The Story of OpenAL

Open Source and Open Standards: Chapters One through Three. A voyage through one of Loki's free software projects.

At Loki Entertainment Software, we deal with lots of different kinds of software. From public domain, to BSD-licensed source, to Free Software under GPL and LGPL, to proprietary code under contract and NDAs, to closed source hardware drivers, we encounter it all on a daily basis. Moreover, we also write a variety of source. The games we port to Linux are not open source, but our own work is available as free software in projects like part of projects like Fenris, Setup, SMPEG or SDL.

Crossing the fences from all sides over and over again, and getting to know both the concerns of free software and open-source advocates as well as the concerns of publishers and developers, our priorities have changed. As important as open-source and free software are, one of our projects in particular developed out of the recognition that there is something even more important: open, well-documented standards. That project is OpenAL, and its history so far is a fair bit of education on how important standards are—and how difficult it is to create them.

A Case of Need

With the Linux ports of Heretic2 and Heavy Gear 2, Loki encountered engines that used spatialized sound above and beyond left-right panning. Heretic2 for a time served as a showcase for audio hardware manufacturers: Aureal's A3D and Creative's EAX, specifically, were both supported. Both competitors at that time had ventured beyond the DirectSound3D feature set and were following very different approaches to spatialized sound. Windows game developers, caught between three different APIs (not counting the large number of commercial audio SDKs offered for Windows), more often than not chose to ignore the issue altogether and refrained from implementing advanced features (unless they decided to go with one of the SDK offerings that promised to provide a convenient abstraction). The manufacturers stepped up and in many cases guided the developer's hand in adding EAX or A3D support to their engines. Spatialized 3-D sound became a checkbox item, a feature to be listed on the game box, and for a while the Windows game developers were offered an easy ride.

Unfortunately, the situation for Linux users was quite different. Driver support for the SB16 de facto standard had stabilized, and PCI sound cards, all in all, worked reasonably well under Linux. Yet, no ALSA or OSS-proprietary or open-source support for the most advanced features and boards was readily available.

Developers targeting Apple users found themselves in a similar situation—while the multimedia APIs on Mac OS might be more sophisticated and complete than the Linux solutions, they were even less compatible with DirectSound and equally oblivious to EAX and A3D features. Windows developers who cared about portability faced an audio feature set full of shining promises, but without a portable API. Given the vastly different approaches of the two main competitors, things were bound to get worse—at a time when Microsoft didn't even provide DirectSound3D for the NT platform that many developers preferred.

Creative, a dominant force in the audio hardware market, found itself challenged by Aureal, a company that attempted to take its experience with high-end spatialized sound solutions into the PC consumer market. Aureal proposed “wavetracing”, a proprietary approach to generating spatialized sound by simulation of sound propagation and acoustic properties of a scene—essentially, by submitting scene geometry to the sound driver. It is important to understand that to a much larger degree than graphics, audio technology was, and still is, software-based. Both the viable market for SDKs, like Miles Sound System or QSound's QMixer, and the evolution of the competing hardware drivers are clear indications of this. Aureal's advantage was their knowledge of simulation, the calculation of early reflections and a lot of optimization work for their drivers. Their disadvantage was the PC consumer market, an environment in which cheap speakers are added to budget solutions as an afterthought. Moving high-end audio processing out of the laboratory and away from the individual Head-Related Transfer Functions (HRTFs), on a desktop where most users and most developers didn't really spend much thought on spatialization, proved to be extraordinarily difficult and, as we now know, ultimately fatal.

Despite operating at diminishing returns for all their efforts, Aureal managed to accomplish one thing: to awaken interest and tentative user demand for better 3-D audio in games. For early adopters, it became a selling point, and publishers and developers followed eventually. Creative, which had their own road map to future audio features, countered by emphasizing their own extended feature set—a much simpler solution focused on stochastic reverberation to emulate the effect of late reflections. In a way, the early and late reflection solutions are complements but the API's and implementations were fundamentally different, and Creative had picked well: EAX was easier and simpler to use, and for all the lack for simulation accuracy, stochastic reverb proved to be a much more effective audio cue for the noisy, low-end speaker desktop.

Both companies were considering Linux initiatives. On one hand, Aureal had a complete software implementation of “wavetracing”, eventually marketed as “A2D” to support the A3D feature set on their competitors' sound cards that could have been the base of a proprietary or even open-source sample implementation. On the other hand, Aureal's code base was a heavily optimized assembly, and a Linux port was not a foregone conclusion. Creative, on the other hand, recruited Linux coders to develop drivers in-house and looked for ways to support EAX under Linux. Both APIs were written with respect to Microsoft's DirectX and COM, but Aureal's geometry-based A3D API had also been inspired by the OpenGL API. This was one of many examples on the influence of OpenGL on the development of OpenAL.

Enter the game developers.


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