Modesty has never been one of Creative's strong suits, but could there be something to all the X-Fi hype? To find out, we subjected the most affordable X-Fi offering, the $110 Sound Blaster X-Fi XtremeMusic, to a punishing battery of performance, quality, and listening tests against competitors ranging from M-Audio's Revolution 7.1 to integrated "Azalia" High Definition Audio. Read on to see how the XtremeMusic fared and why the X-Fi is such a bold departure from audio chips of old.
The X-Fi architecture
Before exploring where the XtremeMusic fits into the Sound Blaster Xtreme Fidelity family, let's take a moment to explore the 51-million transistor X-Fi audio chip that's at the heart of Creative's new Sound Blasters. Manufactured using 0.13-micron process technology, the chip has roughly half the number of transistors of an Athlon 64 and more than 11 times that of the Audigy, so it's quite a leap from previous generations.
The X-Fi's processing power is divided between five internal units: the sample rate converter, digital signal processor, and mixer, filter, and tank engines. Much of the X-Fi's muscle ripples through a sample rate converter (SRC) that Creative claims pushes over 7000 MIPS. The SRC is actually made up of 256 individual sample rate converters, all of which tackle sampling rate conversions in the same manner. First, the sampling rate of an incoming audio stream is doubled. Next, a poly phase Finite Impulse Response (FIR) filter is used to produce a sampling rate four times greater than the desired output sampling rate. Finally, the sampling rate is reduced by a factor of four for output. According to Creative, this process is nearly transparent, and any loss in quality during sample rate conversions is miniscule compared to the noise generated by even the best DACs available on the market. If you're not convinced, the SRC can be bypassed when it's not needed.
Although the X-Fi's sample rate converter has significantly more processing power than the rest of the chip, it's still only one of five main chip components. The next X-Fi component of interest is the Quartet DSP. Quartet, in this case, refers to the fact that the X-Fi's digital signal processor is made up of four SIMD (Single Instruction, Multiple Data) hardware threads. Each of those hardware threads has two data paths, leading Creative to describe it as TIMD, or Thread Interleaved Multiple Data. Giving each DSP thread dual data paths is a clever way to deal with audio data, which generally arrives in multiples of two. With four "stereo" hardware threads, the DSP can tackle eight data streams at onceperfect for an eight-channel sound card. As one might expect, the Quartet DSP's instruction set is audio-centric. It can handle both fixed and floating-point data types, and Creative claims that the interface is programmer friendly.
The Xi-Fi's SRC and Quartet DSP are undoubtedly the stars of the show, but the chip has several other essential components worth mentioning. More than 1200 MIPS of processing power are dedicated to the X-Fi's mixer, which handles the scaling, combining, and, of course, mixing of audio streams. The "Tank" engine handles all of the X-Fi's delay-based effects, including reverb, chorus, reflections, and inter-aural time delays, while a filtering engine dedicates a couple hundred MIPS to environment modeling, equalizers, and positional 3D audio. The X-Fi also has a transport engine that interfaces with onboard memory and an I/O bus, such as PCI. As you might expect, the chip also has an audio I/O component.
A typical audio card would organize the X-Fi's various components in a line, each leading to the next with little flexibility in the signal path. Creative does things differently with the X-Fi, organizing the various chip components around what the company calls an audio ring. The audio ring is actually a pipelined bus with 4096 audio channels. Those audio channels can be used for not only the front, rear, center, and other external channels, but also as internal pathways for effects generation. Each chip component is free to access and route audio streams around the ring, giving the X-Fi a great deal of flexibility. That flexibility, combined with the all those internal audio channels, is perhaps the most exciting upgrade over Creative's previous audio cards. The Audigy, for example, was limited to 64 internal audio channels, all of which were limited to the effects engine.
Speaking of the Audigy2's effects engine, what would a Creative audio chip be without some measure of EAX support? The X-Fi is equipped to handle Creative's latest, EAX Advanced HD 5.0, with hardware acceleration for a total of 128 voicesdouble that of the Audigy2. Battlefield 2 can already take advantage of the X-Fi's support for extra voices, and most games with 3D audio will take advantage of the X-Fi's hardware EAX acceleration. Oddly, though, Creative is pretty much still the only game in town if you want hardware-accelerated 3D audio. Others seem content with the performance hit associated with having the CPU do all the heavy lifting for positional 3D audio.
While the X-Fi architecture is impressive and its ability to accelerate 3D audio unmatched, one of the chip's more interesting features is that it can be paired with onboard memory that Creative calls X-RAM Onboard memory is nothing new in the sound card world (the AWE32 was available with 2, 4, and 8MB of RAM), but X-RAM is different because it's there specifically to be used by applications. Creative hopes that game developers will utilize X-RAM to cache decompressed audio, potentially saving precious CPU cycles in the process. Developers are also encouraged to use X-RAM to store higher quality audio assets that would otherwise be infeasible to decompress on the fly or stream from the hard drive. At least one developer is already taking advantage of X-RAM, with DICE's Battlefield 2 using it to cache in-game audio.
Unfortunately, X-RAM is only available at the top end of the X-Fi lineup on Fatal1ty FPS and Elite Pro cards that cost $300 and $400, respectively. With X-RAM limited to high-end cards, many developers may not see enough of an installed base to justify dedicating development resources to taking advantage of X-RAM. Only time will tell.
Despite its fancy new architecture, onboard memory, support for Dolby Digital ES and DTS-EX output, and THX certification, the X-Fi won't encode Dolby Digital Live. According to Creative, Dolby Digital Live support would make the X-Fi more expensive due to both licensing costs and the need for additional tank RAM on the chip. Creative also asserts that Dolby Digital Live encoding introduces a small amount of latency, and points out that it's limited to 5.1-channel output. Perhaps more notably, Creative says that Dolby Digital Live encoding can't handle DRM-protected content since it passes unprotected data through a digital output. That's an interesting limitation, although PC enthusiasts have never really been enthusiastic about DRM-encrusted content.
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