| title | author | date |
|---|---|---|
OPUS-A2DP-0.5 specification |
Pauli Virtanen <pav@iki.fi> |
Jun 4, 2022 |
In this file, a way to use Opus as an A2DP vendor codec is specified.
We will call this "OPUS-A2DP-0.5". There is no previous public specification for using Opus as an A2DP vendor codec (to my knowledge), which is why we need this one.
[[TOC]]
The Media Codec Specific Information Elements (AVDTP v1.3, §8.21.5) capability and configuration structure is as follows:
| Octet | Bits | Meaning |
|---|---|---|
| 0-5 | 0-7 | Vendor ID Part |
| 6-7 | 0-7 | Channel Configuration |
| 8-11 | 0-7 | Audio Location Configuration |
| 12-14 | 0-7 | Limits Configuration |
| 15-16 | 0-7 | Return Direction Channel Configuration |
| 17-20 | 0-7 | Return Direction Audio Location Configuration |
| 21-23 | 0-7 | Return Direction Limits Configuration |
All integer fields and multi-byte bitfields are laid out in little endian order. All integer fields are unsigned.
Each entry may have different meaning when present as a capability. Below, we indicate this by abbreviations CAP for capability and SEL for the value selected by SRC.
Bits in fields marked RFA (Reserved For Additions) shall be set to zero.
Note
See
a2dp-codec-caps.hfor definition as C structs.
The fixed value
| Octet | Bits | Meaning |
|---|---|---|
| 0-3 | 0-7 | A2DP Vendor ID (0x05F1) |
| 4-5 | 0-7 | A2DP Vendor Codec ID (0x1005) |
Note
The Vendor ID is that of the Linux Foundation, and we are using it here unofficially.
The channel configuration consists of the channel count, and the count of coupled streams. The latter indicates which channels are encoded as left/right pairs, as defined in Sec. 5.1.1 of Opus Ogg Encapsulation RFC7845.
| Octet | Bits | Meaning |
|---|---|---|
| 6 | 0-7 | Channel Count. CAP: maximum number supported. SEL: actual. |
| 7 | 0-7 | Coupled Stream Count. CAP: 0. SEL: actual. |
The Channel Count indicates the number of logical channels encoded in the data stream.
The Coupled Stream Count indicates the number of streams that encode a
coupled (left & right) channel pair. The count shall satisfy
(Channel Count) >= 2*(Coupled Stream Count).
The Stream Count is (Channel Count) - (Coupled Stream Count).
The logical Channels are identified by a Channel Index j such that 0 <= j < (Channel Count). The channels 0 <= j < 2*(Coupled Stream Count)
are encoded in the k-th stream of the payload, where k = floor(j/2) and
j mod 2 determines which of the two channels of the stream the logical
channel is. The channels 2*(Coupled Stream Count) <= j < (Channel Count)
are encoded in the k-th stream of the payload, where k = j - (Coupled Stream Count).
Note
The prescription here is identical to RFC7845 with channel mapping
mapping[j] = j. We do not want to include the mapping table in the A2DP capabilities, so it is assumed to be trivial.
The semantic meaning for each channel is determined by their Audio Location bitfield.
| Octet | Bits | Meaning |
|---|---|---|
| 8-11 | 0-7 | Audio Location bitfield. CAP: available. SEL: actual |
The values specified in CAP are informative, and SEL may contain bits that were not set in CAP. SNK shall handle unsupported audio locations. It may do this for example by ignoring unsupported channels or via suitable up/downmixing. Hence, SRC may transmit channels with audio locations that are not marked supported by SNK.
