[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-terriberry-oggopus) 00 01 02 03

codec                                                      T. Terriberry
Internet-Draft                                       Mozilla Corporation
Intended status: Standards Track                                  R. Lee
Expires: August 11, 2014                                     Voicetronix
                                                                R. Giles
                                                     Mozilla Corporation
                                                        February 7, 2014


               Ogg Encapsulation for the Opus Audio Codec
                      draft-ietf-codec-oggopus-03

Abstract

   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.  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.  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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 11, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




Terriberry, et al.       Expires August 11, 2014                [Page 1]

Internet-Draft                  Ogg Opus                   February 2014


   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Packet Organization . . . . . . . . . . . . . . . . . . . . .   3
   4.  Granule Position  . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Repairing Gaps in Real-time Streams . . . . . . . . . . .   5
     4.2.  Pre-skip  . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  PCM Sample Position . . . . . . . . . . . . . . . . . . .   7
     4.4.  End Trimming  . . . . . . . . . . . . . . . . . . . . . .   8
     4.5.  Restrictions on the Initial Granule Position  . . . . . .   8
     4.6.  Seeking and Pre-roll  . . . . . . . . . . . . . . . . . .   9
   5.  Header Packets  . . . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Identification Header . . . . . . . . . . . . . . . . . .  10
       5.1.1.  Channel Mapping . . . . . . . . . . . . . . . . . . .  14
     5.2.  Comment Header  . . . . . . . . . . . . . . . . . . . . .  19
   6.  Packet Size Limits  . . . . . . . . . . . . . . . . . . . . .  23
   7.  Encoder Guidelines  . . . . . . . . . . . . . . . . . . . . .  24
     7.1.  LPC Extrapolation . . . . . . . . . . . . . . . . . . . .  24
     7.2.  Continuous Chaining . . . . . . . . . . . . . . . . . . .  25
   8.  Implementation Status . . . . . . . . . . . . . . . . . . . .  25
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  26
   10. Content Type  . . . . . . . . . . . . . . . . . . . . . . . .  26
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  27
   13. Copying Conditions  . . . . . . . . . . . . . . . . . . . . .  27
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  27
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  27
     14.2.  Informative References . . . . . . . . . . . . . . . . .  28
     14.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

1.  Introduction

   The IETF Opus codec is a low-latency audio codec optimized for both
   voice and general-purpose audio.  See [RFC6716] for technical
   details.  This document defines the encapsulation of Opus in a
   continuous, logical Ogg bitstream [RFC3533].



Terriberry, et al.       Expires August 11, 2014                [Page 2]

Internet-Draft                  Ogg Opus                   February 2014


   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.

   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
   [RFC3533].

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

   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".

3.  Packet Organization

   An Opus stream is organized as follows.

   There are two mandatory header packets.  The granule position of the
   pages on which these packets complete MUST be zero.

   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 Section 5.1.  It
   MUST be placed alone (without any other packet data) on the first



Terriberry, et al.       Expires August 11, 2014                [Page 3]

Internet-Draft                  Ogg Opus                   February 2014


   page of the logical Ogg bitstream, and must complete on that page.
   This page MUST have its 'beginning of stream' flag set.

   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 Section 5.2.  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.

   All subsequent pages are audio data pages, and the Ogg packets they
   contain are 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 one for multichannel
   audio.  The value N is specified in the ID header (see
   Section 5.1.1), and is fixed over the entire length of the logical
   Ogg bitstream.

   The first N-1 Opus packets, if any, are packed one after another into
   the Ogg packet, using the self-delimiting framing from Appendix B of
   [RFC6716].  The remaining Opus packet is packed at the end of the Ogg
   packet using the regular, undelimited framing from Section 3 of
   [RFC6716].  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.

   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 in Section 3.1 of [RFC6716].  The
   combination of mode, audio bandwidth, and frame size is referred to
   as the configuration of an Opus packet.

   The first audio data page SHOULD NOT have the 'continued packet' flag
   set (which would indicate the first audio data packet is continued
   from a previous page).  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'.




Terriberry, et al.       Expires August 11, 2014                [Page 4]

Internet-Draft                  Ogg Opus                   February 2014


4.  Granule Position

   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.

   The granule position of an audio data page is in units of PCM audio
   samples at a fixed rate of 48 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 kHz decoding rate, and the rest of this
   specification makes the same assumption.

   The duration of an Opus packet may be any multiple of 2.5 ms, up to a
   maximum of 120 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 ms packet fed to a decoder running at
   48 kHz will always return 960 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.

   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.

