XAudio and XAudio2

If DirectSound has been updated, then why create a new game audio API? The answer goes back to both the industry changes and the design planning for Xbox 360.

When Xbox 360 was in its early design phases, we knew that audio processing (except for data compression) was moving to an all-software model. Drastic increases in processing power, combined with the flexibility and ease of simply writing C code to do any arbitrary signal processing, made that clear. What wasn’t clear was how to make it easy and straightforward for developers to harness the flexibility that the game sound designers and composers wanted for modern video games.

We took a hard look at DirectSound and realized that DirectSound was really past its prime; another facelift wasn’t going to cut it for Xbox 360. DirectSound’s metaphor of one-buffer-one-sound wasn’t up to the job for today’s game audio any more than a graphics sprite engine would be up to the job for today’s 3D graphics.

We needed a new audio API, designed from the ground up as a programmable software audio engine. Armed with our vision of what we wanted to enable, along with several years worth of suggestions from the community, we created XAudio for Xbox 360.

Concurrent with development of Xbox 360, Windows Vista was well into its own development. The Windows team developed a whole new audio architecture, top to bottom. A key component in that audio architecture was LEAP, Longhorn Extensible Audio Processor. (Longhorn was the codename for Windows Vista.)

LEAP is a very powerful low-level graph-building audio architecture that offers submixing, software-based DSP effects, and efficient design—all of the things that XAudio for Xbox 360 was designed to do. Although LEAP itself was not released as an API directly to developers, it serves as the underlying audio engine for both DirectSound on Windows Vista and the Microsoft Cross-Platform Audio Creation Tool (XACT) on both Windows XP and Windows Vista.

Well…. Xbox 360 launched, Windows Vista shipped, and we turned ourselves back to the notion of a cross-platform, low-level replacement for DirectSound. We had received good feedback on our XAudio API from Xbox 360 developers.

At the same time, the LEAP architecture lent itself to a more streamlined and efficient engine with a few features that XAudio didn’t provide, like multi-rate graph support. We took the best of both worlds, and XAudio2 was born, taking the philosophy and style of the XAudio API design coupled with the streamlined engine and features of LEAP.

So what is XAudio2? Simply put, it is a flexible, cross-platform, low-level audio API designed to replace DirectSound for game applications on Windows and replace XAudio on Xbox 360. It allows game designers to create audio signal processing paths that range from simple wave playback to complicated audio graphs with submixes and embedded software-based DSP effects.

XAudio2 Features and Architecture

XAudio2 provides many features necessary for the creation of modern game sound design:

• Cross platform between Xbox 360 and Windows-based platforms, including Microsoft Windows XP and Windows Vista
• Arbitrary levels of submixing
• A simple streaming model
• A software-based, dynamic DSP effects model, both local and global
• Simple C language code for arbitrary DSP processing using xAPOs (audio processing objects)
• Native compressed data support: XMA and xWMA on Xbox 360, ADPCM and xWMA on Windows
• A complement of DSP audio effects
• Fully transparent surround sound/3D audio processing
• Clean separation of voices from data
• Non-blocking processing suitable for multi-core, multi-threaded systems
• Efficient and optimized for Windows and Xbox 360
• Optimized in-line filter on each voice