`
`
`
`Microphone Array Support in
`Windows
`
`April 21, 2014 Revision
`
`Abstract
`Under less than ideal conditions, even the best microphone embedded in a laptop or
`monitor does a poor job of capturing sound. An array of microphones can do a better job
`of isolating a sound source and rejecting ambient noise and reverberation. This paper
`provides information about the advantages that microphone arrays can offer, and about
`the support for microphone arrays that was introduced with the Microsoft® Windows
`Vista™ operating system.
`If you are a laptop or computer monitor manufacturer, or a designer working to provide
`better quality captured-sound by integrating microphone arrays, or if you are a hardware
`manufacturer designing Windows-based external USB Audio microphone arrays, then
`this paper provides the design guidelines for building microphone arrays that will work
`well with Windows.
`This information applies to the following operating systems:
`Windows Vista and later
`
`
`References and resources discussed here are listed at the end of this paper.
`The current draft of this paper is available on the WHDC web site at:
`http://www.microsoft.com/whdc/device/audio/default.mspx
`
`Disclaimer: This document is provided “as-is”. Information and views expressed in this document, including
`URL and other Internet website references, may change without notice. Some information relates to pre-
`released product which may be substantially modified before it’s commercially released. Microsoft makes no
`warranties, express or implied, with respect to the information provided here. You bear the risk of using it.
`Some examples depicted herein are provided for illustration only and are fictitious. No real association or
`connection is intended or should be inferred.
`This document does not provide you with any legal rights to any intellectual property in any Microsoft product.
`You may copy and use this document for your internal, reference purposes.
`
` 2014 Microsoft. All rights reserved.
`
` ©
`
`Jawbone's Exhibit No. 2014, IPR2022-01124
`Page 001
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`Microphone Array Support in Windows - 2
`
`
`Contents
`Introduction ............................................................................................................................. 3 
`Microphone Arrays as PC Product Solutions: An Overview .................................................... 3 
`Windows and Microphone Array Solutions .............................................................................. 3 
`About Microphone Arrays ........................................................................................................ 4 
`Beam Forming .................................................................................................................... 4 
`Array Directivity .................................................................................................................. 5 
`Constant Beam Width ........................................................................................................ 5 
`Microphone Array Characteristics ........................................................................................... 6 
`Ambient Noise Gain ........................................................................................................... 6 
`A-Weighted Ambient Noise Gain ........................................................................................ 7 
`Directivity Index .................................................................................................................. 7 
`Supported Microphone Array Geometries ............................................................................... 8 
`Two-Element Arrays ........................................................................................................... 9 
`Four-Element Arrays ........................................................................................................ 10 
`Design Considerations .......................................................................................................... 11 
`Hardware Interface ........................................................................................................... 12 
`Requirements for Microphones and Preamplifiers............................................................ 12 
`Requirements for ADCs ................................................................................................... 12 
`Use of MEMS Microphones for PC Microphone Arrays .................................................... 13 
`Number of Microphones ................................................................................................... 14 
`Microphone Array Geometry ............................................................................................ 14 
`Placement of the Microphone Array ................................................................................. 15 
`Acoustical Design and Construction ................................................................................. 17 
`Next Steps ............................................................................................................................ 18 
`References ............................................................................................................................ 19 
`
`
`© 2014 Microsoft Corporation. All rights reserved.
`
`Jawbone's Exhibit No. 2014, IPR2022-01124
`Page 002
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`Microphone Array Support in Windows - 3
`
`Introduction
`Under less than ideal conditions, even the best microphone embedded in a laptop or
`monitor does a poor job of capturing sound. An array of microphones can do a better job
`of isolating a sound source and rejecting ambient noise and reverberation.
`Because of the advantages that microphone arrays can offer to improve sound-capture
`for PC computing, Microsoft has created support for microphone arrays in the Windows
`operating system. The support includes:
` A class driver to support USB Audio devices.
` Algorithms to support several tested microphone array geometries.
` The ability to identify microphone array geometries based on descriptors as reported
`by a USB device.
`
`This paper describes the research and implementation details that provide the
`foundation for the Windows support for microphone arrays. It also provides specific
`design and implementation guidelines for good quality, cost-effective microphone array
`designs that will work well with Windows.