4.1.  Repairing Gaps in Real-time Streams

   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.  Only gaps that are a multiple of 2.5 ms are
   repairable, as these are the only durations that can be created by
   packet loss or discontinuous transmission.  Muxers need not handle
   other gap sizes.  Creating the necessary packets involves
   synthesizing a TOC byte (defined in Section 3.1 of [RFC6716])--and
   whatever additional internal framing is needed--to indicate the



Terriberry, et al.       Expires August 11, 2014                [Page 5]

Internet-Draft                  Ogg Opus                   February 2014


   packet duration for each stream.  The actual length of each missing
   Opus frame inside the packet is zero bytes, as defined in
   Section 3.2.1 of [RFC6716].

   Zero-byte frames MAY be packed into packets using any of codes 0, 1,
   2, or 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 2 or
   VBR code 3).

   [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.  Synthesized TOC bytes SHOULD maintain the same mode,
   audio bandwidth, channel count, and frame size as the previous packet
   (if any).  This is the simplest and usually the most well-tested case
   for the PLC to handle and it covers all losses that do not include a
   configuration switch, as defined in Section 4.5 of [RFC6716].

   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.  Such changes
   SHOULD be delayed as long as possible to simplify things for PLC
   implementations.

   As an example, a 95 ms gap could be encoded as nineteen 5 ms frames
   in two bytes with a single CBR code 3 packet.  If the previous frame
   size was 20 ms, using four 20 ms frames followed by three 5 ms frames
   requires 4 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 kbps, so the impact on file size
   is minimal.

   Changing modes is discouraged, since this causes some decoder
   implementations to reset their PLC state.  However, SILK and Hybrid
   mode frames cannot fill gaps that are not a multiple of 10 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.

   In the example above, if the previous frame was a 20 ms SILK mode
   frame, the better solution is to synthesize a packet describing four
   20 ms SILK frames, followed by a packet with a single 10 ms SILK
   frame, and finally a packet with a 5 ms CELT frame, to fill the 95 ms



Terriberry, et al.       Expires August 11, 2014                [Page 6]

Internet-Draft                  Ogg Opus                   February 2014


   gap.  This also requires four bytes to describe the synthesized
   packet data (two 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 solution.

   Since medium-band audio is an option only in the SILK mode, wideband
   frames 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.

4.2.  Pre-skip

   There is some amount of latency introduced during the decoding
   process, to allow for overlap in the CELT mode, stereo mixing in the
   SILK mode, and resampling.  The encoder will also introduce latency
   (though the exact amount is not specified).  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.

   A 'pre-skip' field in the ID header (see Section 5.1) signals the
   number of samples which SHOULD be skipped (decoded but discarded) at
   the beginning of the stream.  This provides sufficient history to the
   decoder so that it has already converged before the stream's output
   begins.  It may also be used to perform sample-accurate cropping of
   existing encoded streams.  This amount need not be a multiple of
   2.5 ms, may be smaller than a single packet, or may span the contents
   of several packets.

4.3.  PCM Sample Position

   The PCM sample position is determined from the granule position using
   the formula

         'PCM sample position' = 'granule position' - 'pre-skip' .

   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.






Terriberry, et al.       Expires August 11, 2014                [Page 7]

Internet-Draft                  Ogg Opus                   February 2014


   This can be converted into a playback time using the formula

                                   'PCM sample position'
                 'playback time' = --------------------- .
                                          48000.0

   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 _after_ 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.

   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 must be trimmed from the output (see
   [vorbis-trim]).  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 large packets in streams with a very
   large number of channels might not fit on a single page.

4.4.  End Trimming

   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.

4.5.  Restrictions on the Initial Granule Position

   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



Terriberry, et al.       Expires August 11, 2014                [Page 8]

Internet-Draft                  Ogg Opus                   February 2014


   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.

   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.

   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.  This would indicate that more samples should be
   skipped from the initial decoded output than exist in the stream.  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.

4.6.  Seeking and Pre-roll

   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 [seeking] for general
   implementation guidance.

   When seeking within an Ogg Opus stream, the decoder SHOULD start
   decoding (and discarding the output) at least 3840 samples (80 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 ms prior to the seek target comes
   before the initial PCM sample position, the decoder SHOULD start



Terriberry, et al.       Expires August 11, 2014                [Page 9]

Internet-Draft                  Ogg Opus                   February 2014


   decoding from the beginning of the stream, applying pre-skip as
   normal, regardless of whether the pre-skip is larger or smaller than
   80 ms, and then continue to discard the samples required to reach the
   seek target (if any).

5.  Header Packets

   An Opus stream contains exactly two mandatory header packets: an
   identification header and a comment header.

