Did you ever wonder why certain audio cards sound terrific while others include unwanted background noise when moving a mouse or spinning up a hard drive? This tutorial explains how to design terrific sounding audio for PCs without encountering the gremlins of unwanted noise and distortion. Upon its completion, you will know how to choose the correct components and peripheral circuitry, and the way to create an optimal pc-board layout that helps eliminate noise, crosstalk, and interference.
We'll begin with a basic background in the architecture used to create audio in the PC. But even if you're not creating a sound card or put-ting audio on a PC motherboard, this article will still be useful. Many of the concepts for good layout and measurement apply to other audio applications that require good sound quality in a noisy mixed-signal environment (see "Audio Measurements," p. 102).
PC audio has a well-defined system architecture, making it somewhat different from other em-bedded audio applications. Intel developed an industry standard primarily targeting PCI audio applications known as the Audio Codec Standard, or AC '97. Most new PC audio systems comply with AC '97. The current version of this standard is version 2.1. In addition to Intel's AC '97 standard, Microsoft has included PC audio performance and feature requirements in its PC-99 specification (see "Audio Requirements For Microsoft's PC-99," p. 108).
AC '97 audio has two main components. The first is the hardware audio codec, known as the AC '97 codec. The other is the PCI audio controller (Fig. 1). The AC '97 codec is basically an audio control center on a chip. The codec's main task is to route and mix analog audio signals to and from the PC and the outside world. A combination of several components in the codec makes this feat of audio acrobatics possible. These include analog-to-digital converters (ADCs), digital-to-analog converters (DACs), a digitally controlled analog audio mixer, an input selector that feeds the ADCs, and a digital serial interface known as the AC-Link.
The codec has one or more stereo ADCs for recording signals to a computer's hard-disk drive. Typically, these ADCs are 16- or 18-bit devices. There's a record-select multiplexer that's similar to the selector switch on a home stereo (Fig. 2). A multiplexer selects one of many analog audio sources, including the stereo mix. Users select the source through a Windows recording control panel (Fig. 3).
The analog mixer portion of the AC '97 codec combines signals from the DACs and analog inputs and routes them to the codec's analog output pins (Fig. 4). The corresponding Windows mixer panel controls the mixer circuit (Fig. 5).
Most AC '97 codecs have a well-defined set of analog inputs and outputs. Inputs include Line_In, Mic, Aux, Video, and CD. Outputs include Line_Out, Alt_Line_Out, and Mono_Out. Generally, Line_Out is the main analog output. Alt_Line_Out, combined with an external amplifier, is basically used for driving headphones. The codec can have one or more stereo 16- to 20-bit DACs. The DAC outputs are routed to the audio mixer. As a result, signals being played back from the computer can be mixed with analog inputs from the outside world.
The other component of the PC audio system, PCI audio controller, is responsible for directing audio to and from the PCI bus and the AC '97 codec. Plus, the PCI accelerator often performs additional functions using its DSP core, such as sound synthesis, digital 3D effects, equalization, and sample-rate conversion.
The controller has a standard PCI-bus interface to communicate to the computer's CPU, and a special serial communication interface to the AC '97 codec known as the AC-Link. The AC-Link is a five-wire serial interface specifically designed for communication between AC '97 codecs and audio controllers.
The Right Components
AC '97 codecs provide a good solid foundation for achieving high audio quality in a PC. It's essential too, though, that you choose the correct external components.
For one thing, you want to keep resistor values as low as possible without compromising the performance of the device driving the resistor. There are two basic reasons for keeping resistor values low. First, large resistor values create high-impedance signal traces that are susceptible to both interference and crosstalk.
Second, and to a lesser extent, large resistors contribute to the overall noise figure with Johnson noise. Johnson noise is proportional to the value of the resistor. Typically in PC audio circuitry, Johnson noise is much less significant than susceptibility issues.