages it spans, the comment header packet MUST finish the page on
 which it completes.
</t>
<t>
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All subsequent pages are audio data pages, and the Ogg packets they contain are
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 audio data packets.
Each audio data packet contains one Opus packet for each of N different
 streams, where N is typically one for mono or stereo, but may be greater than
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 one for multichannel audio.
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The value N is specified in the ID header (see
 <xref target="channel_mapping"/>), and is fixed over the entire length of the
 logical Ogg bitstream.
</t>
<t>
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The first N-1 Opus packets, if any, are packed one after another into the Ogg
 packet, using the self-delimiting framing from Appendix&nbsp;B of
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 <xref target="RFC6716"/>.
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The remaining Opus packet is packed at the end of the Ogg packet using the
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 regular, undelimited framing from Section&nbsp;3 of <xref target="RFC6716"/>.
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All of the Opus packets in a single Ogg packet MUST be constrained to have the
 same duration.
A decoder SHOULD treat any Opus packet whose duration is different from that of
 the first Opus packet in an Ogg packet as if it were an Opus packet with an
 illegal TOC sequence.
</t>
<t>
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The coding mode (SILK, Hybrid, or CELT), audio bandwidth, channel count,
 duration (frame size), and number of frames per packet, are indicated in the
 TOC (table of contents) in the first byte of each Opus packet, as described
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 in Section&nbsp;3.1 of&nbsp;<xref target="RFC6716"/>.
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The combination of mode, audio bandwidth, and frame size is referred to as
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 the configuration of an Opus packet.
</t>
<t>
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The first audio data page SHOULD NOT have the 'continued packet' flag set
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 (which would indicate the first audio data packet is continued from a previous
 page).
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Packets MUST be placed into Ogg pages in order until the end of stream.
Audio packets MAY span page boundaries.
A decoder MUST treat a zero-octet audio data packet as if it were an Opus
 packet with an illegal TOC sequence.
The last page SHOULD have the 'end of stream' flag set, but implementations
 should be prepared to deal with truncated streams that do not have a page
 marked 'end of stream'.
The final packet on the last page SHOULD NOT be a continued packet, i.e., the
 final lacing value should be less than 255.
There MUST NOT be any more pages in an Opus logical bitstream after a page
 marked 'end of stream'.
</t>
</section>

<section anchor="granpos" title="Granule Position">
<t>
The granule position of an audio data page encodes the total number of PCM
 samples in the stream up to and including the last fully-decodable sample from
 the last packet completed on that page.
A page that is entirely spanned by a single packet (that completes on a
 subsequent page) has no granule position, and the granule position field MUST
 be set to the special value '-1' in two's complement.
</t>

<t>
The granule position of an audio data page is in units of PCM audio samples at
 a fixed rate of 48&nbsp;kHz (per channel; a stereo stream's granule position
 does not increment at twice the speed of a mono stream).
It is possible to run an Opus decoder at other sampling rates, but the value
 in the granule position field always counts samples assuming a 48&nbsp;kHz
 decoding rate, and the rest of this specification makes the same assumption.
</t>

<t>
The duration of an Opus packet may be any multiple of 2.5&nbsp;ms, up to a
 maximum of 120&nbsp;ms.
This duration is encoded in the TOC sequence at the beginning of each packet.
The number of samples returned by a decoder corresponds to this duration
 exactly, even for the first few packets.
For example, a 20&nbsp;ms packet fed to a decoder running at 48&nbsp;kHz will
 always return 960&nbsp;samples.
A demuxer can parse the TOC sequence at the beginning of each Ogg packet to
 work backwards or forwards from a packet with a known granule position (i.e.,
 the last packet completed on some page) in order to assign granule positions
 to every packet, or even every individual sample.
The one exception is the last page in the stream, as described below.
</t>

<t>
All other pages with completed packets after the first MUST have a granule
 position equal to the number of samples contained in packets that complete on
 that page plus the granule position of the most recent page with completed
 packets.
This guarantees that a demuxer can assign individual packets the same granule
 position when working forwards as when working backwards.
For this to work, there cannot be any gaps.
</t>

