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Remixed audio cards in Windows Vista


Asus' Xonar D2X and Auzentech's Prelude 7.1 go head-to-head
— 11:50 PM on January 14, 2008

Creative has all but dominated the PC sound card market, in no small part thanks to its prowess with hardware-accelerated 3D audio. The EAX positional audio framework born from the SoundBlaster line became the de facto standard for 3D audio in games, giving Creative a distinct advantage over its rivals. Creative licensed EAX, of course, but it kept competitors at version 2.0, limiting them to 32 concurrent 3D voices and standard-definition sampling rates and resolutions. Meanwhile, Creative extended EAX to version 5.0 with support for 128 simultaneous voices and high-definition resolutions and sampling rates.

Despite its positional audio dominance, Creative has faced increased competition of late, largely from sound cards based on C-Media's Oxygen HD audio chip. Microsoft's decision to drop hardware acceleration for DirectSound 3D audio in Vista has also posed a challenge to the SoundBlaster monopoly by blunting some of EAX's appeal. All the while, motherboard makers have diligently worked to improve the quality of onboard audio solutions, creating a perfect storm that has spawned more PC audio choices than we've ever seen before.

Interestingly, two of the most recent additions to the sound card ecosystem use existing audio chips. Asus' new Xonar D2X, for example, is based on a tweaked version of C-Media's Oxygen HD. More interestingly, it comes with a PCI Express interface, finally providing fodder for the scores of empty PCIe x1 slots that dot the enthusiast landscape. On the other side of the fence we have Auzentech's X-Fi Prelude, which is the first third-party card to employ Creative's X-Fi silicon. The Prelude uses a custom board design and upgraded components in an attempt to wring better sound quality from the already impressive X-Fi audio chip.

The Xonar and Prelude both target the high end of the desktop sound card spectrum, so it's only fitting that we face them off against each other in Windows Vista. Read on to see how they fare in a range of gaming, signal quality, and subjective listening tests.


Vista's new Universal Audio Architecture
Before we get into the cards, we should discuss how Windows Vista has changed the audio landscape. Vista includes an all-new audio stack, dubbed the Universal Audio Architecture, which Microsoft wrote new from the ground up with an eye toward improving stability. Windows XP's audio stack ran in kernel mode—the very heart of the OS—making it possible for buggy drivers or other issues to bring down the entire system. In fact, according to 20-year Microsoft veteran Larry Osterman, who worked on the Vista audio engine, "the amount of code that runs in the kernel (coupled with buggy device drivers) causes the audio stack to be one of the leading causes of Windows reliability problems."

To avoid audio stack-related stability issues, UAA was designed to run in user mode rather than kernel mode, putting it at arm's length from the core of the operating system. If UAA crashes, either due to buggy drivers or other issues, the rest of the operating system shouldn't be affected.

An artifact of this new audio stack is the removal of the hardware abstraction layer for DirectSound, distancing the API from a system's sound card. Without a hardware abstraction layer, DirectSound calculations can't be offloaded to an audio chip, effectively killing hardware acceleration for DirectSound. Instead, DirectSound landscapes are pieced together within the UAA's software audio mixer.

The UAA's lack of a hardware abstraction layer for DirectSound also affects Creative's Environmental Audio Extensions (EAX). Because it's an extension of DirectSound 3D, EAX's access to hardware acceleration has been cut off by the Universal Audio Architecture.

Hardware-accelerated audio isn't completely out of the question in Vista, though. Third-party APIs like OpenAL, which doesn't rely on DirectSound, still have direct access to hardware. OpenAL is far less common than DirectSound and Creative's near-ubiquitous EAX, of course, but it's starting to see more widespread support. Bioshock, Call of Duty 4, and Quake Wars all support OpenAL, for example.

OpenAL has also been used by Creative to port EAX to Vista. The company has released ALchemy software that effectively serves as a DirectSound wrapper, converting DirectSound calls for EAX to OpenAL calls that can then be handled in hardware by sound cards based on X-Fi and Audigy audio chips. ALchemy is a free download for X-Fi owners, but those equipped with older Audigy cards have to shell out $10 for an Audigy version of the ALchemy package.

