Analog Audio Classification Using Device Impedance Characteristics

Analog Audio Classification Using Device Impedance Characteristics - 1

Windows Platform Design Notes

Design Information for the Microsoft® Windows® Family of Operating Systems

Abstract

This paper provides information about a taxonomy to classify analog audio devices (microphones, headsets, and speakers) using impedance characteristics of the selected device. This information can be used by PC analog audio systems like the AC’97 CODEC to provide a rudimentary mechanism to detect jack presence and device type enumeration. Topics for this white paper will include testing methodology, data analysis, and a proposal for the classification taxonomy.

Version 1.0 – April 16, 2002

Contents

Introduction

Problem Definition

Analog Audio Devices

Microphones

Headsets

Speakers

PC Sound System

Input Device Characteristics

Output Device Characteristics

Impedance Characteristics

Input Impedance

Output Impedance

Measurement Methodology

Equipment

Test Setup

Data Analysis

Microphones

Headsets

Speakers

Device Classification Taxonomy

Impedance Distribution

Issues

Microsoft Proposal for Analog Audio Device Classification

Resources and Call to Action

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Introduction

Real-Time Communication (RTC) is a key scenario that will be enabled and improved in future Microsoft®Windows® operating systems. In order to effectively configure the audio resources and devices for RTC, a better model for audio device enumeration and characterization needs to be in place. Audio devices that connect to the PC using digital buses such as USB and IEEE 1394 can easily provide device enumeration and characteristics to the operating system. On the other hand, analog audio devices are typically directly connected to an audio CODEC, and don’t have the capability to transmit audio device characteristics over the analog interface.

In a PC, the most common CODEC is AC’97, and is typically implemented as part of the motherboard. The AC’97 CODEC interfaces directly to an analog input or output jack. Without the inclusion of additional sensing circuitry, there is nothing that the operating system can do to query device characteristics, let alone determine device attachment. For more information on the Intel AC’97 specification, see the URL provided at the end of this paper.

Problem Definition

As stated above, today it’s virtually impossible for an operating system to automatically configure and control analog audio devices. However, in the future, it may be possible to at least programmatically categorize connected analog audio devices through revisions to the AC’97 spec along with new hardware. Microsoft conducted an experiment with the goal of determining if there is a way to classify or group the different types of analog audio devices based upon some electronic property. The analog audio devices of interest are microphones, headsets, and speakers. If a unique property was found, then it may be possible for the operating system to use this information to configure the audio ports based upon device type. Additionally, the operating system may configure the audio properties such as audio port type (input or output), gain, and perhaps turn on/off echo cancellation. In this experiment, Microsoft engineers focused on impedance measurements. The balance of this paper reviews the impedance data from a selection of microphones and speakers.

Analog Audio Devices

Microphones

A Microphone is a generic term that refers to any element that transforms acoustic energy (sound) into electrical energy (audio signal). The common microphone types are Dynamic and Condenser.

Dynamic

A dynamic microphone is much like a miniature loudspeaker. A flexibly-mounted diaphragm is coupled to a coil of fine wire that is mounted in the air gap of magnet. When sound strikes the diaphragm, the diaphragm surface vibrates in response. As the diaphragm vibrates back and forth in the magnetic field, a small electrical current is induced in the wire. The magnitude and direction of that current is directly related to the motion of the coil; therefore, the current is an electrical representation of the incident sound wave.

Condenser

Another type of microphone is the classic capacitor (or condenser) microphone. This transducer converts the sound energy by utilizing changes in capacitance due to mechanical vibrations to produce voltage variations proportional to sound waves. Condenser microphones require power to charge the capacitor. A variation of the condenser microphone is the Electret microphone. The Electret has a built in charge, and the few volts needed are to power a built-in FET buffer pre-amplifier.

