Opus audio codec (RFC 6716): API and operations manual
1.2.1

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Typedefs
Opus Multistream API

The multistream API allows individual Opus streams to be combined into a single packet, enabling support for up to 255 channels. More...

## Typedefs

typedef struct OpusMSEncoder | OpusMSEncoder
| Opus multistream encoder state. More...
typedef struct OpusMSDecoder | OpusMSDecoder
| Opus multistream decoder state. More...

## Multistream encoder functions

opus_int32 | opus_multistream_encoder_get_size (int streams, int coupled_streams)
| Gets the size of an OpusMSEncoder structure. More...
opus_int32 | opus_multistream_surround_encoder_get_size (int channels, int mapping_family)
OpusMSEncoder * | opus_multistream_encoder_create (opus_int32 Fs, int channels, int streams, int coupled_streams, const unsigned char *mapping, int application, int *error)
| Allocates and initializes a multistream encoder state. More...
OpusMSEncoder * | opus_multistream_surround_encoder_create (opus_int32 Fs, int channels, int mapping_family, int *streams, int *coupled_streams, unsigned char *mapping, int application, int *error)
int | opus_multistream_encoder_init (OpusMSEncoder *st, opus_int32 Fs, int channels, int streams, int coupled_streams, const unsigned char *mapping, int application)
| Initialize a previously allocated multistream encoder state. More...
int | opus_multistream_surround_encoder_init (OpusMSEncoder *st, opus_int32 Fs, int channels, int mapping_family, int *streams, int *coupled_streams, unsigned char *mapping, int application)
int | opus_multistream_encode (OpusMSEncoder *st, const opus_int16 *pcm, int frame_size, unsigned char *data, opus_int32 max_data_bytes)
| Encodes a multistream Opus frame. More...
int | opus_multistream_encode_float (OpusMSEncoder *st, const float *pcm, int frame_size, unsigned char *data, opus_int32 max_data_bytes)
| Encodes a multistream Opus frame from floating point input. More...
void | opus_multistream_encoder_destroy (OpusMSEncoder *st)
| Frees an `OpusMSEncoder` allocated by opus_multistream_encoder_create(). More...
int | opus_multistream_encoder_ctl (OpusMSEncoder *st, int request,...)
| Perform a CTL function on a multistream Opus encoder. More...

## Multistream decoder functions

opus_int32 | opus_multistream_decoder_get_size (int streams, int coupled_streams)
| Gets the size of an `OpusMSDecoder` structure. More...
OpusMSDecoder * | opus_multistream_decoder_create (opus_int32 Fs, int channels, int streams, int coupled_streams, const unsigned char *mapping, int *error)
| Allocates and initializes a multistream decoder state. More...
int | opus_multistream_decoder_init (OpusMSDecoder *st, opus_int32 Fs, int channels, int streams, int coupled_streams, const unsigned char *mapping)
| Intialize a previously allocated decoder state object. More...
int | opus_multistream_decode (OpusMSDecoder *st, const unsigned char *data, opus_int32 len, opus_int16 *pcm, int frame_size, int decode_fec)
| Decode a multistream Opus packet. More...
int | opus_multistream_decode_float (OpusMSDecoder *st, const unsigned char *data, opus_int32 len, float *pcm, int frame_size, int decode_fec)
| Decode a multistream Opus packet with floating point output. More...
int | opus_multistream_decoder_ctl (OpusMSDecoder *st, int request,...)
| Perform a CTL function on a multistream Opus decoder. More...
void | opus_multistream_decoder_destroy (OpusMSDecoder *st)
| Frees an `OpusMSDecoder` allocated by opus_multistream_decoder_create(). More...

## Detailed Description

The multistream API allows individual Opus streams to be combined into a single packet, enabling support for up to 255 channels.

Unlike an elementary Opus stream, the encoder and decoder must negotiate the channel configuration before the decoder can successfully interpret the data in the packets produced by the encoder. Some basic information, such as packet duration, can be computed without any special negotiation.

The format for multistream Opus packets is defined in RFC 7845 and is based on the self-delimited Opus framing described in Appendix B of RFC 6716. Normal Opus packets are just a degenerate case of multistream Opus packets, and can be encoded or decoded with the multistream API by setting `streams` to `1` when initializing the encoder or decoder.

