| codec | T. Terriberry |
| Internet-Draft | Mozilla Corporation |
| Updates: 5334 (if approved) | R. Lee |
| Intended status: Standards Track | Voicetronix |
| Expires: July 31, 2016 | R. Giles |
| Mozilla Corporation | |
| January 28, 2016 |
Ogg Encapsulation for the Opus Audio Codec
draft-ietf-codec-oggopus-11
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.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 31, 2016.
Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.
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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]. 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.
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 can 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 demultiplexer (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 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].
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].
An Ogg Opus stream is organized as follows.
There are two mandatory header packets. 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 is placed alone (without any other packet data) on the first page of the logical Ogg bitstream, and completes on that page. This page has 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 multiple 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. An implementation of this specification SHOULD treat any Opus packet whose duration is different from that of the first Opus packet in an Ogg packet as if it were a malformed Opus packet with an invalid Table Of Contents (TOC) sequence.
The TOC sequence at the beginning of each Opus packet indicates the coding mode, audio bandwidth, channel count, duration (frame size), and number of frames per packet, as described in Section 3.1 of [RFC6716]. The coding mode is one of SILK, Hybrid, or Constrained Energy Lapped Transform (CELT). The combination of coding mode, audio bandwidth, and frame size is referred to as the configuration of an Opus packet.
Packets are placed into Ogg pages in order until the end of stream. Audio data packets might span page boundaries. The first audio data page could have the 'continued packet' flag set (indicating the first audio data packet is continued from a previous page) if, for example, it was a live stream joined mid-broadcast, with the headers pasted on the front. A demuxer SHOULD NOT attempt to decode the data for the first packet on a page with the 'continued packet' flag set if the previous page with packet data does not end in a continued packet (i.e., did not end with a lacing value of 255) or if the page sequence numbers are not consecutive, unless the demuxer has some special knowledge that would allow it to interpret this data despite the missing pieces. An implementation MUST treat a zero-octet audio data packet as if it were a malformed Opus packet as described in Section 3.4 of [RFC6716].
A logical stream ends with a page with the 'end of stream' flag set, but implementations need to be prepared to deal with truncated streams that do not have a page marked 'end of stream'. There is no reason for the final packet on the last page to be a continued packet, i.e., for the final lacing value to be 255. However, demuxers might encounter such streams, possibly as the result of a transfer that did not complete or of corruption. A demuxer SHOULD NOT attempt to decode the data from a packet that continues onto a subsequent page (i.e., when the page ends with a lacing value of 255) if the next page with packet data does not have the 'continued packet' flag set or does not exist, or if the page sequence numbers are not consecutive, unless the demuxer has some special knowledge that would allow it to interpret this data despite the missing pieces. There MUST NOT be any more pages in an Opus logical bitstream after a page marked 'end of stream'.
The granule position MUST be zero for the ID header page and the page where the comment header completes. That is, the first page in the logical stream, and the last header page before the first audio data page both have a granule position of zero.
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. The granule position of the first audio data page will usually be larger than zero, as described in Section 4.5.
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 is 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 all of them evenly divide 48 kHz. Therefore, 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 as defined in [RFC6716] can 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.
In order to support capturing a real-time stream that has lost or not transmitted packets, a multiplexer (muxer) SHOULD emit packets that explicitly request the use of Packet Loss Concealment (PLC) in place of the missing packets. Implementations that fail to do so still MUST NOT increment the granule position for a page by anything other than the number of samples contained in packets that actually complete on that page.
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 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 sequences 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 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 needed 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.
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 might have introduced additional latency through its own resampling and analysis (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 player 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 that SHOULD be skipped (decoded but discarded) at the beginning of the stream, though some specific applications might have a reason for looking at that data. 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. These samples are not valid audio.
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.
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 samples (80 ms) provides sufficient history to the decoder that it will have converged before the stream's output begins.
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.
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 are 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 might 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.
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.
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.
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 are non-negative. An implementation MUST treat any stream as invalid if 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 treat any stream as invalid if its granule position is smaller than the 'pre-skip' amount. This would indicate that there are more samples to 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.
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 in just one or two bisections on average, even in multi-gigabyte files. See [seeking] for an example of general implementation guidance.
When seeking within an Ogg Opus stream, an implementation 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, an implementation SHOULD start 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 samples to reach the seek target (if any).
An Ogg Opus logical stream contains exactly two mandatory header packets: an identification header and a comment 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
sample *= pow(10, output_gain/(20.0*256)) ,
The fields in the identification (ID) header have the following meaning:
Starting with "Op" helps distinguish it from audio data packets, as this is an invalid TOC sequence.
However, the 'Input Sample Rate' field allows the muxer to pass the sample rate of the original input stream as metadata. This is useful when the user requires the output sample rate to match the input sample rate. For example, when not playing the output, an implementation writing PCM format samples to disk might choose to resample the 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.