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<section anchor="gap-repair" title="Repairing Gaps in Real-time Streams">
<t>
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In order to support capturing a real-time stream that has lost or not
 transmitted packets, a muxer SHOULD emit packets that explicitly request the
 use of Packet Loss Concealment (PLC) in place of the missing packets.
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Only gaps that are a multiple of 2.5&nbsp;ms are repairable, as these are the
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 only durations that can be created by packet loss or discontinuous
 transmission.
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Muxers need not handle other gap sizes.
Creating the necessary packets involves synthesizing a TOC byte (defined in
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Section&nbsp;3.1 of&nbsp;<xref target="RFC6716"/>)&mdash;and whatever
 additional internal framing is needed&mdash;to indicate the packet duration
 for each stream.
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The actual length of each missing Opus frame inside the packet is zero bytes,
 as defined in Section&nbsp;3.2.1 of&nbsp;<xref target="RFC6716"/>.
</t>

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<t>
Zero-byte frames MAY be packed into packets using any of codes&nbsp;0, 1,
 2, or&nbsp;3.
When successive frames have the same configuration, the higher code packings
 reduce overhead.
Likewise, if the TOC configuration matches, the muxer MAY further combine the
 empty frames with previous or subsequent non-zero-length frames (using
 code&nbsp;2 or VBR code&nbsp;3).
</t>

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<t>
<xref target="RFC6716"/> does not impose any requirements on the PLC, but this
 section outlines choices that are expected to have a positive influence on
 most PLC implementations, including the reference implementation.
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Synthesized TOC bytes SHOULD maintain the same mode, audio bandwidth,
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 channel count, and frame size as the previous packet (if any).
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This is the simplest and usually the most well-tested case for the PLC to
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 handle and it covers all losses that do not include a configuration switch,
 as defined in Section&nbsp;4.5 of&nbsp;<xref target="RFC6716"/>.
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</t>

<t>
When a previous packet is available, keeping the audio bandwidth and channel
 count the same allows the PLC to provide maximum continuity in the concealment
 data it generates.
However, if the size of the gap is not a multiple of the most recent frame
 size, then the frame size will have to change for at least some frames.
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Such changes SHOULD be delayed as long as possible to simplify
 things for PLC implementations.
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</t>

<t>
As an example, a 95&nbsp;ms gap could be encoded as nineteen 5&nbsp;ms frames
 in two bytes with a single CBR code&nbsp;3 packet.
If the previous frame size was 20&nbsp;ms, using four 20&nbsp;ms frames
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 followed by three 5&nbsp;ms frames requires 4&nbsp;bytes (plus an extra byte
 of Ogg lacing overhead), but allows the PLC to use its well-tested steady
 state behavior for as long as possible.
The total bitrate of the latter approach, including Ogg overhead, is about
 0.4&nbsp;kbps, so the impact on file size is minimal.
</t>

<t>
Changing modes is discouraged, since this causes some decoder implementations
 to reset their PLC state.
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However, SILK and Hybrid mode frames cannot fill gaps that are not a multiple
 of 10&nbsp;ms.
If switching to CELT mode is needed to match the gap size, a muxer SHOULD do
 so at the end of the gap to allow the PLC to function for as long as possible.
</t>

<t>
In the example above, if the previous frame was a 20&nbsp;ms SILK mode frame,
 the better solution is to synthesize a packet describing four 20&nbsp;ms SILK
 frames, followed by a packet with a single 10&nbsp;ms SILK
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 frame, and finally a packet with a 5&nbsp;ms CELT frame, to fill the 95&nbsp;ms
 gap.
This also requires four bytes to describe the synthesized packet data (two
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 bytes for a CBR code 3 and one byte each for two code 0 packets) but three
 bytes of Ogg lacing overhead are required to mark the packet boundaries.
At 0.6 kbps, this is still a minimal bitrate impact over a naive, low quality
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 solution.
</t>

<t>
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Since medium-band audio is an option only in the SILK mode, wideband frames
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 SHOULD be generated if switching from that configuration to CELT mode, to
 ensure that any PLC implementation which does try to migrate state between
 the modes will be able to preserve all of the available audio bandwidth.
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</t>

</section>

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<section anchor="preskip" title="Pre-skip">
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<t>
There is some amount of latency introduced during the decoding process, to
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 allow for overlap in the CELT mode, stereo mixing in the SILK mode, and
 resampling.
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The encoder may introduce additional latency through its own resampling
 and analysis (though the exact amount is not specified).
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Therefore, the first few samples produced by the decoder do not correspond to
 real input audio, but are instead composed of padding inserted by the encoder
 to compensate for this latency.
These samples need to be stored and decoded, as Opus is an asymptotically
 convergent predictive codec, meaning the decoded contents of each frame depend
 on the recent history of decoder inputs.
However, a decoder will want to skip these samples after decoding them.
</t>

