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DailyTech examines the audio stack in Windows Vista

Microsoft’s long-awaited Windows Vista operating system has plenty of underlying changes. One of the major changes to Windows Vista is a brand new audio stack dubbed Universal Audio Architecture (UAA). UAA completely revamps the way Windows Vista communicates with audio devices and offers a basic audio driver for all UAA compliant devices. Microsoft also introduced early UAA functionality in Windows XP when high definition audio (Intel Azalia) was released a couple years ago. Current UAA compliant audio devices include integrated high definition audio, USB audio and IEEE 1394 AV/C devices.

Basic UAA drivers integrated in Windows Vista are not as feature-less as one would imagine. The basic functionality of UAA-class drivers includes support for 24-bit/96 KHz audio playback and recording across two-channels and six-channel audio support. An early white paper on UAA touts the following benefits:
  • Simpler installation of audio peripherals. The operating system can detect and configure a UAA-compliant audio device when it is connected to the system, without requiring the user to find and load a driver.
  • Performance advantages. UAA class drivers are designed to consume a minimum amount of CPU time during streaming and to take advantage of increased bandwidth in hardware that support data rates comparable to high-end consumer electronics.
  • Glitch-resilient audio. UAA class drivers are designed to follow the planned Vista API real-time coding guidelines for glitch-resilient audio.
  • Security for protected content. UAA class drivers support current and planned content protection technologies in Windows.
Microsoft’s basic UAA driver in Windows Vista provides the above-mentioned features. The default UAA driver automatically installs for USB, IEEE 1394 and integrated high definition audio devices. During the device initialization process the UAA driver works with the Microsoft Bus Driver to figure out the capabilities of the installed codec. From there the audio driver makes the supported features, such as the amount of inputs and outputs, available to the operating system.

DailyTech was able to speak to Sandra Perry, product line manager, Integrated Audio Group, Analog Devices, Inc, regarding the new UAA. "Although it hasn't gotten much publicity, Microsoft made many changes in the audio stack for Vista, and increased the performance requirements for the audio sub-system. This required significant new driver development, but the end result will be the ability to provide a higher quality motherboard audio. Analog Devices looks forward to giving customers an enhanced audio experience under Vista” said Perry.

The new Universal Audio Architecture makes things harder for hardware DSP manufacturers to produce drivers though. As UAA provides basic software-rendered audio functionality, sound cards that have hardware acceleration will require completely new drivers. This includes sound cards such as the Creative Labs Sound Blaster Audigy and X-Fi line of sound cards. When sound card or motherboard manufacturers provide hardware DSPs on sound cards or motherboards, the device must support the UAA class driver for basic functionality.

This is where things become different for hardware DSP manufacturers. Hardware DSP equipped sound cards require a separate driver for the hardware DSP features itself. Windows Vista treats the hardware DSP as an independent device, separate from basic audio output functionality. Creative Labs opted not to partake in creating the UAA specification and was unable to create drivers capable of taking advantage of the hardware DSPs in its Sound Blaster Audigy and X-Fi line. Nevertheless, Creative Labs managed to dodge UAA hardware DSP driver requirement with its ALchemy Project.

It is a bit of a touchy subject, but UAA supports audio content protection schemes. This should allow PC systems to playback protected audio content such as DVD Audio discs. Unfortunately, Realtek is the only provider of hardware content protected high definition audio solutions.

Basic driver aside, the default UAA driver provides a few features not found in drivers released by audio device and codec manufacturers. New features in the UAA driver include bass management, speaker fill, automatic room correction, virtual surround sound, phantom surround, headphone virtualization, loudness equalization and support for microphone arrays.

Bass management is unavailable with the basic Windows Vista UAA driver but the option is available for codec and sound card manufacturers to implement. As with A/V receivers, the Windows-level bass management allows users two options – forward or reverse bass manage. In forward bass management mode Windows can output a full-range signal to large front speakers and a subwoofer or a filtered signal to small speakers and a subwoofer.

In reverse bass management mode, where the PC lacks a subwoofer, Windows will distribute the subwoofer (LFE) signal accordingly to each channel. While most PC speakers and A/V receivers have built in crossovers or bass management, Windows Vista’s bass management will be particular useful in upcoming AMD Live! and Intel Viiv systems – especially those with integrated audio amplifiers such as the AMD Live! Home Cinema systems.

Speaker fill is simply another name for up converting a traditional stereo audio source to output to multiple speakers. Audio purists will scoff, but those that enjoy up converted multi-channel audio will appreciate this feature -- especially users with multi-channel speakers that lack built-in up converting algorithms. Every company has its own methods of up converting stereo sources, Microsoft has opted to use channel manipulation and delays to create a multi-channel audio effect. DailyTech attempted to test out the speaker fill option with Realtek ALC882-based high definition audio codec without luck.

Automatic room correction is a new addition that will become a vital part of AMD Live! Home Cinema systems. As the AMD Live! Home Cinema systems feature integrated amplifiers, automatic room correction will calibrate the speakers so all sound will reach a sweet spot at the same time. A microphone is required to take advantage of automatic room correction features. Using a standard microphone, Windows Vista outputs generated sounds to determine the distance each speaker is away from the sweet spot. From there, time delays and volume levels of each speaker are calculated and adjusted accordingly.

Systems that connect to stereo receivers or speakers via standard red/white analog cables can take advantage of the new virtual surround features. Multi-channel sources are down-mixed into a stereo signal with virtual surround. The down-mixed signal is Dolby Pro Logic compatible, a feature most multi-channel receivers support.

Speaker phantoming is available for users that lack six-channel surround sound speakers. Using speaker phantoming, users with 4.1 speaker systems can enjoy a virtualized center channel for a virtualized 5.1 experience.

Headphone virtualization is a feature targeted towards headphone users that want virtualized surround sound. Microsoft’s headphone virtualization implementation takes advantage of Head Related Transfer Functions (HRTF) technology to enable virtual surround sound through stereo headphones. Headphone virtualization can be enabled when headphones are plugged into the headphone jack.

Lastly on the audio output enhancements is the loudness equalization feature. This feature aims to fix volume level inconsistency between different content sources. Microsoft aims to fix volume level inconsistencies using an equalization technique that simulates human hearing and dynamically adjust gain levels accordingly.

On the new audio input features side of things is native support for microphone arrays. Microsoft caters this feature towards VoIP users for greater voice clarity. With native microphone array support, system manufacturers can integrate microphone arrays in monitors, laptops and other devices.

In addition to the microphone array support, Windows Vista UAA supports the following features:
  • Improved acoustic echo cancellation
  • Microphone array support
  • Stationary noise suppressor
  • Automatic gain control
  • Wideband quality of sound capturing and processing
As with all new operating systems, it will take a while before manufacturers will implement all new audio features in its drivers and hardware. Nevertheless, with the new Universal Audio Architecture Microsoft has raised the bar of basic audio devices.

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