Opus audio codec (RFC 6716): API and operations manual
1.1.2

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Opus Custom

Opus Custom is an optional part of the Opus specification and reference implementation which uses a distinct API from the regular API and supports frame sizes that are not normally supported. Use of Opus Custom is discouraged for all but very special applications for which a frame size different from 2.5, 5, 10, or 20 ms is needed (for either complexity or latency reasons) and where interoperability is less important. More...

## Typedefs

typedef struct OpusCustomEncoder | OpusCustomEncoder
| Contains the state of an encoder. More...
typedef struct OpusCustomDecoder | OpusCustomDecoder
| State of the decoder. More...
typedef struct OpusCustomMode | OpusCustomMode
| The mode contains all the information necessary to create an encoder. More...

## Functions

OpusCustomMode * | opus_custom_mode_create (opus_int32 Fs, int frame_size, int *error)
| Creates a new mode struct. More...
void | opus_custom_mode_destroy (OpusCustomMode *mode)
| Destroys a mode struct. More...
int | opus_custom_encoder_get_size (const OpusCustomMode *mode, int channels)
| Gets the size of an OpusCustomEncoder structure. More...
OpusCustomEncoder * | opus_custom_encoder_create (const OpusCustomMode *mode, int channels, int *error)
| Creates a new encoder state. More...
void | opus_custom_encoder_destroy (OpusCustomEncoder *st)
| Destroys a an encoder state. More...
int | opus_custom_encode_float (OpusCustomEncoder *st, const float *pcm, int frame_size, unsigned char *compressed, int maxCompressedBytes)
| Encodes a frame of audio. More...
int | opus_custom_encode (OpusCustomEncoder *st, const opus_int16 *pcm, int frame_size, unsigned char *compressed, int maxCompressedBytes)
| Encodes a frame of audio. More...
int | opus_custom_encoder_ctl (OpusCustomEncoder *OPUS_RESTRICT st, int request,...)
| Perform a CTL function on an Opus custom encoder. More...
int | opus_custom_decoder_get_size (const OpusCustomMode *mode, int channels)
| Gets the size of an OpusCustomDecoder structure. More...
int | opus_custom_decoder_init (OpusCustomDecoder *st, const OpusCustomMode *mode, int channels)
| Initializes a previously allocated decoder state The memory pointed to by st must be the size returned by opus_custom_decoder_get_size. More...
OpusCustomDecoder * | opus_custom_decoder_create (const OpusCustomMode *mode, int channels, int *error)
| Creates a new decoder state. More...
void | opus_custom_decoder_destroy (OpusCustomDecoder *st)
| Destroys a an decoder state. More...
int | opus_custom_decode_float (OpusCustomDecoder *st, const unsigned char *data, int len, float *pcm, int frame_size)
| Decode an opus custom frame with floating point output. More...
int | opus_custom_decode (OpusCustomDecoder *st, const unsigned char *data, int len, opus_int16 *pcm, int frame_size)
| Decode an opus custom frame. More...
int | opus_custom_decoder_ctl (OpusCustomDecoder *OPUS_RESTRICT st, int request,...)
| Perform a CTL function on an Opus custom decoder. More...

## Detailed Description

Opus Custom is an optional part of the Opus specification and reference implementation which uses a distinct API from the regular API and supports frame sizes that are not normally supported. Use of Opus Custom is discouraged for all but very special applications for which a frame size different from 2.5, 5, 10, or 20 ms is needed (for either complexity or latency reasons) and where interoperability is less important.

In addition to the interoperability limitations the use of Opus custom disables a substantial chunk of the codec and generally lowers the quality available at a given bitrate. Normally when an application needs a different frame size from the codec it should buffer to match the sizes but this adds a small amount of delay which may be important in some very low latency applications. Some transports (especially constant rate RF transports) may also work best with frames of particular durations.

Libopus only supports custom modes if they are enabled at compile time.

The Opus Custom API is similar to the regular API but the opus_encoder_create and opus_decoder_create calls take an additional mode parameter which is a structure produced by a call to opus_custom_mode_create. Both the encoder and decoder must create a mode using the same sample rate (fs) and frame size (frame size) so these parameters must either be signaled out of band or fixed in a particular implementation.

