Technical Editor's Note - How X-Fi works in hardware
Steve's analysis of the X-Fi Elite Pro focusses on the end-user experience and here, for those who want it, is a brief explanation of some of the key technical aspects of Creative's new X-Fi audio processor, which will form the basis of Creative's discrete and embedded audio products for years to come.The first-generation X-Fi audio processor, common to all current X-Fi products, is built on a 130nm silicon process using a fabrication partner Creative has yet to disclose. Using that process, Creative has engineered the X-Fi to carry around 40 million transistors - just a few short of the count in a Northwood core Pentium 4, circa 2003. For a chip dedicated solely to audio processing, it reveals a little of the processing ability that Creative claim the X-Fi to have.
The X-Fi uses an internal ring architecture to allow the internal processing units to talk to each other and pass data around the five processing units joined by audio I/O and transport ring stops on the bus. Each stop on the ring has a bi-directional channel for getting data to the other processing units or the I/O or Transport (used for talking to the X-RAM or the PCI bus).
The five processing units are:
Sample Rate Convertor
Most of the chip is taken up by the Sample Rate Convertor, itself made up of 256 separate sample convertors. This chip is used to transcode sample data into the audio format used within the X-Fi and to the external formats presented out of the chip by the Transport unit.
The Sample Rate Convertor, according to Creative, is capable of over 7G instructions per second at full tilt (depending on the instructions it can execute, of course) - processing the full 256 samples. The convertor first upsamples the audio data, filters it and then downsamples to output the data the chip works on internally during any further processing.
Tank
The Tank is where the X-Fi does its positional audio effects (think EAX) and basic delay-based processing. Around five per cent of the chip's processing ability is spent on the Tank processing unit and it's responsible for the CMSS-3D and positional controls in Game Mode, as Steve showed.
Mixer
As the name suggests, the Mixer unit handles muxing of audio streams being processed by other units on the X-Fi. A good chunk of the total processing power of the X-Fi is contained by the Mixer unit.
Quartet DSP
Finally, the Quartet unit is where the X-Fi's eight channels of audio are processed in real-time by the hardware. The Quartet is split into four essentially stereo processors that deal with data in dual threads - giving eight hardware-accelerated output channels. The Quartet is also programmable, supposedly in an IEEE-compliant fashion, when dealing with the audio data> However it's unclear if Creative will make the Quartet available to programmers to allow them to use it for useful things.
Summary
Basically, at its heart, the X-Fi is a stream processor, much like a GPU is but operating on a completely different type of data. 256 parallel sample convertors feed an eight-way programmable SIMD DSP and, combined with the Tank unit and I/O via a ring bus, the X-Fi has - on paper - all the tools needed to accelerate audio in a more efficient way compared to a CPU. It's overpowered in many respects, and applications to drive the X-Fi to its fullest are some way off.It's hardware that's very limited by the output stage of the board it's placed on, in terms of DACs. Thankfully, Creative seems to be using capable Cirrus DACs and output filters, at least on the high-end boards.
So, it's a big leap for hardware audio processing on a consumer PC, the only barrier being that people are used to getting their sound cheap or free, whether by on-board circuitry or discrete audio boards. Convincing them an X-Fi is worth the money will be a hard task. From a technical standpoint, it's a 40-million-transistor programmable stream processor that happens to be well suited to processing audio. And, it's a nice application of silicon by Creative, very nice indeed.
From my perspective the X-Fi should be highly desirable if you're serious about audio on the PC. That might be as a gamer stepping up from on-board sound or from an Audigy or Audigy 2. Or it might be as a music enthusiast moving up from something like a Terratec DMX Xfire.
