Provided by: libopus-doc_1.3.1-2_all 
NAME
opus_decoder - Opus Decoder
- This page describes the process and functions used to decode Opus.
SYNOPSIS
Typedefs
typedef struct OpusDecoder OpusDecoder
Opus decoder state.
Functions
int opus_decoder_get_size (int channels)
Gets the size of an OpusDecoder structure.
OpusDecoder * opus_decoder_create (opus_int32 Fs, int channels, int *error)
Allocates and initializes a decoder state.
int opus_decoder_init (OpusDecoder *st, opus_int32 Fs, int channels)
Initializes a previously allocated decoder state.
int opus_decode (OpusDecoder *st, const unsigned char *data, opus_int32 len, opus_int16
*pcm, int frame_size, int decode_fec)
Decode an Opus packet.
int opus_decode_float (OpusDecoder *st, const unsigned char *data, opus_int32 len, float
*pcm, int frame_size, int decode_fec)
Decode an Opus packet with floating point output.
int opus_decoder_ctl (OpusDecoder *st, int request,...)
Perform a CTL function on an Opus decoder.
void opus_decoder_destroy (OpusDecoder *st)
Frees an OpusDecoder allocated by opus_decoder_create().
int opus_packet_parse (const unsigned char *data, opus_int32 len, unsigned char *out_toc,
const unsigned char *frames[48], opus_int16 size[48], int *payload_offset)
Parse an opus packet into one or more frames.
int opus_packet_get_bandwidth (const unsigned char *data)
Gets the bandwidth of an Opus packet.
int opus_packet_get_samples_per_frame (const unsigned char *data, opus_int32 Fs)
Gets the number of samples per frame from an Opus packet.
int opus_packet_get_nb_channels (const unsigned char *data)
Gets the number of channels from an Opus packet.
int opus_packet_get_nb_frames (const unsigned char packet[], opus_int32 len)
Gets the number of frames in an Opus packet.
int opus_packet_get_nb_samples (const unsigned char packet[], opus_int32 len, opus_int32
Fs)
Gets the number of samples of an Opus packet.
int opus_decoder_get_nb_samples (const OpusDecoder *dec, const unsigned char packet[],
opus_int32 len)
Gets the number of samples of an Opus packet.
void opus_pcm_soft_clip (float *pcm, int frame_size, int channels, float *softclip_mem)
Applies soft-clipping to bring a float signal within the [-1,1] range.
Detailed Description
This page describes the process and functions used to decode Opus.
The decoding process also starts with creating a decoder state. This can be done with:
int error;
OpusDecoder *dec;
dec = opus_decoder_create(Fs, channels, &error);
where
• Fs is the sampling rate and must be 8000, 12000, 16000, 24000, or 48000
• channels is the number of channels (1 or 2)
• error will hold the error code in case of failure (or OPUS_OK on success)
• the return value is a newly created decoder state to be used for decoding
While opus_decoder_create() allocates memory for the state, it's also possible to
initialize pre-allocated memory:
int size;
int error;
OpusDecoder *dec;
size = opus_decoder_get_size(channels);
dec = malloc(size);
error = opus_decoder_init(dec, Fs, channels);
where opus_decoder_get_size() returns the required size for the decoder state. Note that
future versions of this code may change the size, so no assuptions should be made about
it.
The decoder state is always continuous in memory and only a shallow copy is sufficient to
copy it (e.g. memcpy())
To decode a frame, opus_decode() or opus_decode_float() must be called with a packet of
compressed audio data:
frame_size = opus_decode(dec, packet, len, decoded, max_size, 0);
where
• packet is the byte array containing the compressed data
• len is the exact number of bytes contained in the packet
• decoded is the decoded audio data in opus_int16 (or float for opus_decode_float())
• max_size is the max duration of the frame in samples (per channel) that can fit into the
decoded_frame array
opus_decode() and opus_decode_float() return the number of samples (per channel) decoded
from the packet. If that value is negative, then an error has occurred. This can occur if
the packet is corrupted or if the audio buffer is too small to hold the decoded audio.
