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CVC®
Two Microphone Headset
Document (Miscellaneous)
Issue 2
© Cambridge Silicon Radio Lim2011-2012ited 2011-2012
This material is subject to CSR’s non-disclosure agreement
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CS-218321-DCP2
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Document History
Revision
Date
History
1
22 AUG 11
Original publication of this document
2
01 JAN 12
Updated to latest CSR style
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 2011-2012
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CVC Two Microphone Headset Document (Miscellaneous)
Contacts
Contents
Document History.................................................................................................................................................... 2
Contacts .................................................................................................................................................................. 2
Trademarks, Patents and Licences ......................................................................................................................... 2
Life Support Policy and Use in Safety-critical Compliance ...................................................................................... 2
Contents .................................................................................................................................................................. 3
Tables, Figures and Equations................................................................................................................................ 3
1.
Introduction ...................................................................................................................................................... 4
2.
Headset Design Recommendations ................................................................................................................. 5
3.
2.1.
Microphone Placement ............................................................................................................................. 5
2.2.
Other Design Considerations .................................................................................................................... 7
Recommended Microphone and Speaker Specifications ................................................................................. 8
3.1.
Electrets Microphone ................................................................................................................................ 8
3.2.
Loudspeaker ............................................................................................................................................. 8
Document References ............................................................................................................................................ 9
Terms and Definitions ............................................................................................................................................. 9
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 2011-2012
This material is subject to CSR’s non-disclosure agreement
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CVC Two Microphone Headset Document (Miscellaneous)
Performance and Conformance .............................................................................................................................. 2
1.
Introduction
CSR recommends that you follow the guidelines in this document to ensure that the 2-mic CVC algorithm works
effectively, particularly regarding microphone spacing and orientation.
This document provides guidelines for the effective design of a 2-mic CVC headset.
CVC Two Microphone Headset Document (Miscellaneous)
© Cambridge Silicon Radio Limited 2011-2012
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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-mic headset using the
CVC algorithm.
Note:
PLAN VIEW
SIDE VIEW
Slide Boom Example moves the microphones
closer to the mouth for
maximum performance
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 2011-2012
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CVC Two Microphone Headset Document (Miscellaneous)
Figure 2.1 applies to a sliding boom headset and Figure 2.2 applies to a fixed microphone configuration
headset.
Omni-directional Microphone
placement example
Microphone Spacing
30 mm (+170 / -10 mm)
Secondary Rear
Microphone
Primary Front
Microphone
Figure 2.2: Fixed Microphone Configuration
CSR recommends:
1.
Microphone separation distance is 30mm, 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),
30mm 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 and good frequency response.
4.
Microphones are mounted on the headset centreline so sensitivity is similar when worn on the left
or right ear.
© Cambridge Silicon Radio Limited 2011-2012
This material is subject to CSR’s non-disclosure agreement
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CVC Two Microphone Headset Document (Miscellaneous)
CL
2.2.
Other Design Considerations
As well as the placement recommendations described in section 2.1 CSR recommends you:
1.
Minimise the front microphone-to-mouth distance:
The closer the front microphone is to the mouth helps improve speech SNR, enabling the
CVC software to yield maximum noise suppression and the highest speech quality (lowest
distortion).
1.2.
It also improves frequency response and lowers microphone gain (i.e. less electrical noise)
Attempt mechanical wind protection using microphone placement, porting or the addition of a windscreen.
2.1.
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.
Use of similar microphones where possible:
3.1.
This helps to achieve a similar frequency response between the microphones.
Gasket microphones into position. Benefits include:
4.1.
Isolates vibration between microphone, PCB and case.
4.2.
Avoids multipath leakage by sealing microphone chambers.
4.3.
Baffles the loudspeaker, reducing echo.
Minimise the acoustic echo from the loudspeaker to microphone, i.e:
5.1.
Provide a mechanical design where the hardware delivers > 25 TCLw.
5.2.
Any echo is stronger in the rear microphone.
5.3.
Orients the microphone diaphragm perpendicular to the loudspeaker.
If analogue electrets microphones are used, wire them differentially to reduce common mode noise.
Microphones of various technologies can be supported depending on the CSR silicon chosen.
Microphone types include analogue electrets, MEMS digital and MEMS silicon analogue.
Minimise the acoustic, electric and mechanical crosstalk between microphones to > 40dB.
© Cambridge Silicon Radio Limited 2011-2012
This material is subject to CSR’s non-disclosure agreement
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CVC Two Microphone Headset Document (Miscellaneous)
1.1.
3.
Recommended Microphone and Speaker Specifications
3.1.
Electrets Microphone
Distortion:
< 2%THD from 100Hz to 10kHz at the desired maximum SPL

Sensitivity:
-42 dB and < +/- 3dB part to part variance

Impedance:
2.2KΩ, < 5KΩ to reduce channel crosstalk

Voltage Rating:
2 volts

S/N Ratio:
> 55dB

Direction:
Omni-directional
3.2.

Loudspeaker
Distortion:
< 5%THD from 100Hz to 10kHz at the required maximum SPL
© Cambridge Silicon Radio Limited 2011-2012
This material is subject to CSR’s non-disclosure agreement
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CVC Two Microphone Headset Document (Miscellaneous)

Document References
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
Terms and Definitions
BlueCore®
Group term for CSR’s range of Bluetooth wireless technology chips
Bluetooth®
Set of technologies providing audio and data transfer over short-range radio connections
CSR
Cambridge Silicon Radio
CVC®
Clear Voice Capture
i.e.
Id est, that is
MEMS
Micro-electro-mechanical systems
PCB
Printed Circuit Board
SNR
Signal to Noise Ratio
SPL
Sound Pressure Level
THD
Total Harmonic Distortion
WNR
Wind Noise Reduction
© Cambridge Silicon Radio Limited 2011-2012
This material is subject to CSR’s non-disclosure agreement
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CVC Two Microphone Headset Document (Miscellaneous)
Document