Provided by: libopus-doc_1.4-1build1_all 
NAME
opus_encoder - Opus Encoder
- This page describes the process and functions used to encode Opus.
SYNOPSIS
Typedefs
typedef struct OpusEncoder OpusEncoder
Opus encoder state.
Functions
int opus_encoder_get_size (int channels)
Gets the size of an OpusEncoder structure.
OpusEncoder * opus_encoder_create (opus_int32 Fs, int channels, int application, int *error)
Allocates and initializes an encoder state.
int opus_encoder_init (OpusEncoder *st, opus_int32 Fs, int channels, int application)
Initializes a previously allocated encoder state The memory pointed to by st must be at least the
size returned by opus_encoder_get_size().
opus_int32 opus_encode (OpusEncoder *st, const opus_int16 *pcm, int frame_size, unsigned char *data,
opus_int32 max_data_bytes)
Encodes an Opus frame.
opus_int32 opus_encode_float (OpusEncoder *st, const float *pcm, int frame_size, unsigned char *data,
opus_int32 max_data_bytes)
Encodes an Opus frame from floating point input.
void opus_encoder_destroy (OpusEncoder *st)
Frees an OpusEncoder allocated by opus_encoder_create().
int opus_encoder_ctl (OpusEncoder *st, int request,...)
Perform a CTL function on an Opus encoder.
Detailed Description
This page describes the process and functions used to encode Opus.
Since Opus is a stateful codec, the encoding process starts with creating an encoder state. This can be
done with:
int error;
OpusEncoder *enc;
enc = opus_encoder_create(Fs, channels, application, &error);
From this point, enc can be used for encoding an audio stream. An encoder state must not be used for more
than one stream at the same time. Similarly, the encoder state must not be re-initialized for each frame.
While opus_encoder_create() allocates memory for the state, it's also possible to initialize pre-
allocated memory:
int size;
int error;
OpusEncoder *enc;
size = opus_encoder_get_size(channels);
enc = malloc(size);
error = opus_encoder_init(enc, Fs, channels, application);
where opus_encoder_get_size() returns the required size for the encoder state. Note that future versions
of this code may change the size, so no assuptions should be made about it.
The encoder state is always continuous in memory and only a shallow copy is sufficient to copy it (e.g.
memcpy())
It is possible to change some of the encoder's settings using the opus_encoder_ctl() interface. All these
settings already default to the recommended value, so they should only be changed when necessary. The
most common settings one may want to change are:
opus_encoder_ctl(enc, OPUS_SET_BITRATE(bitrate));
opus_encoder_ctl(enc, OPUS_SET_COMPLEXITY(complexity));
opus_encoder_ctl(enc, OPUS_SET_SIGNAL(signal_type));
where
•
bitrate is in bits per second (b/s)
•
complexity is a value from 1 to 10, where 1 is the lowest complexity and 10 is the highest
•
signal_type is either OPUS_AUTO (default), OPUS_SIGNAL_VOICE, or OPUS_SIGNAL_MUSIC
See Encoder related CTLs and Generic CTLs for a complete list of parameters that can be set or queried.
Most parameters can be set or changed at any time during a stream.
To encode a frame, opus_encode() or opus_encode_float() must be called with exactly one frame (2.5, 5,
10, 20, 40 or 60 ms) of audio data:
len = opus_encode(enc, audio_frame, frame_size, packet, max_packet);
where
• audio_frame is the audio data in opus_int16 (or float for opus_encode_float())
• frame_size is the duration of the frame in samples (per channel)
• packet is the byte array to which the compressed data is written
• max_packet is the maximum number of bytes that can be written in the packet (4000 bytes is
recommended). Do not use max_packet to control VBR target bitrate, instead use the OPUS_SET_BITRATE
CTL.
opus_encode() and opus_encode_float() return the number of bytes actually written to the packet. The
return value can be negative, which indicates that an error has occurred. If the return value is 2 bytes
or less, then the packet does not need to be transmitted (DTX).
Once the encoder state if no longer needed, it can be destroyed with
opus_encoder_destroy(enc);
If the encoder was created with opus_encoder_init() rather than opus_encoder_create(), then no action is
required aside from potentially freeing the memory that was manually allocated for it (calling free(enc)
for the example above)
Typedef Documentation
typedef struct OpusEncoder OpusEncoder
Opus encoder state. This contains the complete state of an Opus encoder. It is position independent and
can be freely copied.