5.1.  Identification Header

      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...               :
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 1: ID Header Packet

   The fields in the identification (ID) header have the following
   meaning:

   1.  *Magic Signature*:

       This is an 8-octet (64-bit) field that allows codec
       identification and is human-readable.  It contains, in order, the
       magic numbers:

          0x4F 'O'

          0x70 'p'

          0x75 'u'

          0x73 's'



Terriberry, et al.       Expires August 11, 2014               [Page 10]

Internet-Draft                  Ogg Opus                   February 2014


          0x48 'H'

          0x65 'e'

          0x61 'a'

          0x64 'd'

       Starting with "Op" helps distinguish it from audio data packets,
       as this is an invalid TOC sequence.



   2.  *Version* (8 bits, unsigned):

       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.



   3.  *Output Channel Count* 'C' (8 bits, unsigned):

       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 Section 5.1.1 for details.



   4.  *Pre-skip* (16 bits, unsigned, little endian):

       This is the number of samples (at 48 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.  When cropping the beginning of existing Ogg




Terriberry, et al.       Expires August 11, 2014               [Page 11]

Internet-Draft                  Ogg Opus                   February 2014


       Opus streams, a pre-skip of at least 3,840 samples (80 ms) is
       RECOMMENDED to ensure complete convergence in the decoder.



   5.  *Input Sample Rate* (32 bits, unsigned, little endian):

       This field is _not_ the sample rate to use for playback of the
       encoded data.

       Opus can switch between internal audio bandwidths of 4, 6, 8, 12,
       and 20 kHz.  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 kHz.  The original sample rate of the encoder input
       is not preserved by the lossy compression.

       An Ogg Opus player SHOULD select the playback sample rate
       according to the following procedure:

       1.  If the hardware supports 48 kHz playback, decode at 48 kHz.

       2.  Otherwise, if the hardware's highest available sample rate is
           a supported rate, decode at this sample rate.

       3.  Otherwise, if the hardware's highest available sample rate is
           less than 48 kHz, decode at the next highest supported rate
           above this and resample.

       4.  Otherwise, decode at 48 kHz and resample.

       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.

       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).





Terriberry, et al.       Expires August 11, 2014               [Page 12]

Internet-Draft                  Ogg Opus                   February 2014


   6.  *Output Gain* (16 bits, signed, little endian):

       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).

       To apply the gain, a decoder could use

                sample *= pow(10, output_gain/(20.0*256)) ,

       where output_gain is the raw 16-bit value from the header.

       Virtually all players and media frameworks should apply it by
       default.  If a player chooses to apply any volume adjustment or
       gain modification, such as the R128_TRACK_GAIN (see Section 5.2)
       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.

       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.

       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 transferred between OpusHead and
       R128_TRACK_GAIN (see below) without saturating.



   7.  *Channel Mapping Family* (8 bits, unsigned):

       This octet indicates the order and semantic meaning of the
       various channels encoded in each Ogg packet.

       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 Section 5.1.1.

   8.  *Channel Mapping Table*: This table defines the mapping from
       encoded streams to output channels.  It is omitted when the



Terriberry, et al.       Expires August 11, 2014               [Page 13]

Internet-Draft                  Ogg Opus                   February 2014


       channel mapping family is 0, but REQUIRED otherwise.  Its
       contents are specified in Section 5.1.1.

   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.

5.1.1.  Channel Mapping

   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.  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 families
   other than family 0, a channel mapping table, as illustrated in
   Figure 2.

      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...               :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 2: Channel Mapping Table

   The fields in the channel mapping table have the following meaning:

   1.  *Stream Count* 'N' (8 bits, unsigned):

       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 Section 3.  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.

       For channel mapping family 0, this value defaults to 1, and is
       not coded.




Terriberry, et al.       Expires August 11, 2014               [Page 14]

Internet-Draft                  Ogg Opus                   February 2014


   2.  *Coupled Stream Count* '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.

       Each packet in an Opus stream has an internal channel count of 1
       or 2, which can change from packet to packet.  This is selected
       by the encoder depending on the bitrate and the audio being
       encoded.  The original channel count of the encoder input is not
       preserved by the lossy compression.

       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 to be
       initialized for stereo output, and the remaining N-M decoders are
       to be initialized for mono only.  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.

       For channel mapping family 0, this value defaults to C-1 (i.e., 0
       for mono and 1 for stereo), and is not coded.



   3.  *Channel Mapping* (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.  If 'index' is 2*M or larger, the
       output MUST be taken from decoding stream ('index'-M) as mono.
       If 'index' is 255, the corresponding output channel MUST contain
       pure silence.

       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.

       For channel mapping family 0, the first index defaults to 0, and
       if C==2, the second index defaults to 1.  Neither index is coded.






Terriberry, et al.       Expires August 11, 2014               [Page 15]

Internet-Draft                  Ogg Opus                   February 2014


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

5.1.1.1.  Channel Mapping Family 0

   Allowed numbers of channels: 1 or 2.  RTP mapping.

   o  1 channel: monophonic (mono).

   o  2 channels: stereo (left, right).