`Microphone Arrays as PC Product Solutions: An Overview
`PCs and other computing devices can usually play sounds well, but they do a poor job of
`capturing sound. With the processing power, storage capacities, broadband
`connections, and speech-recognition engines available today, computing devices can
`use better sound capture to deliver more value to customers.
`With current PC-based audio technology, it is possible to provide better live
`communication than phones, much better record/playback or note-taking devices than
`tape recorders, and better command of the user interface than remote controls. For all
`applications that use sound, end users could benefit from better sound capturing.
`Consider, for example, all of these real-time communication applications:
` Microsoft Windows Messenger, MSN® Messenger, and all other applications built on
`top of the Microsoft Real-Time-Communication stack, such as AOL Instant
`Messenger, other applications for VoIP, and enhanced telephony.
` Enterprise solutions for collaboration and groupware applications, such as Live
`Meeting, the meeting recording capabilities in Microsoft OneNote®, and voice-
`messaging applications.
`
`Robust speech-recognition technologies are still under development, but many
`Windows-based applications already have voice commands integration that work
`satisfactorily, but only when the user wears a headset with a close-talk microphone that
`enables decent sound-capturing quality. Such technologies are convenient for tablet
`PCs and handheld devices, where otherwise users have to type with a stylus.
`Windows and Microphone Array Solutions
`Most PCs or laptops still have just a single microphone. This is a poor solution for
`capturing speech, because the microphone picks up too much ambient noise and adds
`too much electronic noise. The captured signal also includes the room reverberation,
`which decreases intelligibility and confuses speech recognition algorithms. Signal
`processing techniques have their own limitations for removing stationary noise and
`reverberation from a single channel. As a result, users are typically forced into using
`“tethered” or wireless close-proximity microphone headsets to achieve decent sound-
`capturing quality.
`
`© 2014 Microsoft Corporation. All rights reserved.
`
`Jawbone's Exhibit No. 2014, IPR2022-01124
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`Microphone Array Support in Windows - 4
`
`Numerous studies show that users don’t like to wear headsets or to be tethered to the
`computer. In many scenarios, headsets are not an option. For example, walking with a
`headset and a Tablet PC in your hand feels awkward. Using an array of microphones
`with PCs and other computing devices can alleviate the problems caused by using only
`one microphone. The goal—“Wear no close-proximity microphone gear; just talk to your
`computer”—implies mobility and freedom of movement.
`Microphone array solutions should follow these basic design guidelines:
`Implement the characteristics of the tested microphone array geometries that are
`
`supported in Windows as summarized in Table 1 in this paper.
` Meet the design requirements for low noise, directionality, and low manufacturing
`tolerances for microphones and preamplifiers, as summarized in Table 2 in this
`paper.
` Follow the recommendations for analog-to-digital converters for sampling rate,
`sampling synchronization, and anti-aliasing filters as summarized in Table 3 in this
`paper.
` Choose and place the appropriate number of microphones based on the usage
`scenario, with recommended choices illustrated in Figure 11 of this paper.
` Observe the acoustical and construction considerations, to insulate from
`environmental factors that will affect performance, as summarized near the end of
`this paper.
`
`
`Windows includes microphone array support as part of a complete audio subsystem that
`provides these advances:
`Improved acoustic echo cancellation
`
` Microphone array support
` Stationary noise suppressor
` Automatic gain control
` Wideband quality of sound capturing and processing
`
`Microphone array processing is linear and doesn’t introduce distortions to the signal, so
`the microphone array output is good for a human listener and friendly for the speech
`recognition engine. The Windows audio stack can be used both for real-time
`communication applications such as Windows Messenger and for speech recognition-
`enabled applications such as voice commands and dictation.
`The rest of this paper explores the technical details supporting the design and
`implementation of good quality microphone arrays that will work well with Windows.
`About Microphone Arrays
`A microphone array is a set of closely positioned microphones. Microphone arrays
`achieve better directionality than a single microphone by taking advantage of the fact
`that an incoming acoustic wave arrives at each of the microphones at a slightly different
`time. The chief concepts that are used in the design of microphone arrays are beam
`forming, array directivity, and beam width.