See also
opus_encoder_create,opus_encoder_init
Function Documentation
opus_int32 opus_encode (OpusEncoder * st, const opus_int16 * pcm, int frame_size, unsigned char * data,
opus_int32 max_data_bytes)
Encodes an Opus frame.
Parameters
st OpusEncoder*: Encoder state
pcm opus_int16*: Input signal (interleaved if 2 channels). length is
frame_size*channels*sizeof(opus_int16)
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.
data unsigned char*: Output payload. This must contain storage for at least max_data_bytes.
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.
opus_int32 opus_encode_float (OpusEncoder * st, const float * pcm, int frame_size, unsigned char * data,
opus_int32 max_data_bytes)
Encodes an Opus frame from floating point input.
Parameters
st OpusEncoder*: Encoder state
pcm float*: Input in float format (interleaved if 2 channels), 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. length is
frame_size*channels*sizeof(float)
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.
data unsigned char*: Output payload. This must contain storage for at least max_data_bytes.
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.
OpusEncoder * opus_encoder_create (opus_int32 Fs, int channels, int application, int * error)
Allocates and initializes an encoder state. There are three coding modes:
OPUS_APPLICATION_VOIP gives best quality at a given bitrate for voice signals. It enhances the input
signal by high-pass filtering and emphasizing formants and harmonics. Optionally it includes in-band
forward error correction to protect against packet loss. Use this mode for typical VoIP applications.
Because of the enhancement, even at high bitrates the output may sound different from the input.
OPUS_APPLICATION_AUDIO gives best quality at a given bitrate for most non-voice signals like music. Use
this mode for music and mixed (music/voice) content, broadcast, and applications requiring less than 15
ms of coding delay.
OPUS_APPLICATION_RESTRICTED_LOWDELAY configures low-delay mode that disables the speech-optimized mode in
exchange for slightly reduced delay. This mode can only be set on an newly initialized or freshly reset
encoder because it changes the codec delay.
This is useful when the caller knows that the speech-optimized modes will not be needed (use with
caution).
Parameters
Fs opus_int32: Sampling rate of input signal (Hz) This must be one of 8000, 12000, 16000, 24000, or
48000.
channels int: Number of channels (1 or 2) in input signal
application int: Coding mode (one of OPUS_APPLICATION_VOIP, OPUS_APPLICATION_AUDIO, or
OPUS_APPLICATION_RESTRICTED_LOWDELAY)
error int*: Error codes
Note
Regardless of the sampling rate and number channels selected, the Opus encoder can switch to a lower
audio bandwidth or number of channels if the bitrate selected is too low. This also means that it is
safe to always use 48 kHz stereo input and let the encoder optimize the encoding.
int opus_encoder_ctl (OpusEncoder * st, int request, ...)
Perform a CTL function on an Opus encoder. Generally the request and subsequent arguments are generated
by a convenience macro.
Parameters
st OpusEncoder*: Encoder state.
request This and all remaining parameters should be replaced by one of the convenience macros in
Generic CTLs or Encoder related CTLs.
See also
Generic CTLs
Encoder related CTLs
void opus_encoder_destroy (OpusEncoder * st)
Frees an OpusEncoder allocated by opus_encoder_create().
Parameters
st OpusEncoder*: State to be freed.
int opus_encoder_get_size (int channels)
Gets the size of an OpusEncoder structure.
Parameters
channels int: Number of channels. This must be 1 or 2.
Returns
The size in bytes.
int opus_encoder_init (OpusEncoder * st, opus_int32 Fs, int channels, int application)
Initializes a previously allocated encoder state The memory pointed to by st must be at least the size
returned by opus_encoder_get_size(). This is intended for applications which use their own allocator
instead of malloc.
See also
opus_encoder_create(),opus_encoder_get_size() To reset a previously initialized state, use the
OPUS_RESET_STATE CTL.
Parameters
st OpusEncoder*: Encoder state
Fs opus_int32: Sampling rate of input signal (Hz) This must be one of 8000, 12000, 16000, 24000, or
48000.
channels int: Number of channels (1 or 2) in input signal
application int: Coding mode (one of OPUS_APPLICATION_VOIP, OPUS_APPLICATION_AUDIO, or
OPUS_APPLICATION_RESTRICTED_LOWDELAY)
Return values
OPUS_OK Success or Error codes
Author
Generated automatically by Doxygen for Opus from the source code.
Version 1.4 Mon Apr 8 2024 16:14:57 opus_encoder(3)