   *Special mapping*: This channel mapping value also indicates that the
   contents consists of a single Opus stream that is stereo if 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.  When the 'channel mapping family' octet has this
   value, the channel mapping table MUST be omitted from the ID header
   packet.

5.1.1.2.  Channel Mapping Family 1

   Allowed numbers of channels: 1...8.  Vorbis channel order.

   Each channel is assigned to a speaker location in a conventional
   surround arrangement.  Specific locations depend on the number of
   channels, and are given below in order of the corresponding channel
   indicies.

   o  1 channel: monophonic (mono).

   o  2 channels: stereo (left, right).

   o  3 channels: linear surround (left, center, right)

   o  4 channels: quadraphonic (front left, front right, rear left,
      rear right).

   o  5 channels: 5.0 surround (front left, front center, front right,
      rear left, rear right).

   o  6 channels: 5.1 surround (front left, front center, front right,
      rear left, rear right, LFE).

   o  7 channels: 6.1 surround (front left, front center, front right,
      side left, side right, rear center, LFE).





Terriberry, et al.       Expires August 11, 2014               [Page 16]

Internet-Draft                  Ogg Opus                   February 2014


   o  8 channels: 7.1 surround (front left, front center, front right,
      side left, side right, rear left, rear right, LFE)

   This set of surround options and speaker location orderings is the
   same as those used by the Vorbis codec [vorbis-mapping].  The
   ordering is different from the one used by the WAVE
   [wave-multichannel] and FLAC [flac] formats, so correct ordering
   requires permutation of the output channels when decoding to or
   encoding from those formats.  'LFE' here refers to a Low Frequency
   Effects, often mapped to a subwoofer with no particular spatial
   position.  Implementations SHOULD identify 'side' or 'rear' speaker
   locations with 'surround' and 'back' as appropriate when interfacing
   with audio formats or systems which prefer that terminology.

5.1.1.3.  Channel Mapping Family 255

   Allowed numbers of channels: 1...255.  No defined channel meaning.

   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.

5.1.1.4.  Undefined Channel Mappings

   The remaining channel mapping families (2...254) are reserved.  A
   decoder encountering a reserved channel mapping family value SHOULD
   act as though the value is 255.

5.1.1.5.  Downmixing

   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.

   Implementations MAY use the following matricies to implement
   downmixing from multichannel files using Channel Mapping Family 1
   (Section 5.1.1.2), 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.  For 5 or more channels
   they are normalized to 2 as a compromise between clipping and dynamic
   range reduction.





Terriberry, et al.       Expires August 11, 2014               [Page 17]

Internet-Draft                  Ogg Opus                   February 2014


   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.

   L output = ( 0.585786 * left + 0.414214 * center                    )
   R output = (                   0.414214 * center + 0.585786 * right )

   Exact coefficient values are 1 and 1/sqrt(2), multiplied by 1/(1 + 1/
                        sqrt(2)) for normalization.

      Figure 3: Stereo downmix matrix for the linear surround channel
                                  mapping

       /          \   /                                     \ / FL \
       | L output |   | 0.422650 0.000000 0.366025 0.211325 | | FR |
       | R output | = | 0.000000 0.422650 0.211325 0.366025 | | RL |
       \          /   \                                     / \ RR /

    Exact coefficient values are 1, sqrt(3)/2 and 1/2, multiplied by 1/
                 (1 + sqrt(3)/2 + 1/2) for normalization.

   Figure 4: Stereo downmix matrix for the quadraphonic channel mapping

                                                               / 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 |
                                                               \    /

       Exact coefficient values are 1, 1/sqrt(2), sqrt(3)/2 and 1/2,
   multiplied by 2/(1 + 1/sqrt(2) + sqrt(3)/2 + 1/2) for normalization.

       Figure 5: Stereo downmix matrix for the 5.0 surround mapping














Terriberry, et al.       Expires August 11, 2014               [Page 18]

Internet-Draft                  Ogg Opus                   February 2014


                                                                   /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 |
                                                                   \LFE/

       Exact coefficient values are 1, 1/sqrt(2), sqrt(3)/2 and 1/2,
     multiplied by 2/(1 + 1/sqrt(2) + sqrt(3)/2 + 1/2 + 1/sqrt(2)) for
                              normalization.

       Figure 6: Stereo downmix matrix for the 5.1 surround mapping

     /                                                                \
     | 0.455310 0.321953 0.000000 0.394310 0.227655 0.278819 0.321953 |
     | 0.000000 0.321953 0.455310 0.227655 0.394310 0.278819 0.321953 |
     \                                                                /

   Exact coefficient values are 1, 1/sqrt(2), sqrt(3)/2, 1/2 and sqrt(3)
      /2/sqrt(2), multiplied by 2/(1 + 1/sqrt(2) + sqrt(3)/2 + 1/2 +
   sqrt(3)/2/sqrt(2) + 1/sqrt(2)) for normalization.  The coeffients are
     in the same order as in Section 5.1.1.2, and the matricies above.