Opus is a stateful codec with overlapping blocks and as a result Opus packets are not
coded independently of each other. Packets must be passed into the decoder serially and in
the correct order for a correct decode. Lost packets can be replaced with loss concealment
by calling the decoder with a null pointer and zero length for the missing packet.
A single codec state may only be accessed from a single thread at a time and any required
locking must be performed by the caller. Separate streams must be decoded with separate
decoder states and can be decoded in parallel unless the library was compiled with
NONTHREADSAFE_PSEUDOSTACK defined.
Typedef Documentation
typedef struct OpusDecoder OpusDecoder
Opus decoder state. This contains the complete state of an Opus decoder. It is position
independent and can be freely copied.
See also
opus_decoder_create,opus_decoder_init
Function Documentation
int opus_decode (OpusDecoder * st, const unsigned char * data, opus_int32 len, opus_int16 *
pcm, int frame_size, int decode_fec)
Decode an Opus packet.
Parameters
st OpusDecoder*: Decoder state
data char*: Input payload. Use a NULL pointer to indicate packet loss
len opus_int32: Number of bytes in payload*
pcm opus_int16*: Output signal (interleaved if 2 channels). length is
frame_size*channels*sizeof(opus_int16)
frame_size Number of samples per channel of available space in pcm. If this is less
than the maximum packet duration (120ms; 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 decoded samples or Error codes
int opus_decode_float (OpusDecoder * st, const unsigned char * data, opus_int32 len, float *
pcm, int frame_size, int decode_fec)
Decode an Opus packet with floating point output.
Parameters
st OpusDecoder*: Decoder state
data char*: Input payload. Use a NULL pointer to indicate packet loss
len opus_int32: Number of bytes in payload
pcm float*: Output signal (interleaved if 2 channels). length is
frame_size*channels*sizeof(float)
frame_size Number of samples per channel of available space in pcm. If this is less
than the maximum packet duration (120ms; 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 decoded samples or Error codes
OpusDecoder* opus_decoder_create (opus_int32 Fs, int channels, int * error)
Allocates and initializes a decoder state.
Parameters
Fs opus_int32: Sample rate to decode at (Hz). This must be one of 8000, 12000, 16000,
24000, or 48000.
channels int: Number of channels (1 or 2) to decode
error int*: OPUS_OK Success or Error codes
Internally Opus stores data at 48000 Hz, so that should be the default value for Fs.
However, the decoder can efficiently decode to buffers at 8, 12, 16, and 24 kHz so if for
some reason the caller cannot use data at the full sample rate, or knows the compressed
data doesn't use the full frequency range, it can request decoding at a reduced rate.
Likewise, the decoder is capable of filling in either mono or interleaved stereo pcm
buffers, at the caller's request.
int opus_decoder_ctl (OpusDecoder * st, int request, ...)
Perform a CTL function on an Opus decoder. Generally the request and subsequent arguments
are generated by a convenience macro.
Parameters
st OpusDecoder*: Decoder state.
request This and all remaining parameters should be replaced by one of the convenience
macros in Generic CTLs or Decoder related CTLs.
See also
Generic CTLs
Decoder related CTLs
void opus_decoder_destroy (OpusDecoder * st)
Frees an OpusDecoder allocated by opus_decoder_create().
Parameters
st OpusDecoder*: State to be freed.
int opus_decoder_get_nb_samples (const OpusDecoder * dec, const unsigned char packet[],
opus_int32 len)
Gets the number of samples of an Opus packet.
Parameters
dec OpusDecoder*: Decoder state
packet char*: Opus packet
len opus_int32: Length of packet
Returns
Number of samples
Return values
OPUS_BAD_ARG Insufficient data was passed to the function
OPUS_INVALID_PACKET The compressed data passed is corrupted or of an unsupported type
int opus_decoder_get_size (int channels)
Gets the size of an OpusDecoder structure.
Parameters
channels int: Number of channels. This must be 1 or 2.
