The Opus codec is designed for interactive speech and audio transmission over the Internet. It is designed by the IETF Codec Working Group and incorporates technology from Skype's SILK codec and Xiph.Org's CELT codec.

The Opus codec is designed to handle a wide range of interactive audio applications, including Voice over IP, videoconferencing, in-game chat, and even remote live music performances. It can scale from low bit-rate narrowband speech to very high quality stereo music. Its main features are:

Sampling rates from 8 to 48 kHz
Bit-rates from 6 kb/s to 510 kb/s
Support for both constant bit-rate (CBR) and variable bit-rate (VBR)
Audio bandwidth from narrowband to full-band
Support for speech and music
Support for mono and stereo
Support for multichannel (up to 255 channels)
Frame sizes from 2.5 ms to 60 ms
Good loss robustness and packet loss concealment (PLC)
Floating point and fixed-point implementation

Documentation sections:

Changes for TI SimpleLink Implementation

In order to support SimpleLink devices, the TI optimized implementation only supports the CELT codec. Anything relating to SILK and HYBRID modes is unsupported. The original implementation and documentation can be found at the Opus Codec website.

Additionally, optional build options further reduce the functionality to:

Mono
20ms frames
Complexity setting <= 3

Benchmarks for the TI SimpleLink Implementation

The benchmarks below are based on mono, 16kHz, encoder level 3 complexity:

~17 MCPs average encode.
~10 MCPs average decode (no PLC).
< 61 KBytes of program memory (-Os).
14842 Bytes of data tables in FLASH.
104 Bytes of data tables in SRAM.
< 17K Bytes of SRAM per encoder channel. This comprises:
- 1572 Bytes of status.
- 13200 Bytes of temporary storage
- < 2.1K Bytes peak stack usage for encoder.
< 21K Bytes of SRAM per decoder channel. This comprises:
- 9224 Bytes of status.
- 9384 Bytes of temporary storage
- < 2.0K Bytes peak stack usage for decoder

The memory figures are for the CCS tool chain v7.4 with optimization for code size. The MCPs were measured using CC26x2R1 LaunchPad plus CC3200AUDBOOST hardware using real time audio.

Build settings

The above limitations can be overridden by recompiling the Opus library with the following defines set.

Warning: Doing so should be considered experimental and untested:

ALLOW_STEREO: By default only mono audio is supported. Setting this option allows stereo audio to be supported.
ALLOW_NON_20ms: By default only 20ms frames are supported. Setting this option allows 2.5, 5, 10 and 20ms frames to be supported.
ALLOW_FULL_COMPLEXITY: Default maximum complexity is limited to 3. Setting this option allows complexity settings up to 10 to be supported.
ALLOW_FULL_API: By default the generation of multi-frame packets by the encoder is not supported. Setting this option allows this.
DISABLE_FLOAT_API: By default this option is set. The tonality_analysis, is written using single precision floating point arithmetic

The following define must be set when building for the IAR compiler:

HAS_INTRINSICS_H

Memory Usage and Scratch Buffer

Temporary memory requirements are split between the normal C stack and a temporary scratch area. The scratch area can be malloc'd and free'd each frame (external to the codec) or allocated once on start-up.s The encoder and decoder have different scratch requirements and the encoder's requirement is build option dependent.

The following constants are given in this header file:

ENC_DYN_ALLOC_SIZE
DEC_DYN_ALLOC_SIZE

An example allocation is:

int32_t *Scratch_Buffer;

Scratch_Buffer = malloc(ENC_DYN_ALLOC_SIZE*sizeof(int32_t);

The decoder's scratch area can be aliased to the encoder's since the encoder always has a larger requirement, assuming the encoder and decoder are not run concurrently.

This partitioning was performed in order to reduce the stack requirements, it does not however reduce the total memory requirement particularly if the scratch allocation is done on start-up and there are periods when neither encoder/decoder are running.

Runtime stack requirement

2K Bytes for the CCS tool chain and slightly less for the IAR tool chain in the worst case.

Scratch buffer size requirement

The scratch buffer requirements are:

13200 Bytes for the encoder.
9384 Bytes for the decoder.