       Figure 7: Stereo downmix matrix for the 6.1 surround mapping

    /                                                                 \
    | .388631 .274804 .000000 .336565 .194316 .336565 .194316 .274804 |
    | .000000 .274804 .388631 .194316 .336565 .194316 .336565 .274804 |
    \                                                                 /

       Exact coefficient values are 1, 1/sqrt(2), sqrt(3)/2 and 1/2,
     multiplied by 2/(2 + 2/sqrt(2) + sqrt(3)) for normalization.  The
      coeffients are in the same order as in Section 5.1.1.2, and the
                             matricies above.

       Figure 8: Stereo downmix matrix for the 7.1 surround mapping

5.2.  Comment Header













Terriberry, et al.       Expires August 11, 2014               [Page 19]

Internet-Draft                  Ogg Opus                   February 2014


      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'      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      'T'      |      'a'      |      'g'      |      's'      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Vendor String Length                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     :                        Vendor String...                       :
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   User Comment List Length                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 User Comment #0 String Length                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     :                   User Comment #0 String...                   :
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 User Comment #1 String Length                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                                                               :

                      Figure 9: Comment Header Packet

   The comment header consists of a 64-bit magic signature, followed by
   data in the same format as the [vorbis-comment] header used in Ogg
   Vorbis, except (like Ogg Theora and Speex) the final "framing bit"
   specified in the Vorbis spec is not present.

   1.  *Magic Signature*:

       This is an 8-octet (64-bit) field that allows codec
       identification and is human-readable.  It contains, in order, the
       magic numbers:

          0x4F 'O'

          0x70 'p'

          0x75 'u'

          0x73 's'

          0x54 'T'




Terriberry, et al.       Expires August 11, 2014               [Page 20]

Internet-Draft                  Ogg Opus                   February 2014


          0x61 'a'

          0x67 'g'

          0x73 's'

       Starting with "Op" helps distinguish it from audio data packets,
       as this is an invalid TOC sequence.



   2.  *Vendor String Length* (32 bits, unsigned, little endian):

       This field gives the length of the following vendor string, in
       octets.  It MUST NOT indicate that the vendor string is longer
       than the rest of the packet.



   3.  *Vendor String* (variable length, UTF-8 vector):

       This is a simple human-readable tag for vendor information,
       encoded as a UTF-8 string [RFC3629].  No terminating null octet
       is required.

       This tag is intended to identify the codec encoder and
       encapsulation implementations, for tracing differences in
       technical behavior.  User-facing encoding applications can use
       the 'ENCODER' user comment tag to identify themselves.



   4.  *User Comment List Length* (32 bits, unsigned, little endian):

       This field indicates the number of user-supplied comments.  It
       MAY indicate there are zero user-supplied comments, in which case
       there are no additional fields in the packet.  It MUST NOT
       indicate that there are so many comments that the comment string
       lengths would require more data than is available in the rest of
       the packet.



   5.  *User Comment #i String Length* (32 bits, unsigned, little
       endian):

       This field gives the length of the following user comment string,
       in octets.  There is one for each user comment indicated by the



Terriberry, et al.       Expires August 11, 2014               [Page 21]

Internet-Draft                  Ogg Opus                   February 2014


       'user comment list length' field.  It MUST NOT indicate that the
       string is longer than the rest of the packet.



   6.  *User Comment #i String* (variable length, UTF-8 vector):

       This field contains a single user comment string.  There is one
       for each user comment indicated by the 'user comment list length'
       field.

   The vendor string length and user comment list length are REQUIRED,
   and implementations SHOULD reject comment headers that do not contain
   enough data for these fields, or that do not contain enough data for
   the corresponding vendor string or user comments they describe.
   Making this check before allocating the associated memory to contain
   the data helps prevent a possible Denial-of-Service (DoS) attack from
   small comment headers that claim to contain strings longer than the
   entire packet or more user comments than than could possibly fit in
   the packet.

   The user comment strings follow the NAME=value format described by
   [vorbis-comment] with the same recommended tag names.

   One new comment tag is introduced for Ogg Opus:

   R128_TRACK_GAIN=-573

   representing the volume shift needed to normalize the track's volume.
   The gain is a Q7.8 fixed point number in dB, as in the ID header's
   'output gain' field.

   This tag is similar to the REPLAYGAIN_TRACK_GAIN tag in
   Vorbis [replay-gain], except that the normal volume reference is the
   [EBU-R128] standard.