Similar to regular Opus the custom modes support on the fly frame size switching, but the sizes available depend on the particular frame size in use. For some initial frame sizes on a single on the fly size is available.

## Typedef Documentation

typedef struct OpusCustomDecoder OpusCustomDecoder

State of the decoder.

One decoder state is needed for each stream. It is initialized once at the beginning of the stream. Do not re-initialize the state for every frame. Decoder state

typedef struct OpusCustomEncoder OpusCustomEncoder

Contains the state of an encoder.

One encoder state is needed for each stream. It is initialized once at the beginning of the stream. Do not re-initialize the state for every frame. Encoder state

typedef struct OpusCustomMode OpusCustomMode

The mode contains all the information necessary to create an encoder.

Both the encoder and decoder need to be initialized with exactly the same mode, otherwise the output will be corrupted. Mode configuration

## Function Documentation

int opus_custom_decode | ( | OpusCustomDecoder * | st,
|  | const unsigned char * | data,
|  | int | len,
|  | opus_int16 * | pcm,
|  | int | frame_size
| ) |  |

Decode an opus custom frame.

Parameters

  [in] | st | `OpusCustomDecoder*`: Decoder state
  [in] | data | `char*`: Input payload. Use a NULL pointer to indicate packet loss
  [in] | len | `int`: Number of bytes in payload
  [out] | pcm | `opus_int16*`: Output signal (interleaved if 2 channels). length is frame_size*channels*sizeof(opus_int16)
  [in] | frame_size | Number of samples per channel of available space in *pcm.

Returns
  Number of decoded samples or Error codes

int opus_custom_decode_float | ( | OpusCustomDecoder * | st,
|  | const unsigned char * | data,
|  | int | len,
|  | float * | pcm,
|  | int | frame_size
| ) |  |

Decode an opus custom frame with floating point output.

Parameters

  [in] | st | `OpusCustomDecoder*`: Decoder state
  [in] | data | `char*`: Input payload. Use a NULL pointer to indicate packet loss
  [in] | len | `int`: Number of bytes in payload
  [out] | pcm | `float*`: Output signal (interleaved if 2 channels). length is frame_size*channels*sizeof(float)
  [in] | frame_size | Number of samples per channel of available space in *pcm.

Returns
  Number of decoded samples or Error codes

OpusCustomDecoder* opus_custom_decoder_create | ( | const OpusCustomMode * | mode,
|  | int | channels,
|  | int * | error
| ) |  |

Creates a new decoder state.

Each stream needs its own decoder state (can't be shared across simultaneous streams).

Parameters

  [in] | mode | `OpusCustomMode`: Contains all the information about the characteristics of the stream (must be the same characteristics as used for the encoder)
  [in] | channels | `int`: Number of channels
  [out] | error | `int*`: Returns an error code

Returns
  Newly created decoder state.

int opus_custom_decoder_ctl | ( | OpusCustomDecoder *OPUS_RESTRICT | st,
|  | int | request,
|  |  | ...
| ) |  |

Perform a CTL function on an Opus custom decoder.

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

See also
  Generic CTLs

void opus_custom_decoder_destroy | ( | OpusCustomDecoder * | st | ) |

Destroys a an decoder state.

Parameters

  [in] | st | `OpusCustomDecoder*`: State to be freed.

int opus_custom_decoder_get_size | ( | const OpusCustomMode * | mode,
|  | int | channels
| ) |  |

Gets the size of an OpusCustomDecoder structure.

Parameters

  [in] | mode | `OpusCustomMode *`: Mode configuration
  [in] | channels | `int`: Number of channels

Returns
  size

int opus_custom_decoder_init | ( | OpusCustomDecoder * | st,
|  | const OpusCustomMode * | mode,
|  | int | channels
| ) |  |

Initializes a previously allocated decoder state The memory pointed to by st must be the size returned by opus_custom_decoder_get_size.

This is intended for applications which use their own allocator instead of malloc.

See also
  opus_custom_decoder_create(),opus_custom_decoder_get_size() To reset a previously initialized state use the OPUS_RESET_STATE CTL.