All fields in the ID headers are REQUIRED, except for the channel mapping table, which MUST be omitted when the channel mapping family is 0, but is REQUIRED otherwise. Implementations SHOULD treat a stream as invalid if it contains an ID header that does not have enough data for these fields, even if it 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 treat it as invalid for containing additional data not specified here, provided it still completes on the first page.
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:
After producing the output channels, the channel mapping family determines the semantic meaning of each one. There are three defined mapping families in this specification.
Allowed numbers of channels: 1 or 2. RTP mapping. This is the same channel interpretation as [RFC7587].
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.
Allowed numbers of channels: 1...8. Vorbis channel order (see below).
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 indices.
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 Free Lossless Audio Codec (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 channel, 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.
Allowed numbers of channels: 1...255. No defined channel meaning.
Channels are unidentified. General-purpose players SHOULD NOT attempt to play these streams. Offline implementations MAY deinterleave the output into separate PCM files, one per channel. Implementations 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.
The remaining channel mapping families (2...254) are reserved. A demuxer implementation encountering a reserved channel mapping family value SHOULD act as though the value is 255.
An Ogg Opus player MUST support any valid channel mapping 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 matrices to implement downmixing from multichannel files using Channel Mapping Family 1 [channel_mapping_1], which are known to give acceptable results for stereo. Matrices for 3 and 4 channels are normalized so each coefficient 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.
In these matrices 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
/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 coefficients are in the same order as in Section 5.1.1.2, and the matrices 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 coefficients are in the same order as in Section 5.1.1.2, and the matrices above.
Figure 8: Stereo downmix matrix for the 7.1 surround mapping
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.
Starting with "Op" helps distinguish it from audio data packets, as this is an invalid TOC sequence.
The vendor string length and user comment list length are REQUIRED, and implementations SHOULD treat a stream as invalid if it contains a comment header that does not have enough data for these fields, or that does 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.
Immediately following the user comment list, the comment header MAY contain zero-padding or other binary data which is not specified here. If the least-significant bit of the first byte of this data is 1, then editors SHOULD preserve the contents of this data when updating the tags, but if this bit is 0, all such data MAY be treated as padding, and truncated or discarded as desired. This allows informal experimentation with the format of this binary data until it can be specified later.
The comment header can be arbitrarily large and might be spread over a large number of Ogg pages. Implementations MUST avoid attempting to allocate excessive amounts of memory when presented with a very large comment header. To accomplish this, implementations MAY treat a stream as invalid if it has a comment header larger than 125,829,120 octets, and MAY ignore individual comments that are not fully contained within the first 61,440 octets of the comment header.
The user comment strings follow the NAME=value format described by [vorbis-comment] with the same recommended tag names: ARTIST, TITLE, DATE, ALBUM, and so on.
Two new comment tags are introduced here:
First, an optional gain for track normalization:
R128_TRACK_GAIN=-573
representing the volume shift needed to normalize the track's volume during isolated playback, in random shuffle, and so on. 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.
Second, an optional gain for album normalization:
R128_ALBUM_GAIN=111
representing the volume shift needed to normalize the overall volume when played as part of a particular collection of tracks. The gain is also a Q7.8 fixed point number in dB, as in the ID header's 'output gain' field.
An Ogg Opus stream MUST NOT have more than one of each of these tags, and if present their values MUST be an integer from -32768 to 32767, inclusive, represented in ASCII as a base 10 number with no whitespace. A leading '+' or '-' character is valid. Leading zeros are also permitted, but the value MUST be represented by no more than 6 characters. Other non-digit characters MUST NOT be present.
If present, R128_TRACK_GAIN and R128_ALBUM_GAIN 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 or the R128_ALBUM_GAIN tag, it MUST apply those gains in addition to the 'output gain' value. If a tool modifies the ID header's 'output gain' field, it MUST also update or remove the R128_TRACK_GAIN and R128_ALBUM_GAIN comment tags if present. A muxer SHOULD place the gain it wants other tools to use by default into the 'output gain' field, and not the 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, unless they are only to be used in some context where there is guaranteed to be no such confusion. [EBU-R128] normalization is preferred to the earlier REPLAYGAIN schemes because of its clear definition and adoption by industry. Peak normalizations are difficult to calculate reliably for lossy codecs because of variation in excursion heights due to decoder differences. In the authors' investigations they were not applied consistently or broadly enough to merit inclusion here.
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. When encoding, implementations SHOULD limit the use of padding in audio data packets to no more than is necessary to make a variable bitrate (VBR) stream constant bitrate (CBR), unless they have no reasonable way to determine what is necessary. Demuxers SHOULD treat audio data packets as invalid (treat them as if they were malformed Opus packets with an invalid TOC sequence) if they are larger than 61,440 octets per Opus stream, unless they have a specific reason for allowing extra padding. Such packets necessarily contain more padding than needed to make a stream CBR. Demuxers MUST avoid attempting to allocate excessive amounts of memory when presented with a very large packet. Demuxers MAY treat audio data packets as invalid or partially process them if they are larger than 61,440 octets in an Ogg Opus stream with channel mapping families 0 or 1. Demuxers MAY treat audio data packets as invalid or partially process them in any Ogg Opus stream if the packet is larger than 61,440 octets and also larger than 7,680 octets per Opus stream. The presence of an extremely large packet in the stream could indicate a memory exhaustion attack or stream corruption.