`
`Beam Forming
`By combining the signals from all microphones, the microphone array can act like a
`highly directional microphone, forming what is called a “beam.” This microphone array
`beam can be electronically managed to point to the originator of the sound, which is
`referred to in this paper as the “speaker.”
`
`© 2014 Microsoft Corporation. All rights reserved.
`
`Jawbone's Exhibit No. 2014, IPR2022-01124
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`Microphone Array Support in Windows - 5
`
`In real time, the microphone array engine searches for the speaker position and acts as
`if it points a beam at the current speaker. The higher directivity of the microphone array
`reduces the amount of captured ambient noises and reverberated waves. More details
`about the algorithm for beamform design can be found in reference [1]. (Numbered
`references are listed at the end of this paper.)
`
`Array Directivity
`Because the microphone array output contains less noise and has less reverberation
`than a single microphone, the stationary noise suppressor does a better job than it
`would with a signal from a single microphone. A typical directivity pattern of a
`microphone array beam for 1,000 Hz is shown on Figure 1. The pattern is more directive
`than even that of an expensive, high-quality hypercardioid microphone.
`
`
`
`
`
`Figure 1. Microphone array directivity pattern in three dimensions
`
`During sound capturing, the microphone array software searches for the speaker’s
`position and aims the capturing beam in that direction. If the person speaking moves,
`the beam will follow the sound source. The “mechanical” equivalent is to have two highly
`directional microphones: one constantly scans the work space and measures the sound
`level, and the other—the capturing microphone—points to the direction with highest
`sound level; that is, to the speaker.
`
`Constant Beam Width
`The normal work band for speech capturing is from 200 Hz to 7,000 Hz, so wavelengths
`can vary by a factor of 35. This makes it difficult to provide a constant width of the
`microphone array beam across the entire work band. The problem is somewhat simpler
`in a typical office environment, where most of the noise energy is in the lower part of the
`
`© 2014 Microsoft Corporation. All rights reserved.
`
`Jawbone's Exhibit No. 2014, IPR2022-01124
`Page 005
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`Microphone Array Support in Windows - 6
`
`frequency band—that is, below 750 Hz. Reverberation is also much stronger at these
`low frequencies and is practically absent above 4,000 Hz.
`Figure 2 shows the directivity pattern of a four-element linear microphone array as a
`function of the frequency. The combination of this microphone array geometry and the
`related microphone array support in Windows provides nearly constant beam width in
`the diapason of 300 to 5,000 Hz, covering the most important area of the work band.
`
`
`
`
`Figure 2. Microphone array directivity as a function of frequency, horizontal plane
`Microphone Array Characteristics
`The acoustical parameters of a microphone array are measured like those of any
`directional microphone. This section defines a set of parameters that are used later to
`compare different microphone array designs. Because of their directivity, microphone
`arrays offer better signal-to-noise ratio (SNR) and signal-to-reverberation ratio (SRR)
`than a single microphone can.
`
`Ambient Noise Gain
`The isotropic ambient noise gain for a given frequency is the volume of the microphone
`array beam:
`
`G
`
`AN
`
`(
`
`f
`
`)
`
`
`
`1
`V
`
`
`
`V
`
`
`
` dccfB( ),
`
`
`
`
`
`Where:
`V is the microphone array work volume—that is, the set of all coordinates
`
`  ,
`,
`c
` (direction, elevation, distance).
`
`© 2014 Microsoft Corporation. All rights reserved.
`
`Jawbone's Exhibit No. 2014, IPR2022-01124
`Page 006
`
`

`

`
`
`Microphone Array Support in Windows - 7
`
` is the microphone array beam directivity pattern—that is, the gain as a
`( ),cfB
`
`
`function of the frequency and incident angle. An example for one frequency is
`shown on Figure 1. An example in one plane is shown on Figure 2.
`The total ambient noise gain NG in decibels is given by:
`2/
`SF
`
` ( ). ( ).GfHfN
`A
`
`
`
`
`
`(
`
`f
`
`
`
`).df
`
`AN
`
`
`
`
`
`
`
`
`
`0
`
`
`
`NG
`
`
`
`20
`
`log
`
`10
`
`Where:
` is the noise spectrum.
