Windows Vista (formerly codenamed Windows "Longhorn") has many significant new features compared with previous Microsoft Windows versions, covering most aspects of the operating system. In addition to the new user interface, security capabilities, and developer technologies, several major components of the core operating system were redesigned, most notably the audio, print, display, and networking subsystems; while the results of this work will be visible to software developers, end-users will only see what appear to be evolutionary changes in the user interface. As part of the redesign of the networking architecture, IPv6 has been incorporated into the operating system, and a number of performance improvements have been introduced, such as TCP window scaling. Prior versions of Windows typically needed third-party wireless networking software to work properly; this is no longer the case with Windows Vista, as it includes comprehensive wireless networking support. For graphics, Windows Vista introduces a new as well as major revisions to Direct3D. The new display driver model facilitates the new Desktop Window Manager, which provides the tearing-free desktop and special effects that are the cornerstones of the Windows Aero graphical user interface. The new display driver model is also able to offload rudimentary tasks to the GPU, allow users to install drivers without requiring a system reboot, and seamlessly recover from rare driver errors due to illegal application behavior. At the core of the operating system, many improvements have been made to the memory manager, process scheduler, heap manager, and I/O scheduler. A Kernel Transaction Manager has been implemented that can be used by data persistence services to enable atomic transactions. The service is being used to give applications the ability to work with the file system and registry using atomic transaction operations.
Audio Windows Vista features a completely re-written audio stack designed to provide low-latency 32-bit floating point audio, higher-quality digital signal processing, bit-for-bit sample level accuracy, up to 144 dB of dynamic range and new audio APIs created by a team including Steve Ball and Larry Osterman. The new audio stack runs at user level, thus reducing impact on system stability. Also, the new Universal Audio Architecture (UAA) model has been introduced, replacing WDM audio, which allows compliant audio hardware to automatically work under Windows without needing device drivers from the audio hardware vendor. There are three major APIs in the Windows Vista audio architecture:
Windows Audio Session API – Very low-level API for rendering audio, render/capture audio streams, adjust volume etc. This API also provides low latency for audio professionals through WaveRT (wave real-time) port driver. Multimedia Device API – For enumerating and managing audio endpoints. Device Topology API – For discovering the internals of an audio card's topology.
Audio stack architecture
Applications communicate with the audio driver through Sessions, and these Sessions are programmed through the Windows Audio Session API (WASAPI). In general, WASAPI operates in two modes. In exclusive mode (also called DMA mode), unmixed audio streams are rendered directly to the audio adapter and no other application's audio will play and signal processing has no effect. Exclusive mode is useful for applications that demand the least amount of intermediate processing of the audio data or those that want to output compressed audio data such as Dolby Digital, DTS or WMA Pro over S/PDIF. WASAPI exclusive mode is similar to kernel streaming in function, but no kernel mode programming is required. In shared mode, audio streams are rendered by the application and optionally applied per-stream audio effects known as Local Effects (LFX) (such as per-session volume control). Then the streams are mixed by the global audio engine, where a set of Global Audio Effects (GFX) may be applied. Finally, they're rendered on the audio device. After passing through WASAPI, all host-based audio processing, including custom audio processing, can take place. Host-based audio enhancement or sound effect processing modules are referred to as Audio Processing Objects, or APOs. All these components operate in user mode, only the audio driver runs in kernel mode. In exclusive mode, some APOs are bypassed. The Windows Kernel Mixer (KMixer) is completely gone. The WASAPI mixer (audiodg.exe) is running in user mode. DirectSound and MME are emulated as Session instances rather than being directly connected to the audio driver. This does have the effect of preventing DirectSound from being hardware-accelerated, and completely removes support for DirectSound3D and EAX extensions, however APIs such as ASIO and OpenAL are not affected.
Audio performance
Windows Vista also includes a new Multimedia Class Scheduler Service (MMCSS) that allows multimedia applications to register their time-critical processing to run at an elevated thread priority, thus ensuring prioritized access to CPU resources for time-sensitive DSP processing and mixing tasks. For audio professionals, a new WaveRT port driver has been introduced that strives to achieve real-time performance by using the multimedia class scheduler and supports audio applications that reduce the latency of audio streams. All the existing audio APIs have been re-plumbed and emulated to use these APIs internally, all audio goes through these three APIs, so that most applications "just work".
Issues A fault in the MME WaveIn/WaveOut emulation was introduced in Windows Vista: if sample rate conversion is needed, audible noise is sometimes introduced, such as when playing audio in a web browser that uses these APIs. This is because the internal resampler, which is no longer configurable, defaults to linear interpolation, which was the lowest-quality conversion mode that could be set in previous versions of Windows. The resampler can be set to a high-quality mode via a hotfix for Windows 7 and Windows Server 2008 R2 only.
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