A majority (over 90 %) of PC microphones are Electret. A typical Electret condenser microphone capsule is a 2 terminal device (there are also 3 pin capsules), which approximates to a current source when biased with around 1-9 volts. This small amount of current (typically, less than half a milliamp) is consumed by a preamplifier built into the microphone capsule to make the conversion of very high impedance source of the Electret element itself and the cable that needs to be driven.

Headsets

PC headsets comprise of a microphone and a small speaker section. The majority of PC headsets utilize an Electret microphone. The speakers are typically magnetic coil and cone. The headset microphone is usually single channel, whereas the speakers sections may be monophonic or stereophonic.

Speakers

Two varieties of speakers were measured – amplified, and non-amplified.

Passive

Passive speakers require amplification from the PC sound card or CODEC amplifier circuitry.

Amplified

Amplified speakers have built-in amplifiers in the speaker system. The devices tested in this experiment are 2.1 or stereo speakers.

PC Sound System

The sound system implementation varies in PCs. Some are implemented on separate sound add-on cards such as the Creative Labs cards, while others are implemented with CODECs integrated on the motherboard. The following block diagram illustrates an example sound system implemented with an AC’97 CODEC.

Input Device Characteristics

Microphone inputs vary considerably from sound card to sound card. In fact, this is one of the primary reasons why an analog PC audio system is difficult to configure by the operating system. The pre-amplification gain settings vary from sound card to sound card.

The following diagram illustrates a generalized Electret microphone and audio input circuit.

The microphone jack circuit illustrated is considered the most universal jack. This jack supports 2-pin dynamic microphones as well as 3-pin electrets (as shown in the diagram.)

There are two impedances associated with the microphone jack. The standard input impedance illustrated as Zi and the bias impedance Rb. This input impedance (in parallel with the bias impedance Rb) has a direct affect on the voltage sensitivity of Electret microphones and should be as high as possible, typically greater than 20 kW (an order of magnitude greater than Rb) and no lower than 10 k.

The bias impedance Rb has two components −an AC impedance and a DC impedance. If Rb is a single resistor connected to a bias voltage Vb, then the AC and DC impedance are identical. The DC impedance, along with the bias voltage will determine the operating voltage for the Electret microphone.

Output Device Characteristics

Two types of output devices were tested −passive speakers and amplified speakers. Amplified speakers have a built-in amplifier to drive the speakers. The following two diagrams schematically illustrate the output device characteristics of both types of speakers.

A passive speaker can be modeled according to the following diagram, where Rs is a series resistor typically in the 4 to 32 range.

Impedance Characteristics

Impedance is an important parameter used to characterize electronic circuits, components, and the materials used to make components. Impedance (Z) is generally defined as the total opposition a device or circuit offers to the flow of an alternative current (AC) at a given frequency. This opposition comes from the combination of resistance, capacitive reactance and inductive reactance to the alternating current. Reactance varies with the frequency of the alternating current.

Input Impedance

Input impedance is important for interoperability among different audio system components. Input impedance is also critical to microphone inputs as impedance has a direct correlation to Electret microphone sensitivity. Line-level inputs should be designed with impedance higher than 10 k, and preferably around 47 k, which is the typical load level for consumer audio equipment.

Output Impedance

Outputs are designed to drive three types of loads: line-level, headphones, and speakers. Outputs designed for line-level usually have minimal drive capability and the load is set above 40 k. Headphone impedance’s vary from 16  to 92  with 32  being the most common. Speaker loading typically is set at 8 .

Measurement Methodology

Equipment

The test equipment used for the measurements consisted of the following:

HewlettPackard 4294A Precision Impedance Analyzer
HewlettPackard 16047E Test Fixture (Bridge)
3.5mm Audio Jack to Bridge adaptor

The Device Under Test (DUT) consisted of microphones, headsets, and speakers.

Test Setup

The following diagram illustrates the test setup. The test setup is schematically described in the next diagram.

Test Setup Schematic Representation

In regards to the impedance tests, headset speakers were measured with a 400  non-inductive sense resistor. Headset speakers had a drive level of 100 mV at the terminals of the speaker. All device impedance was measured at a frequency response of 40 Hz to 20 KHz.