<t>
A 'pre-skip' field in the ID header (see <xref target="id_header"/>) signals
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 the number of samples which SHOULD be skipped (decoded but discarded) at the
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 beginning of the stream.
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This amount MAY not be a multiple of 2.5&nbsp;ms, MAY be smaller than a single
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 packet, or MAY span the contents of several packets.
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These samples are not valid audio, and should not be played.
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</t>
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<t>
For example, if the first Opus frame uses the CELT mode, it will always
 produce 120 samples of windowed overlap-add data.
However, the overlap data is initially all zeros (since there is no prior
 frame), meaning this cannot, in general, accurately represent the original
 audio.
The SILK mode requires additional delay to account for its analysis and
 resampling latency.
The encoder delays the original audio to avoid this problem.
</t>

<t>
The pre-skip field MAY also be used to perform sample-accurate cropping of
 already encoded streams.
In this case, a value of at least 3840&nbsp;samples (80&nbsp;ms) provides
 sufficient history to the decoder that it will have converged
 before the stream's output begins.
</t>

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</section>
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<section anchor="pcm_sample_position" title="PCM Sample Position">
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<t>
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<figure align="center">
<preamble>
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The PCM sample position is determined from the granule position using the
 formula
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</preamble>
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<artwork align="center"><![CDATA[
'PCM sample position' = 'granule position' - 'pre-skip' .
]]></artwork>
</figure>
</t>

<t>
For example, if the granule position of the first audio data page is 59,971,
 and the pre-skip is 11,971, then the PCM sample position of the last decoded
 sample from that page is 48,000.
<figure align="center">
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<preamble>
This can be converted into a playback time using the formula
</preamble>
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<artwork align="center"><![CDATA[
                  'PCM sample position'
'playback time' = --------------------- .
                         48000.0
]]></artwork>
</figure>
</t>

<t>
The initial PCM sample position before any samples are played is normally '0'.
In this case, the PCM sample position of the first audio sample to be played
 starts at '1', because it marks the time on the clock
 <spanx style="emph">after</spanx> that sample has been played, and a stream
 that is exactly one second long has a final PCM sample position of '48000',
 as in the example here.
</t>

<t>
Vorbis streams use a granule position smaller than the number of audio samples
 contained in the first audio data page to indicate that some of those samples
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 must be trimmed from the output (see <xref target="vorbis-trim"/>).
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However, to do so, Vorbis requires that the first audio data page contains
 exactly two packets, in order to allow the decoder to perform PCM position
 adjustments before needing to return any PCM data.
Opus uses the pre-skip mechanism for this purpose instead, since the encoder
 may introduce more than a single packet's worth of latency, and since very
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 large packets in streams with a very large number of channels might not fit
 on a single page.
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</t>
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</section>
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<section anchor="end_trimming" title="End Trimming">
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<t>
The page with the 'end of stream' flag set MAY have a granule position that
 indicates the page contains less audio data than would normally be returned by
 decoding up through the final packet.
This is used to end the stream somewhere other than an even frame boundary.
The granule position of the most recent audio data page with completed packets
 is used to make this determination, or '0' is used if there were no previous
 audio data pages with a completed packet.
The difference between these granule positions indicates how many samples to
 keep after decoding the packets that completed on the final page.
The remaining samples are discarded.
The number of discarded samples SHOULD be no larger than the number decoded
 from the last packet.
</t>
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</section>
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<section anchor="start_granpos_restrictions"
 title="Restrictions on the Initial Granule Position">
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<t>
The granule position of the first audio data page with a completed packet MAY
 be larger than the number of samples contained in packets that complete on
 that page, however it MUST NOT be smaller, unless that page has the 'end of
 stream' flag set.
Allowing a granule position larger than the number of samples allows the
 beginning of a stream to be cropped or a live stream to be joined without
 rewriting the granule position of all the remaining pages.
This means that the PCM sample position just before the first sample to be
 played may be larger than '0'.
Synchronization when multiplexing with other logical streams still uses the PCM
 sample position relative to '0' to compute sample times.
This does not affect the behavior of pre-skip: exactly 'pre-skip' samples
 should be skipped from the beginning of the decoded output, even if the
 initial PCM sample position is greater than zero.
</t>