   An Ogg Opus file MUST NOT have more than one such tag, and if present
   its value MUST be an integer from -32768 to 32767, inclusive,
   represented in ASCII with no whitespace.  If present, it MUST
   correctly represent the R128 normalization gain relative to the
   'output gain' field specified in the ID header.  If a player chooses
   to make use of the R128_TRACK_GAIN tag, it MUST be applied _in
   addition_ to the 'output gain' value.  If an encoder wishes to use
   R128 normalization, and the output gain is not otherwise constrained
   or specified, the encoder SHOULD write the R128 gain into the 'output
   gain' field and store a tag containing "R128_TRACK_GAIN=0".  That is,
   it should assume that by default tools will respect the 'output gain'
   field, and not the comment tag.  If a tool modifies the ID header's



Terriberry, et al.       Expires August 11, 2014               [Page 22]

Internet-Draft                  Ogg Opus                   February 2014


   'output gain' field, it MUST also update or remove the
   R128_TRACK_GAIN comment tag.

   To avoid confusion with multiple normalization schemes, an Opus
   comment header SHOULD NOT contain any of the REPLAYGAIN_TRACK_GAIN,
   REPLAYGAIN_TRACK_PEAK, REPLAYGAIN_ALBUM_GAIN, or
   REPLAYGAIN_ALBUM_PEAK tags.

   There is no Opus comment tag corresponding to REPLAYGAIN_ALBUM_GAIN.
   That information should instead be stored in the ID header's 'output
   gain' field.

6.  Packet Size Limits

   Technically valid Opus packets can be arbitrarily large due to the
   padding format, although the amount of non-padding data they can
   contain is bounded.  These packets might be spread over a similarly
   enormous number of Ogg pages.  Encoders SHOULD use no more padding
   than required to make a variable bitrate (VBR) stream constant
   bitrate (CBR).  Decoders SHOULD avoid attempting to allocate
   excessive amounts of memory when presented with a very large packet.
   The presence of an extremely large packet in the stream could
   indicate a memory exhaustion attack or stream corruption.  Decoders
   SHOULD reject a packet that is too large to process, and display a
   warning message.

   In an Ogg Opus stream, the largest possible valid packet that does
   not use padding has a size of (61,298*N - 2) octets, or about 60 kB
   per Opus stream.  With 255 streams, this is 15,630,988 octets
   (14.9 MB) and can span up to 61,298 Ogg pages, all but one of which
   will have a granule position of -1.  This is of course a very extreme
   packet, consisting of 255 streams, each containing 120 ms of audio
   encoded as 2.5 ms frames, each frame using the maximum possible
   number of octets (1275) and stored in the least efficient manner
   allowed (a VBR code 3 Opus packet).  Even in such a packet, most of
   the data will be zeros as 2.5 ms frames cannot actually use all
   1275 octets.  The largest packet consisting of entirely useful data
   is (15,326*N - 2) octets, or about 15 kB per stream.  This
   corresponds to 120 ms of audio encoded as 10 ms frames in either SILK
   or Hybrid mode, but at a data rate of over 1 Mbps, which makes little
   sense for the quality achieved.  A more reasonable limit is
   (7,664*N - 2) octets, or about 7.5 kB per stream.  This corresponds
   to 120 ms of audio encoded as 20 ms stereo CELT mode frames, with a
   total bitrate just under 511 kbps (not counting the Ogg encapsulation
   overhead).  With N=8, the maximum number of channels currently
   defined by mapping family 1, this gives a maximum packet size of
   61,310 octets, or just under 60 kB.  This is still quite
   conservative, as it assumes each output channel is taken from one



Terriberry, et al.       Expires August 11, 2014               [Page 23]

Internet-Draft                  Ogg Opus                   February 2014


   decoded channel of a stereo packet.  An implementation could
   reasonably choose any of these numbers for its internal limits.

7.  Encoder Guidelines

   When encoding Opus files, Ogg encoders should take into account the
   algorithmic delay of the Opus encoder.

   In encoders derived from the reference implementation, the number of
   samples can be queried with:

    opus_encoder_ctl(encoder_state, OPUS_GET_LOOKAHEAD, &delay_samples);

   To achieve good quality in the very first samples of a stream, the
   Ogg encoder MAY use linear predictive coding (LPC) extrapolation
   [linear-prediction] to generate at least 120 extra samples at the
   beginning to avoid the Opus encoder having to encode a discontinuous
   signal.  For an input file containing 'length' samples, the Ogg
   encoder SHOULD set the pre-skip header value to
   delay_samples+extra_samples, encode at least
   length+delay_samples+extra_samples samples, and set the granulepos of
   the last page to length+delay_samples+extra_samples.  This ensures
   that the encoded file has the same duration as the original, with no
   time offset.  The best way to pad the end of the stream is to also
   use LPC extrapolation, but zero-padding is also acceptable.

