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CVC
®
Two Microphone Headset Industrial Design
Guideline
Document (Miscellaneous)
Issue 3
© Cambridge Silicon Radio Limited 2011-2013
Page 1 of 9
CS-218321-DCP3
www.csr.com
Document History
Revision
Date
History
1
14 NOV 13
Original publication of this document
2
01 JAN 12
Updated to latest CSR style
3
19 NOV 13
Updated to latest CSR style, added details to sections 2 and 3.
General information
Information on this product
Customer support for this product
More detail on compliance and standards
Help with this document
www.csr.com
[email protected]
www.csrsupport.com
[email protected]
[email protected]
Trademarks, Patents and Licences
Unless otherwise stated, words and logos marked with ™ or ® are trademarks registered or owned by CSR plc
and/or its affiliates.
Bluetooth® and the Bluetooth logos are trademarks owned by Bluetooth SIG, Inc. and licensed to CSR.
Other products, services and names used in this document may have been trademarked by their respective owners.
The publication of this information does not imply that any licence is granted under any patent or other rights owned
by CSR plc or its affiliates.
CSR reserves the right to make technical changes to its products as part of its development programme.
While every care has been taken to ensure the accuracy of the contents of this document, CSR cannot accept
responsibility for any errors.
Life Support Policy and Use in Safety-critical Compliance
CSR’s products are not authorised for use in life-support or safety-critical applications. Use in such applications is
done at the sole discretion of the customer. CSR will not warrant the use of its devices in such applications.
Performance and Conformance
Refer to www.csrsupport.com for compliance and conformance to standards information.
© Cambridge Silicon Radio Limited 2011-2013
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CVC Two Microphone Headset Industrial Design Guideline
Document (Miscellaneous)
Contacts
Contents
Tables, Figures and Equations
Figure 2.1: Microphone Configuration for Sliding Boom Headsets ............................................................................... 5
Figure 2.2: Fixed Microphone Configuration ................................................................................................................. 6
© Cambridge Silicon Radio Limited 2011-2013
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CVC Two Microphone Headset Industrial Design Guideline
Document (Miscellaneous)
Document History ......................................................................................................................................................... 2
Contacts........................................................................................................................................................................ 2
Trademarks, Patents and Licences .............................................................................................................................. 2
Life Support Policy and Use in Safety-critical Compliance............................................................................................ 2
Performance and Conformance .................................................................................................................................... 2
Contents ....................................................................................................................................................................... 3
Tables, Figures and Equations ..................................................................................................................................... 3
1. Introduction ............................................................................................................................................................ 4
2. Headset Design Recommendations ...................................................................................................................... 5
2.1. Microphone Placement ................................................................................................................................... 5
2.2. Design Recommendations ............................................................................................................................. 6
2.3. Additional Design Considerations ................................................................................................................... 7
3. Recommended Microphone and Speaker Specifications ...................................................................................... 8
3.1. Electrets Microphone ...................................................................................................................................... 8
3.2. Analogue MEMS Microphone ......................................................................................................................... 8
3.3. Loudspeaker ................................................................................................................................................... 8
Document References .................................................................................................................................................. 9
Terms and Definitions ................................................................................................................................................... 9
1.
Introduction
CSR recommends that you follow the guidelines in this document to ensure that the 2 microphone CVC algorithm
works effectively, particularly regarding microphone spacing and orientation.
This document provides guidelines for the effective design of a 2 microphone CVC headset.
CVC Two Microphone Headset Industrial Design Guideline
