liboac

Introduction

liboac is the reference implementation of the Open Audio Codec (OAC) specification.

OAC intends to be the successor to Opus and liboac is based on libopus. Both are works in progress.

This package implements a shared library for encoding and decoding raw OAC bitstreams. It also includes a number of test tools used for testing the correct operation of the library. The bitstreams read/written by these tools should not be used for OAC file distribution: they include additional debugging data and cannot support seeking.

Software License

Modifications and additions made to the software by the Alliance for Open Media are licensed as specified in (COPYING)[./COPYING].

See the git commit logs for a list of modifications.

The libopus COPYING file is preserved as (OPUS_LICENSE)[./OPUS_LICENSE].

Compiling

To build from a distribution tarball, you only need to do the following:

% ./configure
% make

To build from the git repository, the following steps are necessary:

0. Set up a development environment:

On an Ubuntu or Debian family Linux distribution:

% sudo apt-get install git autoconf automake libtool gcc make

On a Fedora/Redhat based Linux:

% sudo dnf install git autoconf automake libtool gcc make

Or for older Redhat/Centos Linux releases:

% sudo yum install git autoconf automake libtool gcc make

On Apple macOS, install Xcode and brew.sh, then in the Terminal enter:

% brew install autoconf automake libtool

1. Clone the repository:

   % git clone https://github.com/AOMediaCodec/oac.git % cd oac

2. Compiling the source

   % ./autogen.sh % ./configure % make

On x86, it's a good idea to use a -march= option that allows the use of AVX2.

3. Install the codec libraries (optional)

   % sudo make install

Once you have compiled the codec, there will be a oac_demo executable in the top directory.

Usage: oac_demo [-e] <application> <sampling rate (Hz)> <channels (1/2)>
         <bits per second> [options] <input> <output>
       oac_demo -d <sampling rate (Hz)> <channels (1/2)> [options]
         <input> <output>

mode: voip | audio | restricted-lowdelay
options:
  -e                : only runs the encoder (output the bit-stream)
  -d                : only runs the decoder (reads the bit-stream as input)
  -cbr              : enable constant bitrate; default: variable bitrate
  -cvbr             : enable constrained variable bitrate; default:
                      unconstrained
  -bandwidth <NB|MB|WB|SWB|FB>
                    : audio bandwidth (from narrowband to fullband);
                      default: sampling rate
  -framesize <2.5|5|10|20|40|60>
                    : frame size in ms; default: 20
  -max_payload <bytes>
                    : maximum payload size in bytes, default: 1024
  -complexity <comp>
                    : complexity, 0 (lowest) ... 10 (highest); default: 10
  -inbandfec        : enable SILK inband FEC
  -forcemono        : force mono encoding, even for stereo input
  -dtx              : enable SILK DTX
  -loss <perc>      : simulate packet loss, in percent (0-100); default: 0

input and output are little-endian signed 16-bit PCM files or OAC bitstreams with simple oac_demo custom framing.

Testing

This package includes a collection of automated unit and system tests which SHOULD be run after compiling the package especially the first time it is run on a new platform.

To run the integrated tests:

% make check

Compiling liboac for Windows and alternative build systems

See cmake/README.md or meson/README.md.

Portability notes

This implementation uses floating-point by default but can be compiled to use only fixed-point arithmetic by setting --enable-fixed-point (if using autoconf) or by defining the FIXED_POINT macro (if building manually). The fixed point implementation has somewhat lower audio quality and is slower on platforms with fast FPUs, it is normally only used in embedded environments.

The implementation can be compiled with any compiler supporting C89 or later. While it does not rely on any undefined behavior as defined by C89 or C99, it relies on common implementation-defined behavior for two's complement architectures:

• Right shifts of negative values are consistent with two's complement arithmetic, so that a>>b is equivalent to floor(a/(2^b)),

• For conversion to a signed integer of N bits, the value is reduced modulo 2^N to be within range of the type,

• The result of integer division of a negative value is truncated towards zero, and

• The compiler provides a 64-bit integer type (a C99 requirement which is supported by most C89 compilers).