`)
`( fN A
` is the preamplifier frequency response (ideally flat between 200 and 7,000
`( fH
`)
`Hz, with falling slopes from both sides going to zero at 80 and 7,500 Hz
`respectively).
`SF is the sampling rate (typically 16 kHz for voice applications).
`
`Ambient noise gain gives the proportion of the noise floor RMS in relation to the output
`of the microphone array and to the output of an omnidirectional microphone. A lower
`value is better, and 0 dB means that the microphone array does not suppress ambient
`noise at all.
`
`A-Weighted Ambient Noise Gain
`Because humans hear different frequencies differently, many acoustic parameters are
`weighted by using a standardized A-weighting function. The A-weighted total ambient
`noise gain NGA in decibels is given by:
`
`
`
`
`
`(
`
`f
`
`
`
`).df
`
`AN
`
`2/
`( ). ( ). ( ).GfHfNfA
`A
`
`
`
`
`
`SF
`
`
`0
`
`
`
`NGA
`
`
`
`20
`
`log
`
`10
`
`
`
`
`
`
`
`
`
`Where:
` is the standard A-weighting function; other parameters are the same as
`( fA
`)
`above.
`
`A-weighted ambient noise gain gives the proportion of the noise floor in relation to the
`output of the microphone array and to the output of an omnidirectional microphone as
`they would be compared by a human. In this case, –6 dB NGA means that a human
`would say that the noise on the output of a microphone array is half that of an
`omnidirectional microphone.
`
`Directivity Index
`Another parameter to characterize the beamformer is the directivity index, DI.
`
`In considering the following formula for calculating DI, note that cos θ is used when θ is
`defined to be –π/2 and π/2 at the poles, and 0 at the equator. These limits match the
`definitions of φ and θ in Appendix B of the “How to Build and Use Microphone Arrays for
`Windows Vista” companion document. And these limits also match the definitions for
`wHorizontalAngle (φ) and wVerticalAngle (θ) in the kernel streaming interface
`definitions.
`
`
`This is the power function for a given frequency f and direction (φ, θ), with a
`fixed radius:
`( 2),cfB
`( fP
`
`,
`,
`)
`
`
`
`
`
`
`,
`
` 0
`
`
`
`const
`
`
`
`© 2014 Microsoft Corporation. All rights reserved.
`
`Jawbone's Exhibit No. 2014, IPR2022-01124
`Page 007
`
`

`

`
`
`Microphone Array Support in Windows - 8
`
`This is the average power over all directions (the whole sphere):
`2
`
`1
`
` 
`(
`fP
`4
`
`2
`
`
` 
`This is the power in the “best” direction, called the Main Response Axis:
`
`
`( fP
`
`,
`,
`)
`fP
`
`
`
`T
`T
`T
`Dividing the power in the “best” direction by the average power gives an
`indication of directionality for a particular frequency.
`Averaging this ratio over all frequencies gives the Directivity Index.
`
`
`2/
`SF
`fP
`T
`
`
`fP
`
`df
`
`
`
`0
`
`
`
`f
`
`DI
`
`
`
`10
`
`log
`10
`
`
`fP
`
`
`
`
`
`,
`,
`cos
`)
`dd
`
`
`
`
`The directivity index characterizes how well the microphone array detects sound in the
`direction of the MRA while suppressing sounds that come from other directions, such as
`additional sound sources and reverberation. The DI is measured in decibels, where 0 dB
`means no directivity at all. A higher number means better directivity. An ideal cardioid
`microphone should have DI of 4.8 dB, but in practice cardioid microphones have a DI
`below 4.5 dB.
`Supported Microphone Array Geometries
`The proper microphone array geometry (number, type, and position of the microphones)
`is critical for the final results. To ensure successful design and a good user experience,
`Windows supports a number of carefully analyzed and tested geometries that cover the
`most common scenarios in the office and on the go.
`The summary of these microphone arrays characteristics is shown in Table 1.