Parameters

  [in] | st | `OpusCustomDecoder*`: Decoder state
  [in] | mode | `OpusCustomMode *`: Contains all the information about the characteristics of the stream (must be the same characteristics as used for the encoder)
  [in] | channels | `int`: Number of channels

Returns
  OPUS_OK Success or Error codes

int opus_custom_encode | ( | OpusCustomEncoder * | st,
|  | const opus_int16 * | pcm,
|  | int | frame_size,
|  | unsigned char * | compressed,
|  | int | maxCompressedBytes
| ) |  |

Encodes a frame of audio.

Parameters

  [in] | st | `OpusCustomEncoder*`: Encoder state
  [in] | pcm | `opus_int16*`: PCM audio in signed 16-bit format (native endian). There must be exactly frame_size samples per channel.
  [in] | frame_size | `int`: Number of samples per frame of input signal
  [out] | compressed | `char *`: The compressed data is written here. This may not alias pcm and must be at least maxCompressedBytes long.
  [in] | maxCompressedBytes | `int`: Maximum number of bytes to use for compressing the frame (can change from one frame to another)

Returns
  Number of bytes written to "compressed". If negative, an error has occurred (see error codes). It is IMPORTANT that the length returned be somehow transmitted to the decoder. Otherwise, no decoding is possible.

int opus_custom_encode_float | ( | OpusCustomEncoder * | st,
|  | const float * | pcm,
|  | int | frame_size,
|  | unsigned char * | compressed,
|  | int | maxCompressedBytes
| ) |  |

Encodes a frame of audio.

Parameters

  [in] | st | `OpusCustomEncoder*`: Encoder state
  [in] | pcm | `float*`: PCM audio in float format, 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. There must be exactly frame_size samples per channel.
  [in] | frame_size | `int`: Number of samples per frame of input signal
  [out] | compressed | `char *`: The compressed data is written here. This may not alias pcm and must be at least maxCompressedBytes long.
  [in] | maxCompressedBytes | `int`: Maximum number of bytes to use for compressing the frame (can change from one frame to another)

Returns
  Number of bytes written to "compressed". If negative, an error has occurred (see error codes). It is IMPORTANT that the length returned be somehow transmitted to the decoder. Otherwise, no decoding is possible.

OpusCustomEncoder* opus_custom_encoder_create | ( | const OpusCustomMode * | mode,
|  | int | channels,
|  | int * | error
| ) |  |

Creates a new encoder state.

Each stream needs its own encoder state (can't be shared across simultaneous streams).

Parameters

  [in] | mode | `OpusCustomMode*`: Contains all the information about the characteristics of the stream (must be the same characteristics as used for the decoder)
  [in] | channels | `int`: Number of channels
  [out] | error | `int*`: Returns an error code

Returns
  Newly created encoder state.

int opus_custom_encoder_ctl | ( | OpusCustomEncoder *OPUS_RESTRICT | st,
|  | int | request,
|  |  | ...
| ) |  |

Perform a CTL function on an Opus custom encoder.

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

See also
  Encoder related CTLs

void opus_custom_encoder_destroy | ( | OpusCustomEncoder * | st | ) |

Destroys a an encoder state.

Parameters

  [in] | st | `OpusCustomEncoder*`: State to be freed.

int opus_custom_encoder_get_size | ( | const OpusCustomMode * | mode,
|  | int | channels
| ) |  |

Gets the size of an OpusCustomEncoder structure.

Parameters

  [in] | mode | `OpusCustomMode *`: Mode configuration
  [in] | channels | `int`: Number of channels

Returns
  size

OpusCustomMode* opus_custom_mode_create | ( | opus_int32 | Fs,
|  | int | frame_size,
|  | int * | error
| ) |  |

Creates a new mode struct.

This will be passed to an encoder or decoder. The mode MUST NOT BE DESTROYED until the encoders and decoders that use it are destroyed as well.

Parameters

  [in] | Fs | `int`: Sampling rate (8000 to 96000 Hz)
  [in] | frame_size | `int`: Number of samples (per channel) to encode in each packet (64 - 1024, prime factorization must contain zero or more 2s, 3s, or 5s and no other primes)
  [out] | error | `int*`: Returned error code (if NULL, no error will be returned)

Returns
  A newly created mode

void opus_custom_mode_destroy | ( | OpusCustomMode * | mode | ) |

Destroys a mode struct.

Only call this after all encoders and decoders using this mode are destroyed as well.

Parameters

  [in] | mode | `OpusCustomMode*`: Mode to be freed.

For more information visit the Opus Website. |