Multistream Opus streams can contain up to 255 elementary Opus streams. These may be either "uncoupled" or "coupled", indicating that the decoder is configured to decode them to either 1 or 2 channels, respectively. The streams are ordered so that all coupled streams appear at the beginning.

A `mapping` table defines which decoded channel `i` should be used for each input/output (I/O) channel `j`. This table is typically provided as an unsigned char array. Let `i = mapping[j]` be the index for I/O channel `j`. If `i < 2*coupled_streams`, then I/O channel `j` is encoded as the left channel of stream `(i/2)` if `i` is even, or as the right channel of stream `(i/2)` if `i` is odd. Otherwise, I/O channel `j` is encoded as mono in stream `(i - coupled_streams)`, unless it has the special value 255, in which case it is omitted from the encoding entirely (the decoder will reproduce it as silence). Each value `i` must either be the special value 255 or be less than `streams + coupled_streams`.

The output channels specified by the encoder should use the Vorbis channel ordering. A decoder may wish to apply an additional permutation to the mapping the encoder used to achieve a different output channel order (e.g. for outputing in WAV order).

Each multistream packet contains an Opus packet for each stream, and all of the Opus packets in a single multistream packet must have the same duration. Therefore the duration of a multistream packet can be extracted from the TOC sequence of the first stream, which is located at the beginning of the packet, just like an elementary Opus stream:

int nb_samples;
int nb_frames;
nb_frames = opus_packet_get_nb_frames(data, len);
if (nb_frames < 1)
return nb_frames;
nb_samples = opus_packet_get_samples_per_frame(data, 48000) * nb_frames;

The general encoding and decoding process proceeds exactly the same as in the normal Opus Encoder and Opus Decoder APIs. See their documentation for an overview of how to use the corresponding multistream functions.

## Typedef Documentation

typedef struct OpusMSDecoder OpusMSDecoder

Opus multistream decoder state.

This contains the complete state of a multistream Opus decoder. It is position independent and can be freely copied.

See also
  opus_multistream_decoder_create
  opus_multistream_decoder_init

typedef struct OpusMSEncoder OpusMSEncoder

Opus multistream encoder state.

This contains the complete state of a multistream Opus encoder. It is position independent and can be freely copied.

See also
  opus_multistream_encoder_create
  opus_multistream_encoder_init

## Function Documentation

int opus_multistream_decode | ( | OpusMSDecoder * | st,
|  | const unsigned char * | data,
|  | opus_int32 | len,
|  | opus_int16 * | pcm,
|  | int | frame_size,
|  | int | decode_fec
| ) |  |

Decode a multistream Opus packet.

Parameters

  | st | `OpusMSDecoder*`: Multistream decoder state.
  [in] | data | `const unsigned char*`: Input payload. Use a `NULL` pointer to indicate packet loss.
  | len | `opus_int32`: Number of bytes in payload.
  [out] | pcm | `opus_int16*`: Output signal, with interleaved samples. This must contain room for `frame_size*channels` samples.

  frame_size

  `int`: The number of samples per channel of available space in pcm. If this is less than the maximum packet duration (120 ms; 5760 for 48kHz), this function will not be capable of decoding some packets. In the case of PLC (data==NULL) or FEC (decode_fec=1), then frame_size needs to be exactly the duration of audio that is missing, otherwise the decoder will not be in the optimal state to decode the next incoming packet. For the PLC and FEC cases, frame_size must be a multiple of 2.5 ms.

  | decode_fec | `int`: Flag (0 or 1) to request that any in-band forward error correction data be decoded. If no such data is available, the frame is decoded as if it were lost.

Returns
  Number of samples decoded on success or a negative error code (see Error codes) on failure.

int opus_multistream_decode_float | ( | OpusMSDecoder * | st,
|  | const unsigned char * | data,
|  | opus_int32 | len,
|  | float * | pcm,
|  | int | frame_size,
|  | int | decode_fec
| ) |  |

Decode a multistream Opus packet with floating point output.

Parameters

  | st | `OpusMSDecoder*`: Multistream decoder state.
  [in] | data | `const unsigned char*`: Input payload. Use a `NULL` pointer to indicate packet loss.
  | len | `opus_int32`: Number of bytes in payload.
  [out] | pcm | `opus_int16*`: Output signal, with interleaved samples. This must contain room for `frame_size*channels` samples.

  frame_size

  `int`: The number of samples per channel of available space in pcm. If this is less than the maximum packet duration (120 ms; 5760 for 48kHz), this function will not be capable of decoding some packets. In the case of PLC (data==NULL) or FEC (decode_fec=1), then frame_size needs to be exactly the duration of audio that is missing, otherwise the decoder will not be in the optimal state to decode the next incoming packet. For the PLC and FEC cases, frame_size must be a multiple of 2.5 ms.

  | decode_fec | `int`: Flag (0 or 1) to request that any in-band forward error correction data be decoded. If no such data is available, the frame is decoded as if it were lost.