`Details about the geometries are given in the rest of this section. The table shows:
` Noise gain (NG)
` A-weighted noise gain (NGA)
` Directivity index (DI)
`
`Table 1. Characteristics of supported microphone arrays
`Microphone array
`Elements Type
`NG, dB
`Linear, small
`2 uni-directional
`-12.7
`Linear, big
`2 uni-directional
`-12.9
`Linear, 4el
`4 uni-directional
`-13.1
`L-shaped
`4 uni-directional
`-12.9
`Linear, 4 el second
`4
`integrated
`-12.9
`geometry
`
`Table 1 shows only the noise reduction due to microphone array processing. The
`stationary noise suppressor in the audio stack will add 8 to 13 dB of noise reduction.
`The microphone array not only reduces the amount of ambient noise, but it also helps
`this noise suppressor to do a better job. Suppose that the signal-to-noise-ratio (SNR) in
`the room is 3 dB when captured with an omnidirectional microphone. With this input
`SNR, a stationary noise suppressor cannot do much noise reduction without introducing
`
`DI, dB
`
`7.4
`7.1
`10.1
`10.2
`9.9
`
`NGA, dB
`-6.0
`-6.7
`-7.6
`-7.0
`-7.3
`
`© 2014 Microsoft Corporation. All rights reserved.
`
`Jawbone's Exhibit No. 2014, IPR2022-01124
`Page 008
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`

`
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`Microphone Array Support in Windows - 9
`
`heavy nonlinear distortions and adding audible artifacts called musical noises. The noise
`reduction can add around 3 dB as well, so in this case the output SNR is 6 dB.
`Under the same conditions, the microphone array reduces 13 dB of the ambient noise,
`and now the noise suppressor has a 16 dB SNR on its input. It can easily reduce an
`additional 13 dB of stationary noise without significant distortion in the signal and audible
`musical noises. The output SNR in this case will be 29 dB, which is 23 dB better than a
`system with an omnidirectional microphone. The total noise reduction of the audio stack
`reaches an impressive 26 dB, creating high sound quality with a very low level of
`distortion and artifacts.
`
`Two-Element Arrays
`Two-element microphone arrays can cover a quiet office or cubicle with good sound
`capturing when the speaker is less than 0.6 meters (2 feet) from the microphones.
`These arrays are suitable for integration into laptops and tablets or as standalone
`devices. Both microphones point directly forward to the speaker. These microphone
`arrays steer the beam in a horizontal direction only, in the range of ±50O. The directivity
`in a vertical direction is provided by the uni-directional microphones. The geometries are
`shown on Figure 3.
`
`a) Small two-element array
`
`b) Big two-element microphone array
`
`
`
`
`
`Figure 3. Two-element microphone arrays
`
`Small Two-Element Array. A small two-element array uses two uni-directional
`microphones 100 millimeters (4 inches) apart. Work area: ±50O horizontally, ±60O
`vertically. The channels order is: 1 –left, 2 –right, looking from front of the microphone
`array.
`Big Two-Element Array. A big two-element array uses two uni-directional microphones
`200 millimeters (8 inches) apart. Work area: ±50O horizontally, ±60O vertically. The
`channels order is: 1 – left , 2 – right, looking from front of the microphone array.
`
`© 2014 Microsoft Corporation. All rights reserved.
`
`Jawbone's Exhibit No. 2014, IPR2022-01124
`Page 009
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`Microphone Array Support in Windows - 10
`
`Four-Element Arrays
`A four-element microphone array can cover an office or cubicle with good sound
`capturing when the speaker is up to 2.0 meters (6 feet) away.
`
`a) Linear four-element microphone array
`
`
`
` b) L-shaped four-element microphone array
`
`
`
`
`
`b) Linear four-element microphone array second geometry
`
`
`
`Figure 4. Four-element microphone arrays
`
`These arrays are suitable for both external microphone array implementation and for
`integration into laptops and tablets working under normal noise conditions in the office or
`on the go. All microphones point directly forward to the speaker, as shown in Figure 4.