<t>
On the other hand, a granule position that is smaller than the number of
 decoded samples prevents a demuxer from working backwards to assign each
 packet or each individual sample a valid granule position, since granule
 positions must be non-negative.
A decoder MUST reject as invalid any stream where the granule position is
 smaller than the number of samples contained in packets that complete on the
 first audio data page with a completed packet, unless that page has the 'end
 of stream' flag set.
It MAY defer this action until it decodes the last packet completed on that
 page.
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</t>

<t>
If that page has the 'end of stream' flag set, a demuxer MUST reject as invalid
 any stream where its granule position is smaller than the 'pre-skip' amount.
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This would indicate that more samples should be skipped from the initial
 decoded output than exist in the stream.
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If the granule position is smaller than the number of decoded samples produced
 by the packets that complete on that page, then a demuxer MUST use an initial
 granule position of '0', and can work forwards from '0' to timestamp
 individual packets.
If the granule position is larger than the number of decoded samples available,
 then the demuxer MUST still work backwards as described above, even if the
 'end of stream' flag is set, to determine the initial granule position, and
 thus the initial PCM sample position.
Both of these will be greater than '0' in this case.
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</section>

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<section anchor="seeking_and_preroll" title="Seeking and Pre-roll">
<t>
Seeking in Ogg files is best performed using a bisection search for a page
 whose granule position corresponds to a PCM position at or before the seek
 target.
With appropriately weighted bisection, accurate seeking can be performed with
 just three or four bisections even in multi-gigabyte files.
See <xref target="seeking"/> for general implementation guidance.
</t>

<t>
When seeking within an Ogg Opus stream, the decoder SHOULD start decoding (and
 discarding the output) at least 3840&nbsp;samples (80&nbsp;ms) prior to the
 seek target in order to ensure that the output audio is correct by the time it
 reaches the seek target.
This 'pre-roll' is separate from, and unrelated to, the 'pre-skip' used at the
 beginning of the stream.
If the point 80&nbsp;ms prior to the seek target comes before the initial PCM
 sample position, the decoder SHOULD start decoding from the beginning of the
 stream, applying pre-skip as normal, regardless of whether the pre-skip is
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 larger or smaller than 80&nbsp;ms, and then continue to discard the samples
 required to reach the seek target (if any).
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</t>
</section>

</section>

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<section anchor="headers" title="Header Packets">
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An Opus stream contains exactly two mandatory header packets:
 an identification header and a comment header.
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</t>

<section anchor="id_header" title="Identification Header">

<figure anchor="id_header_packet" title="ID Header Packet" align="center">
<artwork align="center"><![CDATA[
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      'O'      |      'p'      |      'u'      |      's'      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      'H'      |      'e'      |      'a'      |      'd'      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Version = 1  | Channel Count |           Pre-skip            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Input Sample Rate (Hz)                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Output Gain (Q7.8 in dB)    | Mapping Family|               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               :
|                                                               |
:               Optional Channel Mapping Table...               :
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>

