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How the Intel HD Audio system works
Any reputable piece of A/V equipment that is designed to go in the living room almost certainly includes these kinds of diagrams as a key part of the system design process. However, these documents rarely, if ever, exist on a PC system design. As part of your system design, you should create complete audio flowcharts of the audio paths for the primary usability models in your system. Here's the general flow for the Intel HD Audio system.
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By
David Roach, President OptimalSound.net, Wayne Jones Operations Manager, Intel Press
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Audio DesignLine
(09/13/2006 4:05 PM EDT)
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Editor's note: This article and the subsequent articles provide an overview of the Intel HD Audio system. It is excerpted from the book Intel HD Audio for the Digital Home published by Intel Press. This excerpt is from pages 329-340 of the book. All material is copyright Intel.
While different cultures may say it in slightly different ways, a picture can convey certain types of information much better than the written word. In this case, the pictures are audio flow charts and block diagrams. While these visual aids are commonplace for most audio equipment, complete audio flow diagrams including motherboard, audio codec, and software routing all combined into a single picture have not been available. This chapter is in-tended to change that situation.
Any reputable piece of A/V equipment that is designed to go in the living room almost certainly includes these kinds of diagrams as a key part of the system design process. However, these documents rarely, if ever, exist on a PC system design. As part of your system design, you should create complete audio flowcharts of the audio paths for the primary usability models in your system.
Multiple Layers
Unlike most audio equipment, the PC has multiple different layers of signal routing to consider. It must interface to the outside world, establish connections from multiple jacks to one or more audio codecs, route the signals from the audio codecs through the Intel HD Audio controller and a complex software stack to the applications that are consuming or generating audio streams. Each stage along the way could insert gain, attenuation, or some sort of signal processing, and it is important to understand the headroom, noise floor, and resolution of each path, analog or digital, in order to provide reliable high fi-delity audio from a PC.
To see the signal flow through all paths in a single glance, it is most convenient to think of them as layers much like the classic Russian dolls where each doll is enclosed inside a larger doll. The PC has multiple layers: the out-side world, the motherboard, one or more audio codec layers, and one or more software layers, and finally the end user. Each layer fits totally within the next, with the user being in the center, as he or she should be.
The Real World Layer
The intersection between the real world and the motherboard occurs at the colored audio jacks in the system, or at any built-in microphones and speak-ers. The end user decides what device to connect to the PC, and how audio content comes to and goes from the PC. This layer is outermost in the system diagram. In Windows Vista, these connections are referred to as audio end-points, and the user experience is oriented towards the final source or destina-tion in the real world, rather than being oriented to the jacks or the pins or DACs on the codec.
Figure 9.1 The Real World Layer
You should create a separate layer for each real-world usage scenario. For a system which supports 7.1 home theater, you should create a layer which pic-tures a target speaker system and all the jacks contained on it, with the con-nections going to the colored jacks on the PC. For a VoIP phone application using a headset, you should create a layer showing a headset plugged into the front panel jacks. If the user might have an expectation that the system must support both simultaneously, you should create a layer which shows both. Signals in this layer are predominantly analog, though digital signals such S/PDIF or ADAT could be present.
The Motherboard or System Layer
The intersection between the motherboard and the real world occurs at the colored audio jacks in the system, or at any built-in microphones and speak-ers. The intersection between the motherboard and the codec occurs at the co-dec's ports and pin widgets. You could argue that two layers exist here, since sometimes jacks are mounted on the chassis and not on the motherboard. The wiring between these jacks and the motherboard would constitute a fifth layer. Unless otherwise noted, these examples treat this intersection as a single layer and assume that all jacks are mounted directly on the motherboard.
Figure 9.2 The Motherboard Layer
The motherboard design engineer generates the motherboard schematics, which define the routing and connections between the real world and the codec. While these schematics are a formal design document, the audio signal path is often only obvious to an electrical engineer who is trained in analog design and familiar with the chips in the circuit. By themselves, the schemat-ics are not an adequate representation of the audio signal paths in the system, though they provide the underlying basis for this layer.
The motherboard design engineer also must generate a corresponding set of verb tables, which are compiled into the BIOS and used to program the default configuration registers in each pin widget in each codec during system startup. The pin configuration defaults describe the motherboard schematics to the software layer. The Microsoft UAA class driver for Intel HD Audio uses the contents of these configuration registers to determine how to expose audio devices in Windows.
The motherboard layer is fully contained within the real-world layer, and it fully contains one or more codec layers at its center. The audio signal routing on the motherboard is predominantly analog, though digital signals such as S/PDIF, ADAT and I2S may be present. The layer shown in Figure 9.2 is relatively simple, since each signal from a jack is passed directly to a port on the codec. However, this simple, direct flow is not always the case; this layer can have considerable complexity associated with it.
This layer is likely to have the least audio fidelity of any of these layers, primarily because the inside of a computer contains many unintended EMI sources which can compromise the signal quality. Follow the layout tech-niques detailed in Chapters 5 and 6 to maximize the fidelity of this layer.
Next installment: The Intel HD Audio Codec Layer and Vista details.
David Roach has over 20 years of PC audio design experience, having
participated in the development of the first real-time MIDI synthesizers for
the Macintosh and Intel x-86 architectures, first PC-based physical modeling
waveguide synthesizer for Creative Labs and leading development for the
Jabra Earphone, and led the PC audio team at SigmaTel. Mr. Roach is
currently the President of Optimal Sound LLC.
Wayne Jones has over 30 years of engineering experience in professional
audio product development and audio test and measurement applications gained
from the company he founded, Amber, and with Audio Precision. Mr. Jones has
served on standards committees for the Audio Engineering Society, Consumer
Electronics Association and International Electrotechnical Committee. He was
author of the revised AES-6id for PC Audio testing and has authored many
articles and application notes on audio measurement topics.
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