7.1.  LPC Extrapolation

   The first step in LPC extrapolation is to compute linear prediction
   coefficients. [lpc-sample] When extending the end of the signal,
   order-N (typically with N ranging from 8 to 40) LPC analysis is
   performed on a window near the end of the signal.  The last N samples
   are used as memory to an infinite impulse response (IIR) filter.

   The filter is then applied on a zero input to extrapolate the end of
   the signal.  Let a(k) be the kth LPC coefficient and x(n) be the nth
   sample of the signal, each new sample past the end of the signal is
   computed as:

                                  N
                                 ---
                          x(n) = \   a(k)*x(n-k)
                                 /
                                 ---
                                 k=1

   The process is repeated independently for each channel.  It is
   possible to extend the beginning of the signal by applying the same



Terriberry, et al.       Expires August 11, 2014               [Page 24]

Internet-Draft                  Ogg Opus                   February 2014


   process backward in time.  When extending the beginning of the
   signal, it is best to apply a "fade in" to the extrapolated signal,
   e.g. by multiplying it by a half-Hanning window [hanning].

7.2.  Continuous Chaining

   In some applications, such as Internet radio, it is desirable to cut
   a long stream into smaller chains, e.g. so the comment header can be
   updated.  This can be done simply by separating the input streams
   into segments and encoding each segment independently.  The drawback
   of this approach is that it creates a small discontinuity at the
   boundary due to the lossy nature of Opus.  An encoder MAY avoid this
   discontinuity by using the following procedure:

   1.  Encode the last frame of the first segment as an independent
       frame by turning off all forms of inter-frame prediction.  De-
       emphasis is allowed.

   2.  Set the granulepos of the last page to a point near the end of
       the last frame.

   3.  Begin the second segment with a copy of the last frame of the
       first segment.

   4.  Set the pre-skip value of the second stream in such a way as to
       properly join the two streams.

   5.  Continue the encoding process normally from there, without any
       reset to the encoder.

   In encoders derived from the reference implementation, inter-frame
   prediction can be turned off by calling:

     opus_encoder_ctl(encoder_state, OPUS_SET_PREDICTION_DISABLED, 1);

   Prediction should be enabled again before resuming normal encoding,
   even after a reset.

8.  Implementation Status

   A brief summary of major implementations of this draft is available
   at [1], along with their status.

   [Note to RFC Editor: please remove this entire section before final
   publication per [RFC6982].]






Terriberry, et al.       Expires August 11, 2014               [Page 25]

Internet-Draft                  Ogg Opus                   February 2014


9.  Security Considerations

   Implementations of the Opus codec need to take appropriate security
   considerations into account, as outlined in [RFC4732].  This is just
   as much a problem for the container as it is for the codec itself.
   It is extremely important for the decoder to be robust against
   malicious payloads.  Malicious payloads must not cause the decoder to
   overrun its allocated memory or to take an excessive amount of
   resources to decode.  Although problems in encoders are typically
   rarer, the same applies to the encoder.  Malicious audio streams must
   not cause the encoder to misbehave because this would allow an
   attacker to attack transcoding gateways.

   Like most other container formats, Ogg Opus files should not be used
   with insecure ciphers or cipher modes that are vulnerable to known-
   plaintext attacks.  Elements such as the Ogg page capture pattern and
   the magic signatures in the ID header and the comment header all have
   easily predictable values, in addition to various elements of the
   codec data itself.

10.  Content Type

   An "Ogg Opus file" consists of one or more sequentially multiplexed
   segments, each containing exactly one Ogg Opus stream.  The
   RECOMMENDED mime-type for Ogg Opus files is "audio/ogg".

   If more specificity is desired, one MAY indicate the presence of Opus
   streams using the codecs parameter defined in [RFC6381], e.g.,

                            audio/ogg; codecs=opus

   for an Ogg Opus file.

   The RECOMMENDED filename extension for Ogg Opus files is '.opus'.

   When Opus is concurrently multiplexed with other streams in an Ogg
   container, one SHOULD use one of the "audio/ogg", "video/ogg", or
   "application/ogg" mime-types, as defined in [RFC5334].  Such streams
   are not strictly "Ogg Opus files" as described above, since they
   contain more than a single Opus stream per sequentially multiplexed
   segment, e.g. video or multiple audio tracks.  In such cases the the
   '.opus' filename extension is NOT RECOMMENDED.

11.  IANA Considerations

   This document has no actions for IANA.





