3DsoundSurge Review: NVIDIA nForce APU

NVIDIA has done such an excellent job on the nuts and bolts of all the nForce has to offer that we will leave much of the gory details to the materials on their site (linked below). However, a basic understanding of three terms will be critical to understanding what the nForce audio brings to the table and what you are getting if you purchase an nForce motherboard. The terms are APU, SoundStorm and DICE.

Lets start with the core – the APU (http://www.nvidia.com/view.asp?IO=apu). The APU, also found in Microsoft's Xbox, is an integral part of the first- and second-generation nForce media and communications processors. Utilizing five DSPs (three fixed, two programmable) dedicated to audio processing, the NVIDIA nForce APU is both the muscle and brains behind the nForce audio system.

Not all motherboards have the APU enabled, and not all MCPs contain the APU even though they appear similar in physical appearance. Today there are essentially three parts that NVIDIA ships: MCP, MCP-D and MCP-T. Here's the breakdown:

Because there is licensing cost (Dolby) in the APU part, and in the MCP2-T there is also additional technologies (3Com Ethernet and Firewire) there is a cost difference between the parts. So, for motherboard vendors that use the nForce MCP, the option is available to use only an AC 97 CODEC, handing all of the digital audio controller functions to the host CPU.

There is also a third market variant: PCI audio controllers. Chaintech ships an nForce motherboard, the 7NJS ZENITH, with neither the APU nor the AC '97 interface enabled. It's a decision they decided to go with even though they ship the MCP2-T.

If you have an nForce motherboard that is utilizing the APU (something that should be obvious from the specifications but if not then you can tell if the audio control panel such as described in this review loads) then you can be assured that you are going to have a unit capable of delivering on the following:

· 32 DX8 hardware submixers (A submixer is a mix of multiple voices that can be routed back to the voice processor for 3D positioning and/or other effects. It is like an effects send on a pro audio mixer.)

· Per voice parametric equalization in hardware (This means that using DS3D or DLS the APU can alter the EQ on each processed voice, something necessary to do occlusion, obstruction, or standard MIDI playback with controller changes. All EAX 2 capable products do this processing but most are not in hardware);

· ASIO support (not tested in this review but NVIDIA reports that their latency is very low)

· Analog 2-speaker output (typically up to 16 bit / 48 kHz although technically the APU supports up to 20-bit output at 48kHz if the AC 97 part supports this. Internally all processing is done at 24-bits which has nothing to do with ability to play a 24/96 sound source with the nForce can not do).

What can vary is the manner in which these connections are delivered and what additional connections are offered.

For the reference board tested in this review these extra connections are via an ACR card. However, ACR is not required, it is just another way to expose those features. In fact, not one NVIDIA nForce 1 or 2 partner uses ACR due to the cost of losing a slot on the motherboard and expense of adding a card.

They either mount it down (e.g. ASUS A7N8X has 6 channel analog out + SPDIF out right on the motherboard and depending on the revision of board you can in some cases get an add in riser card with Aux, CD In, and S/PDIF In. Some other models also have a front panel connector, so if your case supports mic, line, or headphones you can wire them up) or put it on a riser (e.g. MSI K7NS). Some other models also have a front panel connector, so if your case supports mic, line, or headphones you can wire them up.

In addition to the guarantee of connections offered, all SoundStorm branded boards must go through extensive quality testing with Dolby so that it is certified to work as Dolby and NVIDIA have specified.

There’s nothing particularly special about encoding content to Dolby Digital. Movie studios do this as a matter of course in making the soundtracks for the movies we watch and listen to in theaters and at home on DVDs. There are also a number of consumer based software packages that have offered this feature for quite some time. There are two things to take away from this background. First, the Dolby Digital content in movies and from consumer based software packages is pre-rendered, just like the visuals that go along with the audio. Secondly, anyone who has ever used one of the consumer-based packages will report on the significant processor usage to generate that pre-rendered Dolby Digital content.

What’s special about DICE is the “I” as in interactive. In other words the encoder takes any multichannel output (that same multichannel analog output that we have from all of today’s 3D audio games on the PC) and on the fly, without any appreciable lag, converts it to Dolby Digital 5.1 content so that it can be sent via a S/PDIF output to a Dolby Digital 5.1 decoder. Until very recently the nForce APU was the only product on the market to take advantage of the DICE feature, and it remains the only product to deliver the DICE in hardware.

The Dolby Digital Encoding has been listed as optional on CMedia's site for a while now but we have not seen that anyone has licensed it yet. As you probably know several motherboard feature the CMI9739 solution and you will also find it in Philips new low cost soundcard line but then with technology from QSound instead of Sensaura.

Later in this review we will uncover more on the performance of the DICE feature but suffice to say it does deliver as promised.

I/O Availability
Front	Yes – typically 1/8” stereo
Rear	Only on SoundStorm branded boards – typically 1/8” stereo
Center	Only on SoundStorm branded boards – typically 1/8” stereo shared with subwoofer output.
Sub	Only on SoundStorm branded boards – typically 1/8” stereo shared with center output.
CD Analog	Variable
CD Digital	No dedicated CD Digital connection. However as with any other soundcard using WDM drivers Windows offers the option to transfer the audio digitally over the ATA bus instead of depending on any physical connection. It is a universal feature supported by just about every CD/DVD drive built since 20X. Several media players (including MS Windows Media Player) offer this feature even for soundcards using VXD drivers.
Auxiliary	Variable
Video	Variable
Microphone	Only on SoundStorm branded boards – typically 1/8” stereo
Line input	Only on SoundStorm branded boards – typically 1/8” stereo
Wavetable header	No – typical of the majority of sound cards and integrated systems today
Digital Output	Only on SoundStorm branded boards – typically 1/8” stereo
Digital Input	Variable depending on brand and model but will only be found on SoundStorm branded boards
Joystick connection	Variable

This reference system uses Sigmatel products. The rear and center/subwoofer outputs are using Sigmatel’s 9708 CODEC Product Brief (pdf, 23Kb, 2-9708-P1-2.0-1100), Data Sheet (pdf, 272Kb, 2-9708-D1-2.0-0301) and the front channels are using Sigmatels's 9721 stereo AC97 CODEC Product Brief (pdf, 23Kb, 2-9721-P1-2.0-1100), Data Sheet (pdf, 401Kb, 2-9721-D1-1.0-0400)

The 9708 is an 18 bit four channel general purpose quad audio CODEC that conforms to AC97 2.1 specifications. The 9708 uses Sigmatel’s DeltaSigma technology to achieve a >95 dB signal to noise (S/N) ratio across the mixer and DAC. Specs also describe a THD of 0.02 percent, crosstalk of –70 dB and frequency response through to 20 kHz (+/- 1 dB limit).

The 9721 is 18 bit general purpose stereo audio CODEC that again conforms to the AC97 2.1 specifications. Like the 9708 it uses Sigmatel’s DeltaSigma technology to achieve a >95 dB S/N ratio across the mixer and DAC. Detailed peformance specs are somewhat better than the 9708 with a S/N ratio on analog line out of 101 dB vs 98 dB on the 9708. Specs also describe a THD of 0.02 percent, crosstalk of –70 dB and frequency response through to 20 kHz (+/- 1 dB limit).