Document (Miscellaneous)
© Cambridge Silicon Radio Limited 2011-2013
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2.
Headset Design Recommendations
2.1.
Microphone Placement
This section describes CSR's recommendations for the placement of microphones in a 2 microphone headset using
the CVC algorithm.
Note:
Figure 2.1 applies to a sliding boom headset and Figure 2.2 applies to a fixed microphone configuration
headset.
PLAN VIEW
SIDE VIEW
LOUDSPEAKER
-30 deg
+ 10 mm
Cardioid
Vector
+/-30 deg
- 10 mm
Secondary Rear
Microphone –
Noise + Speech
+30 deg
Microphone
Spacing
30 mm
(+170 / -10mm)
Primary Front
Microphone –
Speech dominant
angled toward the
mouth and shielded
from wind.
NOTES:
1. To achieve best voice quality this examples illustrates the use
of omni-directional microphones.
2. The slide boom design is only a suggestion to move the
microphones closer the mouth providing increased speech SNR
and improved frequency response.
-30 deg
+30 deg
Cardioid Vector
+/- 30 deg
3. Uni-directional microphones can be used for slightly more
noise suppression
4. Wind protection is suggested.
Figure 2.1: Microphone Configuration for Sliding Boom Headsets
© Cambridge Silicon Radio Limited 2011-2013
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CVC Two Microphone Headset Industrial Design Guideline
Document (Miscellaneous)
Slide Boom Example moves the microphones
closer to the mouth for
maximum performance
Omni-directional Microphone
placement example
CL
Secondary Rear
Microphone
Primary Front
Microphone
Figure 2.2: Fixed Microphone Configuration
2.2.
Design Recommendations
1.
Microphone separation distance is 30 mm, with a tolerance of +170 mm and -10 mm (front to rear
arrangement with the mouth). For older algorithm versions (BCSW-CVC-HS-5-4-3 or earlier), 30 mm is
recommended, with a tolerance of ±10 mm.
2.
Microphones are oriented orthogonal to each other. The microphone furthest from the mouth is
oriented to face outwards from the surface of the Headset while the microphone nearest the mouth is
oriented to face towards the direction of the mouth.
3.
Use omni-directional microphones for low cost, design ease and good frequency response.
4.
Mount microphones on the headset centreline, so sensitivity is similar when worn on the left or right
ear.
5.
Minimise the front microphone-to-mouth distance as conceptually illustrated in Figure 2.1
5.1.
The closer the front microphone is to the mouth, this increases the speech power while not
affecting the noise level at the microphone. This helps improve speech SNR, allowing lower
microphone gains to be used. Lower gains reduce the surrounding acoustic noise and
electrical noise signals.
5.2.
Front microphone nearer the mouth enables the CVC software to yield maximum noise
suppression at the highest speech quality (lowest distortion).
5.3.
It also improves frequency response making the users speech sound more natural, reducing
the need for equalisation.
© Cambridge Silicon Radio Limited 2011-2013
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CVC Two Microphone Headset Industrial Design Guideline
Document (Miscellaneous)
Microphone Spacing
30 mm (+170 / -10 mm)
2.3.
Additional Design Considerations
As well as the placement recommendations described in section 2.1 CSR recommends you:
1.
Attempt mechanical wind protection using microphone placement, porting or the addition of a windscreen.
1.1.
2.
Use of similar microphones where possible.
4.
This helps to achieve a similar frequency response, phase and gains between the
microphones. These are underlying assumption to get the maximum performance from the
CVC software algorithm.
Gasket microphones into position, sealing the microphone face and the case. Benefits include:
3.1.
Isolates vibration between microphone, PCB and case.
3.2.
Avoids multipath leakage by sealing microphone chambers.
3.3.
Baffles the loudspeaker, reducing echo.
Minimise the acoustic echo from the loudspeaker to microphone, i.e:
4.1.
Provide a sealant material blocking any cavities where sound could travel internal to the
device from the loudspeaker to the microphones.
4.2.
Provide a mechanical design where the hardware delivers > 25 TCLw.
4.3.
Any acoustic echo should be biased or stronger in the rear microphone.
4.4.
Orients the microphone diaphragm perpendicular to the loudspeaker.
5.
If analogue electrets or MEMS microphones are used, wire them differentially into the CSR ADC inputs
to reduce common mode noise.
6.
A very clean microphone bias is mandatory to achieve maximum algorithm performance. Ensure an
electronic filter is used in the microphone bias circuit, limiting any electrical noise from being inducted
into the ADC inputs of the CSR chip.
7.
Microphones of various technologies can be supported depending on the CSR silicon chosen. The
primary microphone types include analogue electrets, digital and analogue MEMS but always try to
always use the same microphone model in the device.
8.
Minimise the acoustic, electrical and mechanical crosstalk between microphones to > 40 dB.
9.
Good RF design practices should be considered to ensure the radio signals (antenna placement) do
not interfere with the microphone inputs.
© Cambridge Silicon Radio Limited 2011-2013
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CVC Two Microphone Headset Industrial Design Guideline
Document (Miscellaneous)
2.1.
3.
Orienting the front microphone port at a angle towards the mouth, and using the headset
housing to shield the direct wind would have very positive results, allowing the headsets to
perform better at higher wind speeds. The more the mechanical wind reduction, the better the
microphone audio, and the less the CVC WNR software is required to clean speech.
3.
Recommended Microphone and Speaker Specifications
3.1.
Electrets Microphone