<t>
The fields in the identification (ID) header have the following meaning:
<list style="numbers">
<t><spanx style="strong">Magic Signature</spanx>:
<vspace blankLines="1"/>
This is an 8-octet (64-bit) field that allows codec identification and is
 human-readable.
It contains, in order, the magic numbers:
<list style="empty">
<t>0x4F 'O'</t>
<t>0x70 'p'</t>
<t>0x75 'u'</t>
<t>0x73 's'</t>
<t>0x48 'H'</t>
<t>0x65 'e'</t>
<t>0x61 'a'</t>
<t>0x64 'd'</t>
</list>
Starting with "Op" helps distinguish it from audio data packets, as this is an
 invalid TOC sequence.
<vspace blankLines="1"/>
</t>
<t><spanx style="strong">Version</spanx> (8 bits, unsigned):
<vspace blankLines="1"/>
The version number MUST always be '1' for this version of the encapsulation
 specification.
Implementations SHOULD treat streams where the upper four bits of the version
 number match that of a recognized specification as backwards-compatible with
 that specification.
That is, the version number can be split into "major" and "minor" version
 sub-fields, with changes to the "minor" sub-field (in the lower four bits)
 signaling compatible changes.
For example, a decoder implementing this specification SHOULD accept any stream
 with a version number of '15' or less, and SHOULD assume any stream with a
 version number '16' or greater is incompatible.
The initial version '1' was chosen to keep implementations from relying on this
 octet as a null terminator for the "OpusHead" string.
<vspace blankLines="1"/>
</t>
<t><spanx style="strong">Output Channel Count</spanx> 'C' (8 bits, unsigned):
<vspace blankLines="1"/>
This is the number of output channels.
This might be different than the number of encoded channels, which can change
 on a packet-by-packet basis.
This value MUST NOT be zero.
The maximum allowable value depends on the channel mapping family, and might be
 as large as 255.
See <xref target="channel_mapping"/> for details.
<vspace blankLines="1"/>
</t>
<t><spanx style="strong">Pre-skip</spanx> (16 bits, unsigned, little
 endian):
<vspace blankLines="1"/>
This is the number of samples (at 48&nbsp;kHz) to discard from the decoder
 output when starting playback, and also the number to subtract from a page's
 granule position to calculate its PCM sample position.
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When cropping the beginning of existing Ogg Opus streams, a pre-skip of at
 least 3,840&nbsp;samples (80&nbsp;ms) is RECOMMENDED to ensure complete
 convergence in the decoder.
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<vspace blankLines="1"/>
</t>
<t><spanx style="strong">Input Sample Rate</spanx> (32 bits, unsigned, little
 endian):
<vspace blankLines="1"/>
This field is <spanx style="emph">not</spanx> the sample rate to use for
 playback of the encoded data.
<vspace blankLines="1"/>
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Opus can switch between internal audio bandwidths of 4, 6, 8, 12, and
 20&nbsp;kHz.
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Each packet in the stream may have a different audio bandwidth.
Regardless of the audio bandwidth, the reference decoder supports decoding any
 stream at a sample rate of 8, 12, 16, 24, or 48&nbsp;kHz.
The original sample rate of the encoder input is not preserved by the lossy
 compression.
<vspace blankLines="1"/>
An Ogg Opus player SHOULD select the playback sample rate according to the
 following procedure:
<list style="numbers">
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<t>If the hardware supports 48&nbsp;kHz playback, decode at 48&nbsp;kHz.</t>
<t>Otherwise, if the hardware's highest available sample rate is a supported
 rate, decode at this sample rate.</t>
<t>Otherwise, if the hardware's highest available sample rate is less than
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 48&nbsp;kHz, decode at the next highest supported rate above this and
 resample.</t>
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<t>Otherwise, decode at 48&nbsp;kHz and resample.</t>
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</list>
However, the 'Input Sample Rate' field allows the encoder to pass the sample
 rate of the original input stream as metadata.
This may be useful when the user requires the output sample rate to match the
 input sample rate.
For example, a non-player decoder writing PCM format samples to disk might
 choose to resample the output audio back to the original input sample rate to
 reduce surprise to the user, who might reasonably expect to get back a file
 with the same sample rate as the one they fed to the encoder.
<vspace blankLines="1"/>
A value of zero indicates 'unspecified'.
Encoders SHOULD write the actual input sample rate or zero, but decoder
 implementations which do something with this field SHOULD take care to behave
 sanely if given crazy values (e.g., do not actually upsample the output to
 10 MHz if requested).
<vspace blankLines="1"/>
</t>
<t><spanx style="strong">Output Gain</spanx> (16 bits, signed, little
 endian):
<vspace blankLines="1"/>
This is a gain to be applied by the decoder.
It is 20*log10 of the factor to scale the decoder output by to achieve the
 desired playback volume, stored in a 16-bit, signed, two's complement
 fixed-point value with 8 fractional bits (i.e., Q7.8).
<figure align="center">
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<preamble>
To apply the gain, a decoder could use
</preamble>
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<artwork align="center"><![CDATA[
sample *= pow(10, output_gain/(20.0*256)) ,
]]></artwork>
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<postamble>
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 where output_gain is the raw 16-bit value from the header.
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</postamble>
</figure>
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<vspace blankLines="1"/>
Virtually all players and media frameworks should apply it by default.
If a player chooses to apply any volume adjustment or gain modification, such
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 as the R128_TRACK_GAIN, R128_ALBUM_GAIN (see <xref target="comment_header"/>)
 or a user-facing volume knob, the adjustment MUST be applied in addition
 to this output gain in order to achieve playback at the desired volume.
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<vspace blankLines="1"/>
An encoder SHOULD set this field to zero, and instead apply any gain prior to
 encoding, when this is possible and does not conflict with the user's wishes.
The output gain should only be nonzero when the gain is adjusted after
 encoding, or when the user wishes to adjust the gain for playback while
 preserving the ability to recover the original signal amplitude.
<vspace blankLines="1"/>
Although the output gain has enormous range (+/- 128 dB, enough to amplify
 inaudible sounds to the threshold of physical pain), most applications can
 only reasonably use a small portion of this range around zero.
The large range serves in part to ensure that gain can always be losslessly
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 transferred between OpusHead and R128 gain tags (see below) without
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 saturating.
<vspace blankLines="1"/>
</t>
<t><spanx style="strong">Channel Mapping Family</spanx> (8 bits,
 unsigned):
<vspace blankLines="1"/>
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This octet indicates the order and semantic meaning of the output channels.
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<vspace blankLines="1"/>
Each possible value of this octet indicates a mapping family, which defines a
 set of allowed channel counts, and the ordered set of channel names for each
 allowed channel count.
The details are described in <xref target="channel_mapping"/>.
</t>
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<t><spanx style="strong">Channel Mapping Table</spanx>:
This table defines the mapping from encoded streams to output channels.
It is omitted when the channel mapping family is 0, but REQUIRED otherwise.
Its contents are specified in <xref target="channel_mapping"/>.
</t>
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</list>
</t>

