APU: The First Audio 3D Chip In A Console

The sound on the MCP 420-D Media and Communication Processor for the nForce platform is now called MCPX. Even though an Intel processor is used, the IGP and MCPX are interconnected with the HyperTransport bus from AMD, which offers a bandwidth of 800 Mb/s. This high speed channel opens communication for the MCP to the 64 Mb of memory without slowing down the system and gives optimum use of MCPX functionalities (sound, network, hard disk controller).

Sound capacities of MCPX:

• 64 audio channels (up to 256 stereo channels)
• Support for DSL2 libraries
• Real time AC3 encoding

As a comparison, Playstation 2 offers:

• 48 audio channels plus the software channels
• 2 MB of memory

The technology used for 3D positioning in the Xbox is offered by Sensaura. It offers many algorithms for reverberation, reflections, environment, etc. Remember that Sensaura is the big competitor of Sound Blaster's EAX technology. The APU (audio processor unit) in the Xbox can compute all the effects offered by the Sensaura library in real time (http://www.sensauradeveloper.com/ ). Thanks to the DirectSound 3D compatibility, most of the available audio tools can be used.

However, the most interesting aspect of the APU in the Xbox is its capacity to do 5.1 encoding in real time. So, once the audio positional effects have been computed within the APU, the unit encodes them in AC3 format for the best positioning. However, without the external Dolby Digital decoder, the APU downsamples to stereo on two channels. During our audio tests, we noticed that the quality was nowhere near the quality offered by the EAX library for Live or Audigy cards (which often goes unexploited anyway). The CPU usage is very low. And the use of one or another audio 3D positioning library will have to change as the algorithms change.

In comparison, the sound on a PC is a lot more flexible because 3D positioning can happen with 4.1 speaker system kits, and without the use of an external Dolby Digital. In the Xbox, as opposed to the stereo, the only output available is in optical AC3. That's too bad, but it would have been difficult to place outputs for 6 speakers. Remember that the console fits in a TV hi-fi environment, and the explosion of home cinema already allows an installed system of audio-video amplifiers. Of course, it is easier to connect four speakers and a bass-box with the cable mess surrounding the PC than on the Xbox, which is ideally for the living room. Dolby Digital output was a must with the DVD playback allowed on the Xbox.

The Xbox is the first console that really uses sound positioning and integrates all the 3D effects. The audio processor in the Xbox is so interesting that there are rumors that a similar PCI audio card will be released. In the first games using the 3D sound of the Xbox, such as Halo, Max Payne, Project Gotham or Amped, the sound immersion is amazing and gives console gaming a new dimension.