The audio location bit values are:
| Channel Order | Bitmask | Audio Location |
|---|---|---|
| 0 | 0x00000001 | Front Left |
| 1 | 0x00000002 | Front Right |
| 2 | 0x00000400 | Side Left |
| 3 | 0x00000800 | Side Right |
| 4 | 0x00000010 | Back Left |
| 5 | 0x00000020 | Back Right |
| 6 | 0x00000040 | Front Left of Center |
| 7 | 0x00000080 | Front Right of Center |
| 8 | 0x00001000 | Top Front Left |
| 9 | 0x00002000 | Top Front Right |
| 10 | 0x00040000 | Top Side Left |
| 11 | 0x00080000 | Top Side Right |
| 12 | 0x00010000 | Top Back Left |
| 13 | 0x00020000 | Top Back Right |
| 14 | 0x00400000 | Bottom Front Left |
| 15 | 0x00800000 | Bottom Front Right |
| 16 | 0x01000000 | Front Left Wide |
| 17 | 0x02000000 | Front Right Wide |
| 18 | 0x04000000 | Left Surround |
| 19 | 0x08000000 | Right Surround |
| 20 | 0x00000004 | Front Center |
| 21 | 0x00000100 | Back Center |
| 22 | 0x00004000 | Top Front Center |
| 23 | 0x00008000 | Top Center |
| 24 | 0x00100000 | Top Back Center |
| 25 | 0x00200000 | Bottom Front Center |
| 26 | 0x00000008 | Low Frequency Effects 1 |
| 27 | 0x00000200 | Low Frequency Effects 2 |
| 28 | 0x10000000 | RFA |
| 29 | 0x20000000 | RFA |
| 30 | 0x40000000 | RFA |
| 31 | 0x80000000 | RFA |
Each bit value is associated with a Channel Order. The bits set in the bitfield define audio locations for the streams present in the payload. The set bit with the smallest Channel Order value defines the audio location for the Channel Index j=0, the bit with the next lowest Channel Order value defines the audio location for the Channel Index j=1, and so forth.
When the Channel Count is larger than the number of bits set in the Audio Location bitfield, the audio locations of the remaining channels are unspecified. Implementations may handle them as appropriate for their use case, considering them as AUX0–AUXN, or in the case of Channel Count = 1, as the single mono audio channel.
When the Channel Count is smaller than the number of bits set in the Audio Location bitfield, the audio locations for the channels are assigned as above, and remaining excess bits shall be ignored.
Note
The channel audio location specification is similar to the location bitfield of the
Audio_Channel_AllocationLTV structure in Bluetooth SIG Assigned Numbers, Generic Audio used in the LE Audio, and the bitmasks defined above are the same.The channel ordering differs from LE Audio, and is defined here to be compatible with the internal stream ordering in the reference Opus Multistream surround encoder Mapping Family 0 and 1 output. This allows making use of its surround masking and LFE handling capabilities. The stream ordering of the reference Opus surround encoder, although being unchanged since its addition in 2013, is an internal detail of the encoder. Implementations using the surround encoder need to check that the mapping table used by the encoder corresponds to the above channel ordering.
For reference, we list the Audio Location bitfield values corresponding to the different channel counts in Opus Mapping Family 0 and 1 surround encoder output, and the expected mapping table:
Mapping Family Channel Count Audio Location Value Stream Ordering Mapping Table 0 1 0x00000000 mono {0} 0 2 0x00000003 FL, FR {0, 1} 1 1 0x00000000 mono {0} 1 2 0x00000003 FL, FR {0, 1} 1 3 0x00000007 FL, FR, FC {0, 2, 1} 1 4 0x00000033 FL, FR, BL, BR {0, 1, 2, 3} 1 5 0x00000037 FL, FR, BL, BR, FC {0, 4, 1, 2, 3} 1 6 0x0000003f FL, FR, BL, BR, FC, LFE {0, 4, 1, 2, 3, 5} 1 7 0x00000d0f FL, FR, SL, SR, FC, BC, LFE {0, 4, 1, 2, 3, 5, 6} 1 8 0x00000c3f FL, FR, SL, SR, BL, BR, FC, LFE {0, 6, 1, 2, 3, 4, 5, 7} The Mapping Table in the table indicates the mapping table selected by
opus_multistream_surround_encoder_create(Opus 1.3.1). If the encoder outputs a different mapping table in a future Opus encoder release, the channel ordering will be incorrect, and the surround encoder can not be used. We expect that the probability of the Opus encoder authors making such changes is negligible.
