Transcript
Acoustical Design guide for the AllEars voice suite on the BlueCore CSR platform September 2016
Retune DSP ApS Diplomvej 381 DK-2800 Kgs. Lyngby Denmark Email:
[email protected] http://www.retune-dsp.com
1 Acoustical Design guide for the AllEars voice suite on the BlueCore CSR platform
© 2016 Retune DSP ApS
About this document This document describes how to use the AllEars voice suite for headsets and hands free communication systems on the CSR BlueCore platform. It describes the acoustical considerations for successful product deployment, in particular regarding microphone placement, and acoustical test setups.
Document history Revision 1.0 1.1
Date Jan 2015 Sep 2016
Remark Initial version Updated with headphone illustrations, speakerphone examples.
Trademarks AllEars is a trademark of Retune DSP ApS BlueCore is a trademark of CSR / Qualcomm
Disclaimer Retune DSP ApS provides this guideline as a educational service only, there are no guarantuees or warranties from Retune DSP ApS or any of its employees that the principles or examples covered here can be used in commercial products with no conflict of intellectual properties of any third party. Nor can Retune DSP ApS be deemed liable for any damage or loss incurred as a result of following the examples or guidelines described in this document either in whole or in part.
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Contents About this document ......................................................................................................................................... 2 Document history .......................................................................................................................................... 2 Trademarks .................................................................................................................................................... 2 Introduction ....................................................................................................................................................... 4 Guiding Principles for Headsets......................................................................................................................... 5 Speakerphone guiding principles ...................................................................................................................... 7 Example device configurations .......................................................................................................................... 9 Example array configurations for top-mounted (horizontal plane) 3 and 4 microphone speakerphones. .. 9 Design example: down-firing speakerphone ............................................................................................... 10 Design example: stereo loudspeaker and speakerphone ........................................................................... 11 Contact ............................................................................................................................................................ 12
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Introduction This document is intended for manufacturers of headsets and speakerphones, and gives recommended and boundary configurations for microphone placement in various use cases. The document gives guidelines for the typical use cases of a beamforming headset (from boomless designs, designs with a short boom) and covers various configurations of hands free systems such as speakerphones and car audio systems.
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Guiding Principles for Headsets For headsets, best performance is achieved with end-fire configuration of the microphones. Here are some guiding principles for microphone placement. 1. Microphones as close to the mouth as possible. 2. Microphones on a line pointing towards the mouth. 3. Microphone distance should be in the range 10mm - 30mm, recommended 15-20mm for optimal performance. 4. Microphones should be shielded as much as possible from wind noise. 5. Mechanical wind protection is recommended. 6. At least two microphones should have a line-of-sight to the mouth. Further guidelines are 1. Use omni-directional microphones for low cost and good frequency response. 2. Use similar microphones in array. 3. RF shielding: a. PDM microphones provide the best isolation against undesired electromagnetic interferences. b. For analog microphones, ensure bias voltage is clean, and use correct shielding for wires. c. Ensure the RF signals from the antenna do not interfere with the microphone signals. 4. Ensure microphone port openings and surroundings are acoustically as similar as possible. 5. To reduce acoustical echo, the acoustical/mechanical feedback from loudspeaker to microphones should be as low as possible. a. Echo loss, or TCLw measured in dB, should be as high as possible, preferably better than 25dB. b. Use sealant to block direct acoustical path inside the device. c. Orient the membrane of the microphones perpendicular to the motion of the loudspeaker membrane. d. Use gaskets for microphones to reduce multipath leakage, and to protect against mechanically transferred vibrations, from contact of device to head and ear, and handling noise. 6. Wind noise considerations a. Protect the microphone openings as much as possible from direct wind, placement on the rear side, and by and use of a windscreen shielding. b. The AllEars wind noise protection system can take advantage of different amounts of wind noise, so that if one microphone is shielded by sitting on the rear side of the device and a front microphone is shielded by sitting on one side of the device, the two microphones are likely to experience wind noise at different times and the wind noise protection system in the DSP will do a better job.
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Figure 1: Recommended microphone spacing for headset with boom.
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Figure 2: Short boom and boomless design.
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Speakerphone guiding principles The AllEars beam-steering technology allows, in principle, any geometry of microphones and target location. The DSP algorithms will steer the beamforming towards the target speaker and while reducing environment noise and internal microphone noise. Having said that, the performance of the system is always limited by the microphone geometry. First of all, general beamforming principles still apply, for example
End-fire gives potentially better performance than broadside Microphones should be spaced closer than half a wavelength (2.1cm for 8kHz) Microphones should not be spaced closer than necessary, because of white-noise-gain amplification of internal microphone noise. In practice this means reduced beamforming benefit (starting at low frequencies) when microphones are moved close together.
Echo cancellation performance is important to consider as well. To ensure good double talk operation, the DSP echo canceller should have as good a starting point as possible, by minimizing coupling and nonlinearity (distortion products from loudspeaker) in the setup. Here are some specific guidelines
Acoustical echo considerations: o Place microphones far away from loudspeaker as possible to reduce acoustical echo, if possible on another side of the device. o Use microphone gaskets to reduce mechanical coupling from loudspeaker to microphones. o Use an efficient sealant to block direct acoustical path inside the device from the loudspeaker cavity to the microphones. o Orient the membrane of the microphones perpendicular to the motion of the loudspeaker membrane Place microphones close to table surface to avoid coloring from reflection from table surface. o Choose high-quality speaker drivers with low distortion. o For stereo devices, mute the loudspeaker closest to the microphones. Beamformer design considerations: o Microphones should have a clear line of sight to target talker. o For 2-microphone operation, place microphones as close as possible to an end-fire configuration towards user. o For three-microphone operation, and no knowledge of target location, use a triangle design, spacing 10-30mm apart, or use a linear array with a spacing of 10-20mm. o For four microphone operation, see the examples below for suggested patterns. o Broadside configuration is also possible (see example below), however, beamformer performance is slightly less than other configurations. o If target location is assumed known, optimal configuration is on a line that points towards the target speaker. Other considerations o Limit mechanical coupling from button presses to microphones. o Place microphone as far away from noise sources such as ventilation openings and fan ports.
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If device is meant to be used standing on a table surface, place microphones as close as possible (3-4 cm) to the surface to minimize reflections that shape the frequency response.
Please consult Retune for more assistance. We can provide simulation of expected performance using your planned microphone geometry.
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Example device configurations Example array configurations for top-mounted (horizontal plane) 3 and 4 microphone speakerphones.
Figure 3: Suggested patterns for 3 and 4 microphone beamforming for unknown target talker location..
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Design example: down-firing speakerphone Array on top of device
Downfiring speaker Figure 4: The microphones are placed on another side than where sound is generated thus reducing echo coupling. Microphone membranes are mounted orthogonal to motion of speaker membrane. Microphones are placed in a horizontal plane, meaning good beamforming at all talker azimuth (sideways) angles. This design would enable a performance both in terms of beamforming noise suppression and reverberation attenuation, as well as echo cancellation performance (specifically double talk).
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Design example: stereo loudspeaker and speakerphone Mic array here capture target in end-fire when user is in front of device
Active speaker during call
Muted during call Second-best location if top side not possible. Figure 5: Two proposed microphone array locations that maximize the distance from the target talker. The top placement gives an end-fire configuration when the talker is in front of the device, which is optimal. The secondary position shown maximizes the distance from the active loudspeaker, this minimizing echo coupling. The broadside configuration is less optimal.
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Contact Retune DSP ApS Diplomvej 381 DK-2800 Kgs. Lyngby Denmark Email:
[email protected] http://www.retune-dsp.com
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