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<t>
All fields in the ID headers are REQUIRED, except for the channel mapping
 table, which is omitted when the channel mapping family is 0.
Implementations SHOULD reject ID headers which do not contain enough data for
 these fields, even if they contain a valid Magic Signature.
Future versions of this specification, even backwards-compatible versions,
 might include additional fields in the ID header.
If an ID header has a compatible major version, but a larger minor version,
 an implementation MUST NOT reject it for containing additional data not
 specified here.
However, implementations MAY reject streams in which the ID header does not
 complete on the first page.
</t>

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<section anchor="channel_mapping" title="Channel Mapping">
<t>
An Ogg Opus stream allows mapping one number of Opus streams (N) to a possibly
 larger number of decoded channels (M+N) to yet another number of output
 channels (C), which might be larger or smaller than the number of decoded
 channels.
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The order and meaning of these channels are defined by a channel mapping,
 which consists of the 'channel mapping family' octet and, for channel mapping
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 families other than family&nbsp;0, a channel mapping table, as illustrated in
 <xref target="channel_mapping_table"/>.
</t>

<figure anchor="channel_mapping_table" title="Channel Mapping Table"
 align="center">
<artwork align="center"><![CDATA[
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                                +-+-+-+-+-+-+-+-+
                                                | Stream Count  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Coupled Count |              Channel Mapping...               :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>

<t>
The fields in the channel mapping table have the following meaning:
<list style="numbers" counter="8">
<t><spanx style="strong">Stream Count</spanx> 'N' (8 bits, unsigned):
<vspace blankLines="1"/>
This is the total number of streams encoded in each Ogg packet.
This value is required to correctly parse the packed Opus packets inside an
 Ogg packet, as described in <xref target="packet_organization"/>.
This value MUST NOT be zero, as without at least one Opus packet with a valid
 TOC sequence, a demuxer cannot recover the duration of an Ogg packet.
<vspace blankLines="1"/>
For channel mapping family&nbsp;0, this value defaults to 1, and is not coded.
<vspace blankLines="1"/>
</t>
<t><spanx style="strong">Coupled Stream Count</spanx> 'M' (8 bits, unsigned):
This is the number of streams whose decoders should be configured to produce
 two channels.
This MUST be no larger than the total number of streams, N.
<vspace blankLines="1"/>
Each packet in an Opus stream has an internal channel count of 1 or 2, which
 can change from packet to packet.
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This is selected by the encoder depending on the bitrate and the audio being
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 encoded.
The original channel count of the encoder input is not preserved by the lossy
 compression.
<vspace blankLines="1"/>
Regardless of the internal channel count, any Opus stream can be decoded as
 mono (a single channel) or stereo (two channels) by appropriate initialization
 of the decoder.
The 'coupled stream count' field indicates that the first M Opus decoders are
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 to be initialized for stereo output, and the remaining N-M decoders are to be
 initialized for mono only.
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The total number of decoded channels, (M+N), MUST be no larger than 255, as
 there is no way to index more channels than that in the channel mapping.
<vspace blankLines="1"/>
For channel mapping family&nbsp;0, this value defaults to C-1 (i.e., 0 for mono
 and 1 for stereo), and is not coded.
<vspace blankLines="1"/>
</t>
<t><spanx style="strong">Channel Mapping</spanx> (8*C bits):
This contains one octet per output channel, indicating which decoded channel
 should be used for each one.
Let 'index' be the value of this octet for a particular output channel.
This value MUST either be smaller than (M+N), or be the special value 255.
If 'index' is less than 2*M, the output MUST be taken from decoding stream
 ('index'/2) as stereo and selecting the left channel if 'index' is even, and
 the right channel if 'index' is odd.
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If 'index' is 2*M or larger, but less than 255, the output MUST be taken from
 decoding stream ('index'-M) as mono.
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If 'index' is 255, the corresponding output channel MUST contain pure silence.
<vspace blankLines="1"/>
The number of output channels, C, is not constrained to match the number of
 decoded channels (M+N).
A single index value MAY appear multiple times, i.e., the same decoded channel
 might be mapped to multiple output channels.
Some decoded channels might not be assigned to any output channel, as well.
<vspace blankLines="1"/>
For channel mapping family&nbsp;0, the first index defaults to 0, and if C==2,
 the second index defaults to 1.
Neither index is coded.
</t>
</list>
</t>