Returns
  Number of samples decoded on success or a negative error code (see Error codes) on failure.

OpusMSDecoder* opus_multistream_decoder_create | ( | opus_int32 | Fs,
|  | int | channels,
|  | int | streams,
|  | int | coupled_streams,
|  | const unsigned char * | mapping,
|  | int * | error
| ) |  |

Allocates and initializes a multistream decoder state.

Call opus_multistream_decoder_destroy() to release this object when finished.

Parameters

  | Fs | `opus_int32`: Sampling rate to decode at (in Hz). This must be one of 8000, 12000, 16000, 24000, or 48000.
  | channels | `int`: Number of channels to output. This must be at most 255. It may be different from the number of coded channels (`streams + coupled_streams`).
  | streams | `int`: The total number of streams coded in the input. This must be no more than 255.
  | coupled_streams | `int`: Number of streams to decode as coupled (2 channel) streams. This must be no larger than the total number of streams. Additionally, The total number of coded channels (`streams + coupled_streams`) must be no more than 255.
  [in] | mapping | `const unsigned char[channels]`: Mapping from coded channels to output channels, as described in Opus Multistream API.
  [out] | error | `int *`: Returns OPUS_OK on success, or an error code (see Error codes) on failure.

int opus_multistream_decoder_ctl | ( | OpusMSDecoder * | st,
|  | int | request,
|  |  | ...
| ) |  |

Perform a CTL function on a multistream Opus decoder.

Generally the request and subsequent arguments are generated by a convenience macro.

Parameters

  st | `OpusMSDecoder*`: Multistream decoder state.
  request | This and all remaining parameters should be replaced by one of the convenience macros in Generic CTLs, Decoder related CTLs, or Multistream specific encoder and decoder CTLs.

See also
  Generic CTLs
  Decoder related CTLs
  Multistream specific encoder and decoder CTLs

void opus_multistream_decoder_destroy | ( | OpusMSDecoder * | st | ) |

Frees an `OpusMSDecoder` allocated by opus_multistream_decoder_create().

Parameters

  st | `OpusMSDecoder`: Multistream decoder state to be freed.

opus_int32 opus_multistream_decoder_get_size | ( | int | streams,
|  | int | coupled_streams
| ) |  |

Gets the size of an `OpusMSDecoder` structure.

Parameters

  streams | `int`: The total number of streams coded in the input. This must be no more than 255.
  coupled_streams | `int`: Number streams to decode as coupled (2 channel) streams. This must be no larger than the total number of streams. Additionally, The total number of coded channels (`streams + coupled_streams`) must be no more than 255.

Returns
  The size in bytes on success, or a negative error code (see Error codes) on error.

int opus_multistream_decoder_init | ( | OpusMSDecoder * | st,
|  | opus_int32 | Fs,
|  | int | channels,
|  | int | streams,
|  | int | coupled_streams,
|  | const unsigned char * | mapping
| ) |  |

Intialize a previously allocated decoder state object.

The memory pointed to by st must be at least the size returned by opus_multistream_encoder_get_size(). This is intended for applications which use their own allocator instead of malloc. To reset a previously initialized state, use the OPUS_RESET_STATE CTL.

See also
  opus_multistream_decoder_create
  opus_multistream_deocder_get_size

Parameters

  | st | `OpusMSEncoder*`: Multistream encoder state to initialize.
  | Fs | `opus_int32`: Sampling rate to decode at (in Hz). This must be one of 8000, 12000, 16000, 24000, or 48000.
  | channels | `int`: Number of channels to output. This must be at most 255. It may be different from the number of coded channels (`streams + coupled_streams`).
  | streams | `int`: The total number of streams coded in the input. This must be no more than 255.
  | coupled_streams | `int`: Number of streams to decode as coupled (2 channel) streams. This must be no larger than the total number of streams. Additionally, The total number of coded channels (`streams + coupled_streams`) must be no more than 255.
  [in] | mapping | `const unsigned char[channels]`: Mapping from coded channels to output channels, as described in Opus Multistream API.