In an Ogg Opus stream, the largest possible valid packet that does not use padding has a size of (61,298*N - 2) octets. With 255 streams, this is 15,630,988 octets 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. 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. 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). For channel mapping family 1, N=8 provides a reasonable upper bound, as it allows for each of the 8 possible output channels to be decoded from a separate stereo Opus stream. This gives a size of 61,310 octets, which is rounded up to a multiple of 1,024 octets to yield the audio data packet size of 61,440 octets that any implementation is expected to be able to process successfully.
When encoding Opus streams, Ogg muxers SHOULD take into account the algorithmic delay of the Opus encoder.
In encoders derived from the reference implementation [RFC6716], 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, implementations MAY use linear predictive coding (LPC) extrapolation to generate at least 120 extra samples at the beginning to avoid the Opus encoder having to encode a discontinuous signal. For more information on linear prediction, see [linear-prediction]. For an input file containing 'length' samples, the implementation SHOULD set the pre-skip header value to (delay_samples + extra_samples), encode at least (length + delay_samples + extra_samples) samples, and set the granule position 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.
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 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].
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. A muxer MAY avoid this discontinuity by using the following procedure:
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));
For best results, this implementation requires that prediction be explicitly enabled again before resuming normal encoding, even after a reset.
A brief summary of major implementations of this draft is available at <https://wiki.xiph.org/OggOpusImplementation>, along with their status.
[Note to RFC Editor: please remove this entire section before final publication per [RFC6982], along with its references.]
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. Robustness against malicious payloads is extremely important. Malicious payloads MUST NOT cause an implementation to overrun its allocated memory or to take an excessive amount of resources to decode. Although problems in encoding applications are typically rarer, the same applies to the muxer. Malicious audio input streams MUST NOT cause an implementation to overrun its allocated memory or consume excessive resources because this would allow an attacker to attack transcoding gateways.
Like most other container formats, Ogg Opus streams 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.
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] and [RFC5334], 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.
In either case, this document updates [RFC5334] to add 'opus' as a codecs parameter value with char[8]: 'OpusHead' as Codec Identifier.
This document updates the IANA Media Types registry to add .opus as a file extension for "audio/ogg", and to add itself as a reference alongside [RFC5334] for "audio/ogg", "video/ogg", and "application/ogg" Media Types.
This document defines a new registry "Opus Channel Mapping Families" to indicate how the semantic meanings of the channels in a multi-channel Opus stream are described. IANA is requested to create a new name space of "Opus Channel Mapping Families". This will be a new registry on the IANA Matrix, and not a subregistry of an existing registry. Modifications to this registry follow the "Specification Required with Expert Review" registration policy as defined in [RFC5226]. Each registry entry consists of a Channel Mapping Family Number, which is specified in decimal in the range 0 to 255, inclusive, and a Reference (or list of references) Each Reference must point to sufficient documentation to describe what information is coded in the Opus identification header for this channel mapping family, how a demuxer determines the Stream Count ('N') and Coupled Stream Count ('M') from this information, and how it determines the proper interpretation of each of the decoded channels.
This document defines three initial assignments for this registry.
| Value | Reference |
|---|---|
| 0 | [RFCXXXX] Section 5.1.1.1 |
| 1 | [RFCXXXX] Section 5.1.1.2 |
| 255 | [RFCXXXX] Section 5.1.1.3 |
The designated expert will determine if the Reference points to a specification that meets the requirements for permanence and ready availability laid out in [RFC5226] and that it specifies the information described above with sufficient clarity to allow interoperable implementations.
Thanks to Ben Campbell, Mark Harris, Greg Maxwell, Christopher "Monty" Montgomery, Jean-Marc Valin, and Mo Zanaty for their valuable contributions to this document. Additional thanks to Andrew D'Addesio, Greg Maxwell, and Vincent Penquerc'h for their feedback based on early implementations.
In Section 11, "RFCXXXX" is to be replaced with the RFC number assigned to this draft.
In the Copyright Notice at the start of the document, the following paragraph is to be appended after the regular copyright notice text:
"The licenses granted by the IETF Trust to this RFC under Section 3.c of the Trust Legal Provisions shall also include the right to extract text from Sections 1 through 14 of this RFC and create derivative works from these extracts, and to copy, publish, display, and distribute such derivative works in any medium and for any purpose, provided that no such derivative work shall be presented, displayed, or published in a manner that states or implies that it is part of this RFC or any other IETF Document."