Low Complexity Communication Codec (LC3)

LC3 and LC3 Plus are audio codecs designed for low-latency audio transport.

• LC3 is specified by the Bluetooth Special Interset Group for the LE Audio protocol

• LC3 Plus is defined by ETSI TS 103 634

In addition to LC3, following features of LC3 Plus are proposed:

• Frame duration of 2.5 and 5ms.
• High-Resolution mode, 48 KHz, and 96 kHz sampling rates.

Overview

The directory layout is as follows :

• include: Library interface
• src: Source files
• tools: Standalone encoder/decoder tools
• python: Python wrapper
• test: Unit testing framework
• fuzz: Roundtrip fuzz testing harness
• build: Building outputs
• bin: Compilation output

How to build

The default toolchain used is GCC. Invoke make to build the library.

$ make -j

Compiled library liblc3.so will be found in bin directory.

LC3 Plus features can be selectively disabled :

• LC3_PLUS=0 disable the support of 2.5ms and 5ms frame durations.
• LC3_PLUS_HR=0 turns off the support of the High-Resolution mode.

Only Bluetooth LC3 features will be included using the following command:

$ make LC3_PLUS=0 LC3_PLUS_HR=0 -j

Cross compilation

The cc, as, ld and ar can be selected with respective Makefile variables CC, AS, LD and AR. The AS and LD selections are optionnal, and fallback to CC selection when not defined.

The LIBC must be set to bionic for android cross-compilation. This switch prevent link with pthread and rt libraries, that is included in the bionic libc.

Following example build for android, using NDK toolset.

$ make -j CC=path_to_android_ndk_prebuilt/toolchain-prefix-clang LIBC=bionic

Compiled library will be found in bin directory.

Web Assembly (WASM)

Web assembly compilation is supported using LLVM WebAssembly backend. Installation of LLVM compiler and linker is needed:

# apt install clang lld

The webasm object is compiled using:

$ make CC="clang --target=wasm32"

Tools

Tools can be all compiled, while invoking make as follows :

$ make tools

The standalone encoder elc3 take a wave file as input and encode it according given parameter. The LC3 binary file format used is the non standard format described by the reference encoder / decoder tools. The standalone decoder dlc3 do the inverse operation.

Refer to elc3 -h or dlc3 -h for options.

Note that elc3 output bitstream to standard output when output file is omitted. On the other side dlc3 read from standard input when input output file are omitted. In such way you can easly test encoding / decoding loop with :

$ alias elc3="LD_LIBRARY_PATH=`pwd`/bin `pwd`/bin/elc3"
$ alias dlc3="LD_LIBRARY_PATH=`pwd`/bin `pwd`/bin/dlc3"
$ elc3 <in.wav> -b <bitrate> | dlc3 > <out.wav>

Adding Linux aplay tools, you will be able to instant hear the result :

$ alias elc3="LD_LIBRARY_PATH=`pwd`/bin `pwd`/bin/elc3"
$ alias dlc3="LD_LIBRARY_PATH=`pwd`/bin `pwd`/bin/dlc3"
$ elc3 <in.wav> -b <bitrate> | dlc3 | aplay -D pipewire

Test

A python implementation of the encoder is provided in test diretory. The C implementation is unitary validated against this implementation and intermediate values given in Appendix C of the LC3 specification.

Prerequisite

# apt install python3 python3-dev python3-pip
$ pip3 install scipy numpy

Running test suite

$ make test

Fuzzing

Roundtrip fuzz testing harness is available in fuzz directory. LLVM clang and clang++ compilers are needed to run fuzzing.

The encoder and decoder fuzzers can be run, for 1 million iterations, using target respectively dfuzz and efuzz. The fuzz target runs both.

$ make efuzz    # Run encoder fuzzer for 1M iteration
$ make dfuzz    # Run decoder fuzzer for 1M iteration
$ make fuzz -j  # Run encoder and decoder fuzzers in parallel

Qualification / Conformance

The implementation is qualified under the QDID 194161 as part of Google Fluoride 1.5.

The conformance reports can be found here

Listening Test

The codec was here subjectively evaluated in a blind listening test.

Meson build system

Meson build system is also available to build and install lc3 codec in Linux environment.

$ meson setup build
$ cd build && meson install

Python wrapper

A python wrapper, installed as follows, is available in the python directory.

$ python3 -m pip install .

Decoding and encoding tools are provided in python/tools, like C tools, you can easly test encoding / decoding loop with :

$ python3 ./python/tools/encoder.py <in.wav> --bitrate <bitrate> | \
  python3 ./python/tools/decoder.py > <out.wav>