`Linear Four-Element Array. A linear four-element array uses four uni-directional
`microphones 190 millimeters and 55 millimeters apart. Work area: ±50O horizontally,
`±60O vertically. This microphone array implemented as external USB device is shown on
`Figure 5. It steers the beam in a horizontal direction only, in the range of ±50O. Directivity
`in a vertical direction is provided by the uni-directional microphones. The channels order
`is from left to right looking from front of the microphone array, that is, 1 is the most left, 4
`is the most right microphone.
`
`© 2014 Microsoft Corporation. All rights reserved.
`
`Jawbone's Exhibit No. 2014, IPR2022-01124
`Page 010
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`Microphone Array Support in Windows - 11
`
`
`Figure 5. Linear four-element USB microphone array
`
`L-shaped Four-Element Array. An L-shaped four-element array uses four uni-
`directional microphones and is designed especially for laptop/tablet convertibles. Two of
`the microphones are positioned on the upper bezel of the screen at 95 and 25
`millimeters from the right edge; the second pair is positioned on the right bezel of the
`screen at 45 and 135 millimeters from the upper horizontal edge.
`With this geometry, the microphones are not
`covered by the hand for either left-handed or
`right-handed users, and they are away from the
`keyboard and the speakers. These positions
`are for laptop mode.
`Work area: ±50O horizontally, ±50O vertically.
`This microphone array integrated into a tablet
`is shown on Figure 6. Note that in tablet mode,
`after the screen rotation, the microphones are
`positioned along the left bezel of the screen.
`L-shaped four-element arrays steer the beam
`in horizontal direction only, in the range of
`±50O, but having microphones with different
`vertical coordinates improves the directivity in
`vertical direction.
`The channels order: 1 and 2 are the left and
`right microphones in the horizontal side, 3 and
`4 are respectively the upper and lower
`microphones in the vertical side.
`
`Linear Four-Element Array Second Geometry. A linear four-element array uses four
`omnidirectional microphones 160 millimeters and 70 millimeters apart. Otherwise it looks
`the same as the one on Figure 4a). The channels order is from left to right looking from
`front of the microphone array, that is, 1 is the most left, 4 is the most right microphone.
`Design Considerations
`The Microsoft microphone array algorithm is CPU time-efficient. It moves signal
`processing inside the PC and enables the manufacture of very inexpensive microphone
`array devices. The device itself is nothing more than a multichannel microphone
`connected to an audio hardware solution. The device captures the signals from the
`microphones, converts them to digital form, and sends them to the computer. The
`integrated microphone array support in Windows will do all the processing, and then
`combine the signals to prove high-quality audio output to the application layer.
`
`Figure 6. L-shaped microphone
`array integrated into a tablet PC
`
`
`
`© 2014 Microsoft Corporation. All rights reserved.
`
`Jawbone's Exhibit No. 2014, IPR2022-01124
`Page 011
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`Microphone Array Support in Windows - 12
`
`Hardware Interface
`There are two ways to connect the microphone array to a personal computer:
` Using a digital USB interface. The advantages to this approach are guaranteed
`sound-capturing quality, uniformly good user experience through the discoverability
`of USB devices, and compatibility with most computers available today. This
`solution is suitable for both external and integrated microphone arrays. The device
`consists of microphones, preamplifiers, analog-to-digital-converters (ADCs), and a
`USB controller.
` Using an analog multichannel audio interface provided by the next-generation
`integrated-PC audio solution, HD Audio. This is a less expensive solution;
`microphones are connected to the HD Audio codec already on the board. This
`solution is more suitable for integrated microphone arrays, but if a standard
`multichannel analog connector becomes available, external microphone arrays can
`be built for computers equipped with HD Audio codecs.
`Requirements for Microphones and Preamplifiers
`The microphone elements used in a design solution should be low noise, directional,
`and with low manufacturing tolerances. Under any circumstances, ribbon, carbon,
`crystal, ceramic, and dynamic microphones must not be used for building microphone
`arrays.
`For microphone arrays integrated into tablets and laptops, it is acceptable to use
`omnidirectional microphones. The laptop, tablet body, or screen provides certain
`directivity of the microphone; however, in general such microphone array will have lower
`noise suppression. For external microphone arrays, using omnidirectional microphones
`is unacceptable.