Data Analysis

The devices were randomly selected by going to a local computer retail store and picking up at least 10 of each type of device. This strategy was selected to approximate a data set that includes popular analog audio devices for the PC.

The next section shows a snapshot of the impedance measurement for two of each device type − microphone, headset, and speaker (passive and amplified).

Microphones

Plantronics Audio 40 Headset (Microphone portion) Impedance Plot

Telex M-40 Microphone Impedance Plot

Headsets

Plantronics Audio 40 Headset (Speaker portion) Impedance Plot

CompUSA brand Headset (speaker portion) Impedance Plot

Speakers

Passive

CTX-55 Passive Speaker Impedance Plot

Yamaha NS-100M Passive Speaker Impedance Plot

Amplified

Labtec LCS-2414 Amplified Speaker Impedance Plot

Compaq JBL-Pro Amplified Speaker Impedance Plot

Device Classification Taxonomy

Impedance Distribution

A total of 10 devices in the categories of Microphones, Speakers, and Headsets were measured for impedance characteristics. All device impedance was measured at a frequency response of 40 Hz to 20 KHz.

For devices that included a volume control adjustment, such as some headsets, the volume control was adjusted to maximum to remove the effect of the volume control on the impedance measurement. The following graph is a scatter plot of the measurements using minimum/maximum impedance values of each device.

As can be seen from the graph, the impedance buckets fall into the following ranges for the tested frequency range of 40 Hz to 20KHz:

Microphones – 460 to 1140 .
Amplified speakers – 3100 to 11500 . There is an exception for the Radio Shack amplified speakers, which has a value of 100 .
Passive speakers – 4.5 to 19 
Headset (speaker section) – 25 to 62 with a special case 210 to 350measured on the Diva brand headset. The Diva headset is a more expensive model used in commercial avionics applications versus standard PC usage. In another case, the Jabra headset is a cell-phone headset with what appears to be a 200  resistor placed in series to reduce the gain output from the cell phone. Our conjecture is that this resistor functioned as a fixed volume control because the earpiece was mechanically designed to be inserted tighter into the ear versus externally through a foam pad.

The following table lists the impedance measurements of each device tested.

Issues

During the experiment, we encountered the following issues that affected the impedance measurements.

Some headsets (microphone and speaker) have volume controls that cause the impedance to vary to the high range (for example, 1000 ). This could be problematic. The data collected for these headsets were taken with the volume control set to maximum to remove the effect of the volume control on the impedance measurement.
Oneamplified speaker (for example, Radio Shack) had relatively low impedance in the 100 versus above 3000 .

Microsoft Proposal for Analog Audio Device Classification

In order for operating systems to properly configure analog audio devices, based on their device type, Microsoft recommends that manufacturers design their devices to the following impedance criteria:

Microphones – 400 to 1500 .
Powered speakers – 3000 to 12000 .
Passive speakers – 4 to 16 
Headset (speaker section) – 32 to 100 

The data collected for Headset (speaker section) included impedance measured above the 100 range (Jabra cell-phone headset and Diva headset brands). Our recommendation is for Headset devices to be designed to fit in the narrower 32 to 100 range.

Resources and Call to Action

Call to Action:

For analog audio device manufacturers:
Design your devices to present the impedance in the ranges defined in the Microsoft Proposal for Analog Audio Device Classification section of this white paper.
For sound card and analog CODEC manufacturers:
Design your devices to measure the impedance in the ranges defined in the Microsoft Proposal for Analog Audio Device Classification section of this white paper.

Feedback:

To provide feedback about Analog Audio Impedance Classification, send e-mail to

Resources:

Windows Platform Development white papers and resources:
Windows Driver Development Kit:
Windows Logo Program for Hardware:
WHQL Test Specifications, HCTs, and testing notes:
AC’97 Specification (Intel)