Distortion:
< 2%THD from 100 Hz to 10 kHz at the desired maximum SPL

Sensitivity:
-42 dB and < ±3 dB part to part variance at (1 kHz 0 dB=1 V/Pa)

Impedance:
2.2 KΩ, < 5 KΩ to reduce channel crosstalk

Voltage Rating:
Typical < 1.8 volts

S/N Ratio:
Typical 55dB 1kHz, 0 dB=1 V/Pa, A weighted or lower

Directivity:
Omni-directional
CVC Two Microphone Headset Industrial Design Guideline
Document (Miscellaneous)
3.2.
Analogue MEMS Microphone
Typical specifications for analogue MEMS microphones.

Distortion:
< 1%THD @ 100 dB SPL @ 1 kHz
Typical < 3%, Max <10% @ 115 dB SPL @ 1 kHz

Sensitivity:
-40 dB and < ±3 dB part to part variance at (1 kHz 0 dB=1 V/Pa)

Voltage Rating:
operational range 1.5 V to 3.3 Volts DC

S/N Ratio:
Typical 62 dB 1 kHz, 0 dB=1 V/Pa, A weighted or lower

Frequency range:
70-15000 Hz + 5 dB Ref Sens. @1 KHz

Directivity:
Omni-directional
3.3.
Loudspeaker

Distortion:
< 5%THD from 100 Hz to 10 kHz at the required maximum SPL

Impedance:
Typical 16 – 32 Ω, if driven directly from the CSR Bluetooth IC.
See CSR datasheets for limitations.
© Cambridge Silicon Radio Limited 2011-2013
Page 8 of 9
CS-218321-DCP3
www.csr.com
Document References
Document
Reference
CSR8620 BGA Datasheet
CS-212920-DS
CSR8620 WLCSP Datasheet
CS-218111-DS
CSR8645 BGA Datasheet
CS-218182-DS
CSR8640 BGA Datasheet
CS-209182-DS
ADC
BlueCore
Analogue to Digital Converter
®
Group term for CSR’s range of Bluetooth wireless technology chips
®
Set of technologies providing audio and data transfer over short-range radio connections
Bluetooth
CSR
®
Cambridge Silicon Radio
CVC
Clear Voice Capture
i.e.
Id est, that is
MEMS
Micro-electro-mechanical systems
PCB
Printed Circuit Board
RF
Radio Frequency
SNR
Signal to Noise Ratio
SPL
Sound Pressure Level
TCLw
Terminal Coupling Loss - weighted
THD
Total Harmonic Distortion
WNR
Wind Noise Reduction
© Cambridge Silicon Radio Limited 2011-2013
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CVC Two Microphone Headset Industrial Design Guideline
Document (Miscellaneous)
Terms and Definitions
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