Terriberry, et al.       Expires August 11, 2014               [Page 26]

Internet-Draft                  Ogg Opus                   February 2014


12.  Acknowledgments

   Thanks to Greg Maxwell, Christopher "Monty" Montgomery, and Jean-Marc
   Valin for their valuable contributions to this document.  Additional
   thanks to Andrew D'Addesio, Greg Maxwell, and Vincent Penqeurc'h for
   their feedback based on early implementations.

13.  Copying Conditions

   The authors agree to grant third parties the irrevocable right to
   copy, use, and distribute the work, with or without modification, in
   any medium, without royalty, provided that, unless separate
   permission is granted, redistributed modified works do not contain
   misleading author, version, name of work, or endorsement information.

14.  References

14.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3533]  Pfeiffer, S., "The Ogg Encapsulation Format Version 0",
              RFC 3533, May 2003.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC5334]  Goncalves, I., Pfeiffer, S., and C. Montgomery, "Ogg Media
              Types", RFC 5334, September 2008.

   [RFC6381]  Gellens, R., Singer, D., and P. Frojdh, "The 'Codecs' and
              'Profiles' Parameters for "Bucket" Media Types", RFC 6381,
              August 2011.

   [RFC6716]  Valin, JM., Vos, K., and T. Terriberry, "Definition of the
              Opus Audio Codec", RFC 6716, September 2012.

   [EBU-R128]
              EBU Technical Committee, "Loudness Recommendation EBU
              R128", August 2011, <https://tech.ebu.ch/loudness>.

   [vorbis-comment]
              Montgomery, C., "Ogg Vorbis I Format Specification:
              Comment Field and Header Specification", July 2002,
              <https://www.xiph.org/vorbis/doc/v-comment.html>.





Terriberry, et al.       Expires August 11, 2014               [Page 27]

Internet-Draft                  Ogg Opus                   February 2014


14.2.  Informative References

   [RFC4732]  Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
              Service Considerations", RFC 4732, December 2006.

   [RFC6982]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", RFC 6982, July
              2013.

   [flac]     Coalson, J., "FLAC - Free Lossless Audio Codec Format
              Description", January 2008, <https://xiph.org/flac/
              format.html>.

   [hanning]  Wikipedia, "Hann window", May 2013, <https://
              en.wikipedia.org/wiki/
              Hamming_function#Hann_.28Hanning.29_window>.

   [linear-prediction]
              Wikipedia, "Linear Predictive Coding", January 2014,
              <https://en.wikipedia.org/wiki/Linear_predictive_coding>.

   [lpc-sample]
              Degener, J. and C. Bormann, "Autocorrelation LPC coeff
              generation algorithm (Vorbis source code)", November 1994,
              <https://svn.xiph.org/trunk/vorbis/lib/lpc.c>.

   [replay-gain]
              Parker, C. and M. Leese, "VorbisComment: Replay Gain",
              June 2009, <https://wiki.xiph.org/
              VorbisComment#Replay_Gain>.

   [seeking]  Pfeiffer, S., Parker, C., and G. Maxwell, "Granulepos
              Encoding and How Seeking Really Works", May 2012, <https:/
              /wiki.xiph.org/Seeking>.

   [vorbis-mapping]
              Montgomery, C., "The Vorbis I Specification, Section 4.3.9
              Output Channel Order", January 2010, <https://www.xiph.org
              /vorbis/doc/Vorbis_I_spec.html#x1-800004.3.9>.

   [vorbis-trim]
              Montgomery, C., "The Vorbis I Specification, Appendix A:
              Embedding Vorbis into an Ogg stream", November 2008,
              <https://xiph.org/vorbis/doc/
              Vorbis_I_spec.html#x1-130000A.2>.






Terriberry, et al.       Expires August 11, 2014               [Page 28]

Internet-Draft                  Ogg Opus                   February 2014


   [wave-multichannel]
              Microsoft Corporation, "Multiple Channel Audio Data and
              WAVE Files", March 2007, <http://msdn.microsoft.com/en-us/
              windows/hardware/gg463006.aspx>.

14.3.  URIs

   [1] https://wiki.xiph.org/OggOpusImplementation

Authors' Addresses

   Timothy B. Terriberry
   Mozilla Corporation
   650 Castro Street
   Mountain View, CA  94041
   USA

   Phone: +1 650 903-0800
   Email: tterribe@xiph.org


   Ron Lee
   Voicetronix
   246 Pulteney Street, Level 1
   Adelaide, SA  5000
   Australia

   Phone: +61 8 8232 9112
   Email: ron@debian.org


   Ralph Giles
   Mozilla Corporation
   163 West Hastings Street
   Vancouver, BC  V6B 1H5
   Canada

   Phone: +1 778 785 1540
   Email: giles@xiph.org












Terriberry, et al.       Expires August 11, 2014               [Page 29]


Html markup produced by rfcmarkup 1.107, available from http://tools.ietf.org/tools/rfcmarkup/