Returns
  OPUS_OK on success, or an error code (see Error codes) on failure.

int opus_multistream_encode | ( | OpusMSEncoder * | st,
|  | const opus_int16 * | pcm,
|  | int | frame_size,
|  | unsigned char * | data,
|  | opus_int32 | max_data_bytes
| ) |  |

Encodes a multistream Opus frame.

Parameters

  | st | `OpusMSEncoder*`: Multistream encoder state.
  [in] | pcm | `const opus_int16*`: The input signal as interleaved samples. This must contain `frame_size*channels` samples.
  | frame_size | `int`: Number of samples per channel in the input signal. This must be an Opus frame size for the encoder's sampling rate. For example, at 48 kHz the permitted values are 120, 240, 480, 960, 1920, and 2880. Passing in a duration of less than 10 ms (480 samples at 48 kHz) will prevent the encoder from using the LPC or hybrid modes.
  [out] | data | `unsigned char*`: Output payload. This must contain storage for at least max_data_bytes.
  [in] | max_data_bytes | `opus_int32`: Size of the allocated memory for the output payload. This may be used to impose an upper limit on the instant bitrate, but should not be used as the only bitrate control. Use OPUS_SET_BITRATE to control the bitrate.

Returns
  The length of the encoded packet (in bytes) on success or a negative error code (see Error codes) on failure.

int opus_multistream_encode_float | ( | OpusMSEncoder * | st,
|  | const float * | pcm,
|  | int | frame_size,
|  | unsigned char * | data,
|  | opus_int32 | max_data_bytes
| ) |  |

Encodes a multistream Opus frame from floating point input.

Parameters

  | st | `OpusMSEncoder*`: Multistream encoder state.
  [in] | pcm | `const float*`: The input signal as interleaved samples with a normal range of +/-1.0. Samples with a range beyond +/-1.0 are supported but will be clipped by decoders using the integer API and should only be used if it is known that the far end supports extended dynamic range. This must contain `frame_size*channels` samples.
  | frame_size | `int`: Number of samples per channel in the input signal. This must be an Opus frame size for the encoder's sampling rate. For example, at 48 kHz the permitted values are 120, 240, 480, 960, 1920, and 2880. Passing in a duration of less than 10 ms (480 samples at 48 kHz) will prevent the encoder from using the LPC or hybrid modes.
  [out] | data | `unsigned char*`: Output payload. This must contain storage for at least max_data_bytes.
  [in] | max_data_bytes | `opus_int32`: Size of the allocated memory for the output payload. This may be used to impose an upper limit on the instant bitrate, but should not be used as the only bitrate control. Use OPUS_SET_BITRATE to control the bitrate.

Returns
  The length of the encoded packet (in bytes) on success or a negative error code (see Error codes) on failure.

OpusMSEncoder* opus_multistream_encoder_create | ( | opus_int32 | Fs,
|  | int | channels,
|  | int | streams,
|  | int | coupled_streams,
|  | const unsigned char * | mapping,
|  | int | application,
|  | int * | error
| ) |  |

Allocates and initializes a multistream encoder state.

Call opus_multistream_encoder_destroy() to release this object when finished.

Parameters

  | Fs | `opus_int32`: Sampling rate of the input signal (in Hz). This must be one of 8000, 12000, 16000, 24000, or 48000.
  | channels | `int`: Number of channels in the input signal. This must be at most 255. It may be greater than the number of coded channels (`streams + coupled_streams`).
  | streams | `int`: The total number of streams to encode from the input. This must be no more than the number of channels.
  | coupled_streams | `int`: Number of coupled (2 channel) streams to encode. This must be no larger than the total number of streams. Additionally, The total number of encoded channels (`streams + coupled_streams`) must be no more than the number of input channels.
  [in] | mapping | `const unsigned char[channels]`: Mapping from encoded channels to input channels, as described in Opus Multistream API. As an extra constraint, the multistream encoder does not allow encoding coupled streams for which one channel is unused since this is never a good idea.

  application

  `int`: The target encoder application. This must be one of the following:
  OPUS_APPLICATION_VOIP
    Process signal for improved speech intelligibility.
  OPUS_APPLICATION_AUDIO
    Favor faithfulness to the original input.
  OPUS_APPLICATION_RESTRICTED_LOWDELAY
    Configure the minimum possible coding delay by disabling certain modes of operation.

