OpenAL

History

OpenAL was originally developed in 2000 by Loki Software to help them in their business of porting Windows games to Linux. After the demise of Loki, the project was maintained for a time by the free software/open source community, and implemented on NVIDIA nForce sound cards and motherboards. It is now hosted (and largely developed) by Creative Technology with on-going support from Apple, Blue Ripple Sound, and individual open-source developers.

While the OpenAL charter says that there will be an "Architecture Review Board" (ARB) modeled on the OpenGL ARB, no such organization has ever been formed and the OpenAL specification is generally handled and discussed via email on its public mailing list.

Since 1.1, the implementation by Creative has turned proprietary, with the last releases in free licenses still accessible through the project's Subversion source-code repository. However, OpenAL Soft is a widely used open source alternative.

API structure and functionality

The general functionality of OpenAL is encoded in source objects, audio buffers and a single listener. A source object contains a pointer to a buffer, the velocity, position and direction of the sound, and the intensity of the sound. The listener object contains the velocity, position and direction of the listener, and the general gain applied to all sound. Buffers contain audio data in PCM format, either 8- or 16-bit, in either monaural or stereo format. The rendering engine performs all necessary calculations as far as distance attenuation, Doppler effect, etc.

The net result of all of this for the end user is that in a properly written OpenAL application, sounds behave quite naturally as the user moves through the three-dimensional space of the virtual world. From a programmer's perspective, very little additional work is required to make this happen in an existing OpenGL-based 3D graphical application.

Unlike the OpenGL specification, the OpenAL specification includes two subsections of the API: the core consisting of the actual OpenAL function calls, and the ALC (Audio Library Context) API which is used to manage rendering contexts, resource usage and locking in a cross platform manner. There is also an 'ALUT' (Audio Library Utility Toolkit) library that provides higher level 'convenience' functions — exactly analogous to OpenGL's 'GLUT'.

In order to provide additional functionality in the future, OpenAL utilizes an extension mechanism. Individual vendors are thereby able to include their own extensions into distributions of OpenAL, commonly for the purpose of exposing additional functionality on their proprietary hardware. Extensions can be promoted to ARB (Architecture Review Board) status, indicating a standard extension which will be maintained for backwards compatibility. ARB extensions have the prospect of being added to the core API after a period of time.

For advanced digital signal processing and hardware-accelerated sound effects, the EFX (Effects Extension) or environmental audio extensions (EAX) can be used.

Limitations

The single listener model in OpenAL is tailored to a single human user and is not fit for artificial intelligence or robotic simulations or multiple human participants as in collaborative musical performances. In these cases a multiple listener model is required. OpenAL also fails to take into account sound propagation delays (the speed of sound is used for the Doppler effect only). The distance to a sound source only translates into an amplitude effect (attenuation) and not a delay. Hence OpenAL cannot be used for time difference of arrival calculations unless that functionality is added in separately.

Supported platforms

The API is available on the following platforms:

Operating systems:

1. Android (supports OpenSL ES)
2. AmigaOS 3.x and 4.x
3. Bada
4. BlackBerry 10
5. BlackBerry PlayBook
6. BSD
7. iOS (supports Core Audio)
8. IRIX
9. Linux (supports ALSA, OSS, PortAudio and PulseAudio)
10. Mac OS 8, Mac OS 9 and Mac OS X (Core Audio)
11. Microsoft Windows (supports DirectSound, Windows Multimedia API and Windows Multimedia Device (MMDevice) API)
12. MorphOS
13. OpenBSD
14. Solaris
15. QNX
16. AROS

Gaming devices:

1. GameCube
2. PlayStation 2
3. PlayStation 3
4. Xbox
5. Xbox 360
6. Wii
7. PlayStation Portable

Games

The following video games are known to use OpenAL:

Other applications

Blender – 3D modelling and rendering tool uses OpenAL for its built-in game engine

3DMark06 – Gamer's benchmarking tool

Dolphin (emulator) – GameCube and Wii emulator

Vanda Engine – uses OpenAL 1.1 to simulate 2D and 3D sounds

Croquet Project

Implementations

OpenAL SI

The OpenAL Sample Implementation is the original implementation, from Loki, and is not currently maintained.

OpenAL Soft

OpenAL Soft is an LGPL-licensed, cross-platform, software implementation. The library is meant as a compatible update/replacement to the deprecated OpenAL Sample Implementation, as well as a free alternative to the now-proprietary OpenAL. OpenAL Soft supports mono, stereo, 4-channel, 5.1, 6.1, 7.1 and HRTF output.

AeonWave-OpenAL

AeonWave-OpenAL is an LGPL-licensed OpenAL emulation layer that takes advantage of the hardware acceleration provided by the non-free but low cost AeonWave 4D-audio library for Linux and Windows made by Adalin B.V.. The author claims that AeonWave-OpenAL implementation renders 3D audio five (on an AMD Athlon 64 X2) to seven (on an Intel Atom N270) times faster than either OpenAL SI or OpenAL Soft under the same conditions. By using the AeonWave library this implementation supports HRTF as well as spatialised surround sound for up to eight speakers.

Rapture3D OpenAL Driver

The Rapture3D OpenAL Driver is a non-free, commercial, Windows only, software implementation made by Blue Ripple Sound. The library is intended as a high performance drop-in replacement for other implementations. It features:

32bit floating point audio path.

High quality sample rate conversion (used for various purposes including Doppler shift).

High quality effects and filters.

Directional sound sources.

Support for multi-channel sound sources (including assets encoded using Ambisonics).

The only limit on the number of sources or effects is CPU power, can render hundreds of sound sources and multiple effects on relatively old hardware.

Higher-order Ambisonics (HOA) bus running at up to fourth order.