Careful Design Helps Exorcise Noise Demons From PC Audio

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.

So what should external resistor values normally be? In general, keep input resistor values high enough that the input impedance is at least 10 kΩ. Try not to let resistor values in the audio paths get above 47 kΩ.

Always use metal-film resistors. Generally, most surface-mount resistors are of the metal-film variety. It's worth mentioning that you should stay away from wire-wound and carbon-composition resistors. Wire-wound resistors can induce noise and interference because they tend to be inductive. Carbon-composition resistors tend to be inherently noisier.

In addition, make liberal use of decoupling and bulk capacitors. Place decoupling capacitors at every power pin on the codec, the controller, and all op amps. These capacitors will help reduce high-frequency noise and interference. Employ good-quality 0.1-µF ceramic capacitors. Bulk capacitors will help provide reserve current that will reduce the impact caused by high-current switching circuitry on the PC. Implement bulk capacitors on power pins that demand current, such as a headphone amplifier.

Implement tantalum or electrolytic capacitors for ac coupling of audio signals, if possible. Otherwise, use high-quality NPO ceramic capacitors.

In general, inputs and outputs should be ac coupled to prevent unwanted dc biases from finding their way into the audio signal path. In addition, ac coupling capacitors are required on the inputs of an AC '97 codec, because the codecs are internally dc biased. Use good-quality electrolytic or tantalum capacitors wherever possible. Pay careful attention to the capacitor's polarization. The "+" terminal should always be connected to the side of the circuit with the higher dc bias. Use high-quality ceramic capacitors if the others aren't available. Low-quality ceramics can cause increased audio distortion and should be avoided.

The value of coupling capacitors will determine the low-frequency performance of the audio signal path. This is generally determined by the formula:

f₀ =1/(2πRC)

where C is the coupling capacitor value, R is the load resistance, and f₀ is the 3-dB corner frequency.

For headphone outputs, this capacitor can be quite large because the load can be as low as 32 Ω. Using this formula, the capacitor value for a 3-dB roll-off frequency of 20 Hz is:

C = 1/(2πRf₀)
= 1/(2π↔ 32 ↔ 20)
= 249 µF

Typically, ac coupling capacitors for headphone drivers are 220 to 470 µF. A 10-µF value is adequate for line outputs that normally drive a 10-kΩ load. Because AC '97 codecs have input impedances of 10 kΩ or greater, usual values for inputs are 1 to 10 µF.

Also, employ only high-quality op amps. For good audio quality, use an op amp with a slew rate of at least 4 V/ms. The voltage noise and gain-bandwidth product should be better than 5 nV/Ö*Hz and 10 MHz for input stages with a significant amount of gain, such as microphone preamplifiers. Output amplifiers intended to drive headphones must be capable of driving loads as low as 32 Ω without severe distortion. Choose an op amp that can run on a single +5-V supply rail to greatly simplify powering the analog audio circuitry.

Furthermore, give consideration to external circuits that drive your audio circuitry. One common mistake that designers make is not taking the limitations of external circuitry into account.

Don't put mute circuitry on inputs that may short-circuit the external amplifiers driving your PC audio inputs. This includes microphone inputs because some microphones may have active circuitry.

Don't place large EMI capacitors between the signal trace and ground on the inputs. Many op amps can't drive over a few hundred picofarads. Try to keep EMI capacitors under 220 pF.

Finally, keep input impedances over 10 kΩ. See the previous section for a discussion on resistor values.

Your pc-board layout will probably make or break your design. Good layout will help ensure that you don't have digital noise or interference in your analog audio. It also will minimize crosstalk between analog signals. It can even aid with EMI emissions testing.

So, what are the principles of good pc-board layout for PC audio designs? The most important principle for creating a high-performance audio system in the noisy PC environment is to keep digital and analog separated. The easiest way to do this is to partition the analog audio components away from all the digital components on the add-in card or motherboard. Start by creating a separate analog and digital ground plane (Fig. 6). Connect these planes only at one point, usually under the codec (Fig. 7).

Create a separate analog power supply by using a linear voltage regulator. Normally, +12 V from the PCI bus is regulated down to +5 V analog. Add a ferrite bead in series with the input of the regulator to help reduce high-frequency noise. Don't use a dc-dc converter! That will create more problems than it will solve by introducing switching noise.

Don't let digital traces or power find their way over the analog ground plane, or near analog components. In this situation, the digital signals capacitively couple into the analog ground plane. Remember that a capacitor is simply two parallel plates of metal with a dielectric between them. A digital signal trace is a long thin metal plate. The pc-board material is the dielectric, and the analog ground plane is the other metal plate. This also implies that you shouldn't allow analog traces or power to find their way over the digital ground plane.

Learn to identify sensitive signal traces and take special care in routing them. By doing so, risk of interference and crosstalk may further be reduced. Low-level signals or signals that are to be amplified are susceptible to interference or crosstalk because the offending signals will be amplified along with the audio signal. Especially sensitive are the low-level signals that require gain, like the microphone preamplifier and high-impedance signals.

Inputs are sensitive in general, because they represent the beginning of the signal chain. Anything picked up in the input will have a greater chance of cascading through the audio chain.

High-impedance signals are extremely sensitive to noise and crosstalk. One of the easiest ways to identify them is to look for large resistors (Fig. 8).

Use shield traces in order to avoid crosstalk. Crosstalk occurs when one signal finds its way into another signal path. The main mechanism for crosstalk is capacitive. Two traces that run parallel to each other will form a capacitor. Because it isn't always practical to avoid parallel traces, crosstalk can be significantly reduced by placing shield traces between the signal traces. A shield trace is made by creating a trace that's connected to analog ground on one side (Fig. 9). Avoid tying the trace to ground on both sides because that can create ground loops.

The basic rules for layout may be summed up as follows: Create separate analog and digital ground planes. Separate the ground planes by at least 0.08 in. The analog and digital ground planes should be connected at only one point. All analog traces should run over the analog ground plane only. All digital traces should run over the digital ground plane only. All analog components should be placed over only the analog ground plane. Furthermore, mixed signal components, like the AC '97 codec, should have their analog pins partitioned over the analog ground plane and their digital pins partitioned over the digital ground plane. Also, decoupling capacitors should be placed as close as possible to the IC pins and have minimum trace lengths to ground.

Reduce the chance of crosstalk between the analog audio signals by placing shielding between the traces. A shield trace can be created by adding a trace that's tied to analog ground on one end and floating on the other end. Remember to keep high-impedance signals, low-level signals, and other sensitive signal traces as short as possible. Use a linear regulator to create a separate analog supply. Fi-nally, avoid using a dc-dc converter because it can introduce unwanted noise and distortion.

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