Vista's Universal Audio Architecture hasn't just affected hardware acceleration for DirectSound. The new stack also enables a number of new features, including automatic port detection, per-application volume control, and a protected audio path that allows for a whole new world of insidious Digital Rights Management schemes.

Comparing the cards
Now that we've addressed Vista's new audio architecture, it's time to move onto the cards we'll be looking at today. However, before we dig too deep into specifics of the Xonar D2X and X-Fi Prelude, let's take a moment to compare a few key specifications.

One of the biggest differences between these cards is the audio chips that they use; the Prelude employs Creative's X-Fi audio processor, while the Xonar uses a tweaked version of C-Media's Oxygen HD. We've covered both chips in depth before, so I won't spend too much time dwelling on their capabilities. For a more detailed look at each, I suggest reading our initial review of the X-Fi audio chip and our Oxygen HD sound card comparison.

Asus Xonar D2X Auzentech X-Fi Prelude
Audio chip C-Media Oxygen HD Creative X-Fi
Digital-to-analog converter TI Burr Brown PCM1796 AKM AK4396VF
Analog-to-digital converter Cirrus Logic CS5381 AKM AK5394AVS
Maximum recording quality 24-bit/192kHz 24-bit/96kHz
Maximum playback quality 24-bit/192kHz 24-bit/96kHz (multichannel)
24-bit/192kHz (stereo)
Signal-to-noise rating 118dB 120dB*
Output channels 7.1 7.1
Multichannel digital output Dolby Digital Live, DTS Dolby Digital Live, DTS**
Interface PCI Express x1 PCI
Street price
*Auzentech lists 120dB as a "parts spec" but doesn't give an SNR for the card as a whole
**DTS encoding is scheduled for a Q1 driver release

One of the key differences between the X-Fi and Oxygen HD is their support for 3D positional audio. Like most other audio chips, the Oxygen HD lacks hardware acceleration for DirectSound, and it's limited to 32 concurrent voices via EAX 2.0. The X-Fi, on the other hand, is capable of offloading up to 128 voices in hardware via EAX Advanced HD 5.0. EAX 5.0 can also handle higher definition sampling rates and resolutions than 2.0, giving the X-Fi a distinct edge. We've seen that advantage play out in Windows XP, where cards based on the X-Fi offer higher in-game frame rates than those based on the Oxygen HD.

Rather than focusing on hardware-accelerated positional audio, the Oxygen HD uses its horsepower to drive real-time DTS and Dolby Digital Live encoding. This allows the Xonar to take any multi-channel audio content—be it from a game or media playback application—and output it via a single digital connection rather than a collection of analog cables. You'll need a compatible receiver or set of digital speakers to take advantage of this capability, though.

The X-Fi audio chip is also capable of outputting DTS multi-channel audio, but it can't encode on the fly. Without real-time encoding capabilities, the X-Fi is only capable of passing along an existing DTS multi-channel track. That's fine for DVD movies that include a pre-encoded tracks, but it's essentially useless for games, which do not.

Auzentech actually makes sound cards based on the Oxygen HD, so they're familiar with the chip's real-time encoding capabilities. In fact, they consider those capabilities so important that they're bringing them to the Prelude. With the latest drivers, the Prelude is capable of encoding Dolby Digital Live in real time. A driver revision that includes DTS encoding capabilities is also due out in the first quarter of this year.

Moving away from multi-channel, note that the Xonar is capable of playing back and recording 24-bit audio at up to 192kHz. The Prelude also supports audio at up to 24 bits, but the card's sampling rates top out at 96kHz for both multichannel playback and recording. These limitations are inherited from the X-Fi audio chip, whose support for 192kHz sampling rates is restricted to stereo playback. Don't worry, though; even DVD-Audio tops out at 24 bits and 96kHz for multichannel playback, only supporting 192kHz for stereo playback, which the Prelude can handle.