btstack/3rd-party/lc3-google/README.md

# 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_](https://www.bluetooth.org/DocMan/handlers/DownloadDoc.ashx?doc_id=502107&vId=542963)

- LC3 Plus is defined by [_ETSI TS 103 634_](https://www.etsi.org/deliver/etsi_ts/103600_103699/103634/01.04.01_60/ts_103634v010401p.pdf)

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.

```sh
$ 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:

```sh
$ 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.

```sh
$ 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:

```sh
# apt install clang lld
```

The webasm object is compiled using:
```sh
$ make CC="clang --target=wasm32"
```

## Tools

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

```sh
$ 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 :

```sh
$ 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 :

```sh
$ 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

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

#### Running test suite

```sh
$ 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.

```sh
$ 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_](https://launchstudio.bluetooth.com/ListingDetails/160904) as part of Google Fluoride 1.5.

The conformance reports can be found [here](conformance/README.md)

## Listening Test

The codec was [_here_](https://hydrogenaud.io/index.php/topic,122575.0.html)
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.

```sh
$ meson setup build
$ cd build && meson install
```

## Python wrapper

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

```sh
$ python3 -m pip install .
```

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

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