Although each logical device can have only one audio engine—LFX sAPO, and GFX sAPO—the system can support multiple logical devices. Each has its own audio engine and LFX and GFX sAPOs. Figure 2 shows a system with three logical devices.
Figure 2. Windows Vista audio architecture with multiple devices
An audio session is a group of related audio streams that a client can manage collectively. Each session represents a subset of the streams that form the global mix that plays through a particular audio device such as headphones. The global mix combines all of the sessions from all of the applications. Clients control the volume level and mute state of each individual session, and the system applies these settings uniformly to all of the streams in the session.
Typically, a session consists of one or more streams from a single process. However, applications can define cross-process sessions that combine streams from two or more processes. Figure 3 shows how sAPOs are integrated into audio sessions. Note that:
Each audio stream has its own instance of the LFX sAPO.
Streams from different processes can belong to the same session.
Multiple streams from the same process can belong to the same or different sessions.
The streams for all the device’s sessions pass through a single GFX.
Figure 3. Audio session architecture
For further information on audio sessions and the core audio API, see the white paper titled Device Finish-Install Actions in Windows Vista.
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