<t>
After producing the output channels, the channel mapping family determines the
 semantic meaning of each one.
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Currently there are three defined mapping families, although more may be added.
</t>

<section anchor="channel_mapping_0" title="Channel Mapping Family 0">
<t>
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Allowed numbers of channels: 1 or 2.
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RTP mapping.
</t>
<t>
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<list style="symbols">
<t>1 channel: monophonic (mono).</t>
<t>2 channels: stereo (left, right).</t>
</list>
<spanx style="strong">Special mapping</spanx>: This channel mapping value also
 indicates that the contents consists of a single Opus stream that is stereo if
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 and only if C==2, with stream index 0 mapped to output channel 0 (mono, or
 left channel) and stream index 1 mapped to output channel 1 (right channel)
 if stereo.
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When the 'channel mapping family' octet has this value, the channel mapping
 table MUST be omitted from the ID header packet.
</t>
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</section>

<section anchor="channel_mapping_1" title="Channel Mapping Family 1">
<t>
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Allowed numbers of channels: 1...8.
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Vorbis channel order.
</t>
<t>
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Each channel is assigned to a speaker location in a conventional surround
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 arrangement.
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Specific locations depend on the number of channels, and are given below
 in order of the corresponding channel indicies.
<list style="symbols">
  <t>1 channel: monophonic (mono).</t>
  <t>2 channels: stereo (left, right).</t>
  <t>3 channels: linear surround (left, center, right)</t>
  <t>4 channels: quadraphonic (front&nbsp;left, front&nbsp;right, rear&nbsp;left, rear&nbsp;right).</t>
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  <t>5 channels: 5.0 surround (front&nbsp;left, front&nbsp;center, front&nbsp;right, rear&nbsp;left, rear&nbsp;right).</t>
  <t>6 channels: 5.1 surround (front&nbsp;left, front&nbsp;center, front&nbsp;right, rear&nbsp;left, rear&nbsp;right, LFE).</t>
  <t>7 channels: 6.1 surround (front&nbsp;left, front&nbsp;center, front&nbsp;right, side&nbsp;left, side&nbsp;right, rear&nbsp;center, LFE).</t>
  <t>8 channels: 7.1 surround (front&nbsp;left, front&nbsp;center, front&nbsp;right, side&nbsp;left, side&nbsp;right, rear&nbsp;left, rear&nbsp;right, LFE)</t>
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</list>
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</t>
<t>
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This set of surround options and speaker location orderings is the same
 as those used by the Vorbis codec <xref target="vorbis-mapping"/>.
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The ordering is different from the one used by the
 WAVE <xref target="wave-multichannel"/> and
 FLAC <xref target="flac"/> formats,
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 so correct ordering requires permutation of the output channels when decoding
 to or encoding from those formats.
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'LFE' here refers to a Low Frequency Effects, often mapped to a subwoofer
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 with no particular spatial position.
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Implementations SHOULD identify 'side' or 'rear' speaker locations with
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 'surround' and 'back' as appropriate when interfacing with audio formats
 or systems which prefer that terminology.
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</t>
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</section>