  [out] | error | `int *`: Returns OPUS_OK on success, or an error code (see Error codes) on failure.

int opus_multistream_encoder_ctl | ( | OpusMSEncoder * | st,
|  | int | request,
|  |  | ...
| ) |  |

Perform a CTL function on a multistream Opus encoder.

Generally the request and subsequent arguments are generated by a convenience macro.

Parameters

  st | `OpusMSEncoder*`: Multistream encoder state.
  request | This and all remaining parameters should be replaced by one of the convenience macros in Generic CTLs, Encoder related CTLs, or Multistream specific encoder and decoder CTLs.

See also
  Generic CTLs
  Encoder related CTLs
  Multistream specific encoder and decoder CTLs

void opus_multistream_encoder_destroy | ( | OpusMSEncoder * | st | ) |

Frees an `OpusMSEncoder` allocated by opus_multistream_encoder_create().

Parameters

  st | `OpusMSEncoder*`: Multistream encoder state to be freed.

opus_int32 opus_multistream_encoder_get_size | ( | int | streams,
|  | int | coupled_streams
| ) |  |

Gets the size of an OpusMSEncoder structure.

Parameters

  streams | `int`: The total number of streams to encode from the input. This must be no more than 255.
  coupled_streams | `int`: Number of coupled (2 channel) streams to encode. This must be no larger than the total number of streams. Additionally, The total number of encoded channels (`streams + coupled_streams`) must be no more than 255.

Returns
  The size in bytes on success, or a negative error code (see Error codes) on error.

int opus_multistream_encoder_init | ( | OpusMSEncoder * | st,
|  | opus_int32 | Fs,
|  | int | channels,
|  | int | streams,
|  | int | coupled_streams,
|  | const unsigned char * | mapping,
|  | int | application
| ) |  |

Initialize a previously allocated multistream encoder state.

The memory pointed to by st must be at least the size returned by opus_multistream_encoder_get_size(). This is intended for applications which use their own allocator instead of malloc. To reset a previously initialized state, use the OPUS_RESET_STATE CTL.

See also
  opus_multistream_encoder_create
  opus_multistream_encoder_get_size

Parameters

  | st | `OpusMSEncoder*`: Multistream encoder state to initialize.
  | Fs | `opus_int32`: Sampling rate of the input signal (in Hz). This must be one of 8000, 12000, 16000, 24000, or 48000.
  | channels | `int`: Number of channels in the input signal. This must be at most 255. It may be greater than the number of coded channels (`streams + coupled_streams`).
  | streams | `int`: The total number of streams to encode from the input. This must be no more than the number of channels.
  | coupled_streams | `int`: Number of coupled (2 channel) streams to encode. This must be no larger than the total number of streams. Additionally, The total number of encoded channels (`streams + coupled_streams`) must be no more than the number of input channels.
  [in] | mapping | `const unsigned char[channels]`: Mapping from encoded channels to input channels, as described in Opus Multistream API. As an extra constraint, the multistream encoder does not allow encoding coupled streams for which one channel is unused since this is never a good idea.

  application

  `int`: The target encoder application. This must be one of the following:
  OPUS_APPLICATION_VOIP
    Process signal for improved speech intelligibility.
  OPUS_APPLICATION_AUDIO
    Favor faithfulness to the original input.
  OPUS_APPLICATION_RESTRICTED_LOWDELAY
    Configure the minimum possible coding delay by disabling certain modes of operation.

Returns
  OPUS_OK on success, or an error code (see Error codes) on failure.

OpusMSEncoder* opus_multistream_surround_encoder_create | ( | opus_int32 | Fs,
|  | int | channels,
|  | int | mapping_family,
|  | int * | streams,
|  | int * | coupled_streams,
|  | unsigned char * | mapping,
|  | int | application,
|  | int * | error
| ) |  |

opus_int32 opus_multistream_surround_encoder_get_size | ( | int | channels,
|  | int | mapping_family
| ) |  |

int opus_multistream_surround_encoder_init | ( | OpusMSEncoder * | st,
|  | opus_int32 | Fs,
|  | int | channels,
|  | int | mapping_family,
|  | int * | streams,
|  | int * | coupled_streams,
|  | unsigned char * | mapping,
|  | int | application
| ) |  |

For more information visit the Opus Website. |