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Windows Platform Design Notes
Design Information for the Microsoft ® Windows® Family of Operating Systems
Analog Audio Classification Using
Device Impedance Characteristics
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............................................................................................................................................. 3
Problem Definition .................................................................................................................................. 3
Analog Audio Devices............................................................................................................................. 3
Microphones...................................................................................................................................... 3
Headsets ........................................................................................................................................... 4
Speakers ........................................................................................................................................... 4
PC Sound System .................................................................................................................................. 5
Input Device Characteristics .............................................................................................................. 5
Output Device Characteristics ........................................................................................................... 6
Impedance Characteristics ..................................................................................................................... 7
Input Impedance ............................................................................................................................... 7
Output Impedance ............................................................................................................................. 7
Measurement Methodology .................................................................................................................... 7
Equipment ......................................................................................................................................... 7
Test Setup......................................................................................................................................... 7
Data Analysis ......................................................................................................................................... 8
Microphones...................................................................................................................................... 9
Headsets ......................................................................................................................................... 10
Speakers ......................................................................................................................................... 11
Device Classification Taxonomy ........................................................................................................... 13
Impedance Distribution .................................................................................................................... 13
Issues ............................................................................................................................................. 14
Microsoft Proposal for Analog Audio Device Classification .............................................................. 14
Resources and Call to Action................................................................................................................ 15
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Analog Audio Classification Using Device Impedance Characteristics - 2
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© 2002 Microsoft Corporation. All rights reserved.
© 2002 Microsoft Corporation. All rights reserved. Version 1.0 – April 16, 2002
Analog Audio Classification Using Device Impedance Characteristics - 3
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.
© 2002 Microsoft Corporation. All rights reserved. Version 1.0 – April 16, 2002
Analog Audio Classification Using Device Impedance Characteristics - 4
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.
RL
Typical Electret Capsule
+Vb
Term. 1
ECM Unit
Output
C
Term. 2
Ground
RL = 2.2k (External resistor)
C = External Capacitor
RL
+Vb
Term. 1
Output
C
Ground
Term. 2
Electret Microphone
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.
© 2002 Microsoft Corporation. All rights reserved. Version 1.0 – April 16, 2002
Analog Audio Classification Using Device Impedance Characteristics - 5
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.
Stereo Headphone out
AC-Link
Digital
Interface
PCM to DAC
Stereo Line out
Mixer
Mono out
DAC
Registers
64x16 bits
MIC 1
MUX
ADC
MIC 2
GPIO
GPIO
AC97 Codec Block Diagram
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.)
© 2002 Microsoft Corporation. All rights reserved. Version 1.0 – April 16, 2002
Analog Audio Classification Using Device Impedance Characteristics - 6
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 R s is
a series resistor typically in the 4 to 32 range.
© 2002 Microsoft Corporation. All rights reserved. Version 1.0 – April 16, 2002
Analog Audio Classification Using Device Impedance Characteristics - 7
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:

Hewlett Packard 4294A Precision Impedance Analyzer

Hewlett Packard 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.
© 2002 Microsoft Corporation. All rights reserved. Version 1.0 – April 16, 2002
Analog Audio Classification Using Device Impedance Characteristics - 8
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).
© 2002 Microsoft Corporation. All rights reserved. Version 1.0 – April 16, 2002
Analog Audio Classification Using Device Impedance Characteristics - 9
Microphones
Plantronics Audio 40 Headset (Microphone portion) Impedance Plot
Telex M-40 Microphone Impedance Plot
© 2002 Microsoft Corporation. All rights reserved. Version 1.0 – April 16, 2002
Analog Audio Classification Using Device Impedance Characteristics - 10
Headsets
Plantronics Audio 40 Headset (Speaker portion) Impedance Plot
CompUSA brand Headset (speaker portion) Impedance Plot
© 2002 Microsoft Corporation. All rights reserved. Version 1.0 – April 16, 2002
Analog Audio Classification Using Device Impedance Characteristics - 11
Speakers
Passive
CTX-55 Passive Speaker Impedance Plot
Yamaha NS-100M Passive Speaker Impedance Plot
© 2002 Microsoft Corporation. All rights reserved. Version 1.0 – April 16, 2002
Analog Audio Classification Using Device Impedance Characteristics - 12
Amplified
Labtec LCS-2414 Amplified Speaker Impedance Plot
Compaq JBL-Pro Amplified Speaker Impedance Plot
© 2002 Microsoft Corporation. All rights reserved. Version 1.0 – April 16, 2002
Analog Audio Classification Using Device Impedance Characteristics - 13
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.
Impedance Distribution
Microphone
100000
Speaker-Amplified
Speaker-Passive
Impedance Ohm (Log scale)
10000
Headset-Speaker
section
1000
Note: Amplified
Radio Shack
speaker
100
10
1
0
5
10
15
20
25
Devices (min and max values taken)
As can be seen from the graph, the impedance buckets fall into the following ranges
for the tested frequency range of 40 Hz to 20 KHz:

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
© 2002 Microsoft Corporation. All rights reserved. Version 1.0 – April 16, 2002
Analog Audio Classification Using Device Impedance Characteristics - 14
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.

One amplified 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 
© 2002 Microsoft Corporation. All rights reserved. Version 1.0 – April 16, 2002
Analog Audio Classification Using Device Impedance Characteristics - 15

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 [email protected]
Resources:

Windows Platform Development white papers and resources:
http://www.microsoft.com/hwdev/

Windows Driver Development Kit:
http://www.microsoft.com/ddk/

Windows Logo Program for Hardware:
http://www.microsoft.com/winlogo/hardware/

WHQL Test Specifications, HCTs, and testing notes:
http://www.microsoft.com/hwtest/

AC’97 Specification (Intel)
http://developer.intel.com/ial/scalableplatforms/audio/index.htm
© 2002 Microsoft Corporation. All rights reserved. Version 1.0 – April 16, 2002