Returns
The size in bytes.
int opus_decoder_init (OpusDecoder * st, opus_int32 Fs, int channels)
Initializes a previously allocated decoder state. The state must be at least the size
returned by opus_decoder_get_size(). This is intended for applications which use their own
allocator instead of malloc.
See also
opus_decoder_create,opus_decoder_get_size To reset a previously initialized state, use
the OPUS_RESET_STATE CTL.
Parameters
st OpusDecoder*: Decoder state.
Fs opus_int32: Sampling rate to decode to (Hz). This must be one of 8000, 12000,
16000, 24000, or 48000.
channels int: Number of channels (1 or 2) to decode
Return values
OPUS_OK Success or Error codes
int opus_packet_get_bandwidth (const unsigned char * data)
Gets the bandwidth of an Opus packet.
Parameters
data char*: Opus packet
Return values
OPUS_BANDWIDTH_NARROWBAND Narrowband (4kHz bandpass)
OPUS_BANDWIDTH_MEDIUMBAND Mediumband (6kHz bandpass)
OPUS_BANDWIDTH_WIDEBAND Wideband (8kHz bandpass)
OPUS_BANDWIDTH_SUPERWIDEBAND Superwideband (12kHz bandpass)
OPUS_BANDWIDTH_FULLBAND Fullband (20kHz bandpass)
OPUS_INVALID_PACKET The compressed data passed is corrupted or of an unsupported type
int opus_packet_get_nb_channels (const unsigned char * data)
Gets the number of channels from an Opus packet.
Parameters
data char*: Opus packet
Returns
Number of channels
Return values
OPUS_INVALID_PACKET The compressed data passed is corrupted or of an unsupported type
int opus_packet_get_nb_frames (const unsigned char packet[], opus_int32 len)
Gets the number of frames in an Opus packet.
Parameters
packet char*: Opus packet
len opus_int32: Length of packet
Returns
Number of frames
Return values
OPUS_BAD_ARG Insufficient data was passed to the function
OPUS_INVALID_PACKET The compressed data passed is corrupted or of an unsupported type
int opus_packet_get_nb_samples (const unsigned char packet[], opus_int32 len, opus_int32 Fs)
Gets the number of samples of an Opus packet.
Parameters
packet char*: Opus packet
len opus_int32: Length of packet
Fs opus_int32: Sampling rate in Hz. This must be a multiple of 400, or inaccurate
results will be returned.
Returns
Number of samples
Return values
OPUS_BAD_ARG Insufficient data was passed to the function
OPUS_INVALID_PACKET The compressed data passed is corrupted or of an unsupported type
int opus_packet_get_samples_per_frame (const unsigned char * data, opus_int32 Fs)
Gets the number of samples per frame from an Opus packet.
Parameters
data char*: Opus packet. This must contain at least one byte of data.
Fs opus_int32: Sampling rate in Hz. This must be a multiple of 400, or inaccurate
results will be returned.
Returns
Number of samples per frame.
int opus_packet_parse (const unsigned char * data, opus_int32 len, unsigned char * out_toc,
const unsigned char * frames[48], opus_int16 size[48], int * payload_offset)
Parse an opus packet into one or more frames. Opus_decode will perform this operation
internally so most applications do not need to use this function. This function does not
copy the frames, the returned pointers are pointers into the input packet.
Parameters
data char*: Opus packet to be parsed
len opus_int32: size of data
out_toc char*: TOC pointer
frames char*[48] encapsulated frames
size opus_int16[48] sizes of the encapsulated frames
payload_offset int*: returns the position of the payload within the packet (in bytes)
Returns
number of frames
void opus_pcm_soft_clip (float * pcm, int frame_size, int channels, float * softclip_mem)
Applies soft-clipping to bring a float signal within the [-1,1] range. If the signal is
already in that range, nothing is done. If there are values outside of [-1,1], then the
signal is clipped as smoothly as possible to both fit in the range and avoid creating
excessive distortion in the process.
Parameters
pcm float*: Input PCM and modified PCM
frame_size int Number of samples per channel to process
channels int: Number of channels
softclip_mem float*: State memory for the soft clipping process (one float per
channel, initialized to zero)
Author
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