`To prevent adding electrical noises, shielded cables should be used to connect the
`microphones with preamplifiers. The preamplifiers should be low noise and should
`provide high-pass filtering.
`The tolerances of elements used should provide close matching of the preamplifiers’
`parameters. The Microsoft adaptive microphone-array software can compensate to
`some degree for variations in the manufacturing tolerances [2]. For best results, it is
`recommended that solutions meet the requirements specified in Table 2.
`Table 2. Requirements for microphones and preamplifiers
`Component
`Requirement
`Work band
`200-7,000 Hz
`Microphone type
`Uni-directional
`Microphone SNR
`Better than 60 dB
`Sensitivity tolerances,
`In the range of ±4 dB or better for all frequencies
`microphone and preamplifier
`in the work band
`Phase response tolerances,
`In the range of ±10O for all frequencies
`microphone and preamplifier
`in the work band
`High-pass filter cut-off
`150 Hz at –3 dB
`High-pass filter slope
`Better than 18 dB/oct
`
`Requirements for ADCs
`We assume that ADCs used in microphone arrays have integrated anti-aliasing filters
`and do not place any requirements for low-pass filtering.
`
`© 2014 Microsoft Corporation. All rights reserved.
`
`Jawbone's Exhibit No. 2014, IPR2022-01124
`Page 012
`
`

`

`
`
`Microphone Array Support in Windows - 13
`
`For capturing voice for telecommunication and speech recognition, a bandwidth of 200
`to 7,000 Hz is considered sufficient. This means that a minimum sampling rate of 16 kHz
`is required. Increasing the sampling rate beyond this minimum only increases the
`processing time without bringing any additional benefits.
`The sampling rate of the ADCs used should be synchronized for all microphone
`channels. Not only should all ADCs use the same sampling frequency (a common clock
`generator can ensure this), but they should also all be in the same phase with a
`precision of better than 1/64th of the sampling period. This last point requires taking
`some additional measures that depend on the ADCs used. For a four-channel
`microphone array, a typical solution uses two stereo ADCs with a synchronized
`sampling rate.
`The requirements for ADCs are specified in Table 3.
`Table 3. Requirements for the ADCs
` Component
` Requirement
` Sampling rate
` 16,000 Hz, synchronized for all ADCs
` Sampling synchronization
` Better than 1/64th of the sampling period
` Anti-aliasing filter
` Integrated
`
`Use of MEMS Microphones for PC Microphone Arrays
`The recent improvements in development of micro-electromechanical systems (MEMS)
`technology has made possible manufacturing of MEMS microphones into a single chip
`with prices comparable to electret microphones. So-called digital MEMS microphones
`contain the analog-to-digital converter and directly output a digital signal. The benefits of
`MEMS microphones for microphone arrays include the following:
` Single-chip MEMS microphones have low manufacturing tolerances, which makes
`them more suitable for microphone-array applications where microphone matching
`is important.
` MEMS microphones are small and therefore can be used in very compact products
`or when product design must accommodate space limitations in areas such as the
`bezel of a laptop monitor.
` Digital MEMS microphones are less affected by radio frequency and
`electromagnetic interferences, which makes them attractive for mobile PCs or for
`placement near the RF antenna in tablets or laptops.
` The digital audio signal can be carried over normal wires, replacing the shielded
`cables necessary for analog microphones. This makes digital microphones attractive
`for integration into laptops where putting several shielded cables trough the hinge
`might be a problem. Designers can place one or more of these microphones in
`positions based solely on the optimization of acoustic performance and functionality.
`
`The requirements described in this paper for noise and directivity also apply to analog
`and digital MEMS microphones. For digital MEMS microphones, as with analog
`microphones, it is important that the sample rate of the ADCs used on each digital
`MEMS microphone is synchronized and in phase, as described in this paper.
`
`© 2014 Microsoft Corporation. All rights reserved.
`
`Jawbone's Exhibit No. 2014, IPR2022-01124
`Page 013
`
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`
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`Microphone Array Support in Windows - 14
`
`Number of Microphones
`The number of microphones depends of the scenario. More microphones means better
`noise suppression and greater ability to provide good sound quality in noisy
`environments. On the other hand, it also means a more exp