The limits for allowed frame durations and maximum bitrate can also be configured.
| Octet | Bits | Meaning |
|---|---|---|
| 16 | 0 | Frame duration 2.5ms. CAP: supported, SEL: selected |
| 16 | 1 | Frame duration 5ms. CAP: supported, SEL: selected |
| 16 | 2 | Frame duration 10ms. CAP: supported, SEL: selected |
| 16 | 3 | Frame duration 20ms. CAP: supported, SEL: selected |
| 16 | 4 | Frame duration 40ms. CAP: supported, SEL: selected |
| 16 | 5-7 | RFA |
| Octet | Bits | Meaning |
|---|---|---|
| 17-18 | 0-7 | Maximum bitrate. CAP: supported, SEL: selected |
The maximum bitrate is given in units of 1024 bits per second.
The maximum bitrate field in CAP may contain value 0 to indicate everything is supported.
Bidirectional audio may be supported. Its Channel Configuration, Audio Location Configuration, and Limits Configuration have identical form to the forward direction, and represented by exactly similar structures.
Namely:
| Octet | Bits | Meaning |
|---|---|---|
| 19-20 | 0-7 | Channel Configuration fields, for return direction |
| 21-28 | 0-7 | Audio Location fields, for return direction |
| 29-31 | 0-7 | Limits Configuration fields, for return direction |
If no return channel is supported or selected, the number of channels is set to 0 in CAP or SEL.
Note
This is a nonstandard extension to A2DP. The return direction audio data is simply sent back via the underlying L2CAP connection, which is bidirectional, in the same format as the forward direction audio. This is similar to what aptX-LL and FastStream do.
Each packet consists of an RTP header, an RTP payload header, and a payload containing Opus Multistream data.
| Octet | Bits | Meaning |
|---|---|---|
| 0-11 | 0-7 | RTP header |
| 12 | 0-7 | RTP payload header |
| 13-N | 0-7 | Opus Multistream payload |
For each Bluetooth packet, the payload shall contain exactly one Opus Multistream packet, or a fragment of one. The Opus Multistream packet may be fragmented to several consecutive Bluetooth packets.
The format of the Multistream data is the same as in the audio packets of RFC7845, or, as produced/consumed by the Opus Multistream API.
Note
We DO NOT follow RFC7587, as we want fragmentation and multichannel support.
See RFC3550.
The RTP payload type is pt=96 (dynamic).
The RTP payload header is used to indicate if and how the Opus Multistream packet is fragmented across several consecutive Bluetooth packets.
| Octet | Bits | Meaning |
|---|---|---|
| 0 | 0-3 | Frame Count |
| 4 | 4 | RFA |
| 4 | 5 | Is Last Fragment |
| 4 | 6 | Is First Fragment |
| 4 | 7 | Is Fragmented |
In each packet, Frame Count indicates how many Bluetooth packets are still to be received (including the present packet) before the Opus Multistream packet is complete.
The Is Fragment flag indicates whether the present packet contains fragmented payload.
The Is Last Fragment flag indicates whether the present packet is the last part of fragmented payload.
The Is First Fragment flag indicates whether the present packet is the first part of fragmented payload.
In non-fragmented packets, Frame Count shall be (1), and the other bits in the header zero.
The Opus payload is a single Opus Multistream packet, or its fragment.
In case of fragmentation, as indicated by the RTP payload header, concatenating the payloads of the fragment Bluetooth packets shall yield the total Opus Multistream packet.
The SRC should choose encoder parameters such that Bluetooth bandwidth limitations are not exceeded.
The SRC may include FEC data. The SNK may enable forward error correction instead of PLC.
- Bluetooth AVDTP v1.3
- IETF RFC3550
- IETF RFC7587
- IETF RFC7845
- Bluetooth Assigned Numbers, Generic Audio