<section anchor="channel_mapping_255"
 title="Channel Mapping Family 255">
<t>
Allowed numbers of channels: 1...255.
No defined channel meaning.
</t>
<t>
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Channels are unidentified.
General-purpose players SHOULD NOT attempt to play these streams, and offline
 decoders MAY deinterleave the output into separate PCM files, one per channel.
Decoders SHOULD NOT produce output for channels mapped to stream index 255
 (pure silence) unless they have no other way to indicate the index of
 non-silent channels.
</t>
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</section>

<section anchor="channel_mapping_undefined"
 title="Undefined Channel Mappings">
<t>
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The remaining channel mapping families (2...254) are reserved.
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A decoder encountering a reserved channel mapping family value SHOULD act as
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</t>
</section>

<section anchor="downmix" title="Downmixing">
<t>
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An Ogg Opus player MUST play any Ogg Opus stream with a channel mapping family
 of 0 or 1, even if the number of channels does not match the physically
 connected audio hardware.
Players SHOULD perform channel mixing to increase or reduce the number of
 channels as needed.
</t>

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<t>
Implementations MAY use the following matricies to implement downmixing from
 multichannel files using <xref target="channel_mapping_1">Channel Mapping
 Family 1</xref>, which are known to give acceptable results for stereo.
Matricies for 3 and 4 channels are normalized so each coefficent row sums
 to 1 to avoid clipping.
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For 5 or more channels they are normalized to 2 as a compromise between
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 clipping and dynamic range reduction.
</t>
<t>
In these matricies the front left and front right channels are generally
passed through directly.
When a surround channel is split between both the left and right stereo
 channels, coefficients are chosen so their squares sum to 1, which
 helps preserve the perceived intensity.
Rear channels are mixed more diffusely or attenuated to maintain focus
 on the front channels.
</t>

<figure anchor="downmix-matrix-3"
 title="Stereo downmix matrix for the linear surround channel mapping"
 align="center">
<artwork align="center"><![CDATA[
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L output = ( 0.585786 * left + 0.414214 * center                    )
R output = (                   0.414214 * center + 0.585786 * right )
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]]></artwork>
<postamble>
Exact coefficient values are 1 and 1/sqrt(2), multiplied by
 1/(1 + 1/sqrt(2)) for normalization.
</postamble>
</figure>

<figure anchor="downmix-matrix-4"
 title="Stereo downmix matrix for the quadraphonic channel mapping"
 align="center">
<artwork align="center"><![CDATA[
/          \   /                                     \ / FL \
| L output |   | 0.422650 0.000000 0.366025 0.211325 | | FR |
| R output | = | 0.000000 0.422650 0.211325 0.366025 | | RL |
\          /   \                                     / \ RR /
]]></artwork>
<postamble>
Exact coefficient values are 1, sqrt(3)/2 and 1/2, multiplied by
 1/(1&nbsp;+&nbsp;sqrt(3)/2&nbsp;+&nbsp;1/2) for normalization.
</postamble>
</figure>

<figure anchor="downmix-matrix-5"
 title="Stereo downmix matrix for the 5.0 surround mapping"
 align="center">
<artwork align="center"><![CDATA[
                                                         / FL \
/   \   /                                              \ | FC |
| L |   | 0.650802 0.460186 0.000000 0.563611 0.325401 | | FR |
| R | = | 0.000000 0.460186 0.650802 0.325401 0.563611 | | RL |
\   /   \                                              / | RR |
                                                         \    /
]]></artwork>
<postamble>
Exact coefficient values are 1, 1/sqrt(2), sqrt(3)/2 and 1/2, multiplied by
 2/(1&nbsp;+&nbsp;1/sqrt(2)&nbsp;+&nbsp;sqrt(3)/2&nbsp;+&nbsp;1/2)
 for normalization.
</postamble>
</figure>

<figure anchor="downmix-matrix-6"
 title="Stereo downmix matrix for the 5.1 surround mapping"
 align="center">
<artwork align="center"><![CDATA[
                                                                /FL \
/ \   /                                                       \ |FC |
|L|   | 0.529067 0.374107 0.000000 0.458186 0.264534 0.374107 | |FR |
|R| = | 0.000000 0.374107 0.529067 0.264534 0.458186 0.374107 | |RL |
\ /   \                                                       / |RR |
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