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A daptive D ual M icrophone. ADM Executive Summary The theory and the prior art ADM approach Comparison of ADM with shotgun ADM in Bluetooth headsets ADM in mobile phones ADM and other Alango technologies Alango Ltd contact information. ADM technology white paper v3.2.

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A daptive d ual m icrophone l.jpg

Adaptive Dual Microphone

ADM Executive Summary

The theory and the prior art

ADM approach

Comparison of ADM with shotgun

ADM in Bluetooth headsets

ADM in mobile phones

ADM and other Alango technologies

Alango Ltd contact information

ADM technology white paper v3.2


Adm executive summary l.jpg
ADM Executive Summary

Adaptive DualMicrophone (ADM) is a patent pending, digital signal processing technology creating the best possible directional or noise canceling microphone using only two omni-directional microphones.

Standard, fixed directivity microphones (figure eight, cardioid, super-cardioid and others) cannot be optimal when the microphone and/or acoustic interference are changing direction. ADMautomatically adapts itself to provide the best noise attenuation and no signal distortion in varying environments. The adaptation process is very fast and frequency selective so that multiple interferences may be cancelled simultaneously. Besides, ADM is much less affected by wind than other directional microphones, it has no proximity effect and it is much easier to integrate into mobile and other devices than acoustic directional microphones.

ADM technology allows two types of microphones’ configuration: “endfire” and “broadside”. In the endfire configuration the sound of interest is supposed to be on the axis (line connecting the two microphone). In the broadside configuration it is supposed to be on the line transverse to the axis.

In the endfire configurationADMhas two modes of operation: “far-talk” and “close-talk”. In the far-talk modeADMworks as the best possible directional microphone attenuating signals arriving from back and side directions while preserving the front signal. It is ideal for applications like conference, reporter and surveillance microphones, camcorders, hearing aids, assistive listening devices and mobile phones in “speakerphone mode”.

In the near-talk modeADMworks as the best possible noise canceling microphone creating a virtual “bubble of silence” around it effectively eliminating distant sounds without affecting the close sound. Relative freedom of acoustic design makes is ideal for mobile handsets with “soft” switch between speakerphone (far) and handset (close) modes of operation.

ADM technology real time demos are available

ADM technology white paper v3.2


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The theory and the prior art

Theory.Creating a fixed directivity microphone is (theoretically) simple. It may be built electronically from two omnidirectional sound pressure sensors (left figure) or acoustically by opening the microphone diaphragm from two sides (middle figure) .

Microphone directivity is defined by time delay  .If T is the sound propagation time between the sensors (ports) then, varying  between 0 and T it is possible to steer the most attenuated direction between 90 and 270 degrees. The right figure shows attenuation as a function of angle  (“microphone polar pattern”) for =0 (blue), =T/2 (green), =T (magenta). Note also a change in sensitivity as a function of  . If not compensated, sensitivity of a directional microphone drops as 6dB/octave with decrease in frequency.

State of the art. In a simple microphone design the delay  is fixed resulting in one of possible polar patterns (figure eight, cardioid, super or hyper cardioid). Some professional microphones have an option to switch manually between several, predefined polar patterns to provide the best attenuation of unwanted sound in a specific environment. The decrease in sensitivity in low frequency region is compensated by acoustic or electronic means to provide flat frequency response for far-field sound waves. A noise canceling microphone is generally just a bi-directional microphone without any frequency compensation. Thus it provides no additional sound attenuation for on-axis sounds except natural attenuation with distance and low frequency sensitivity decrease.

  • Problems of directional microphones:

  • They are not optimal in changing environments where interferences are coming from different directions at different times and have different spectral content;

  • Due to low frequency sensitivity compensation, they amplify sound field irregularities and thus, they are very sensitive to wind. This requires big wind screens or other protective measures.

  • The frequency response depends on the distance between the sound source and the microphone (proximity effect).

  • They are difficult to build into mobile devices.

ADM technology white paper v3.2


Adm approach l.jpg

AdaptiveDualMicrophone (ADM) patent pending technology provides a solution for all the problems of fixed directivity microphones specified before. It adapts its directional characteristics to a situation at hand, it is robust to wind noise better than omnidirectional microphones, it does not have a proximity effect and it is easy to build into mobile or other devices. This is achieved by Alango proprietary digital signal processing of signals recorded by two sound pressure sensors. ADM technology has low latency and it can be easily implemented on any low cost, 16 bit DSP.

ADM approach

Digitized signals recorded by the two sound pressure sensors are divided on several frequency bands. Optimal directivity gradient microphone is built in every frequency by varying the corresponding delays Fi , Ri. The objective of optimization is direct the null of the polar pattern in a specific band to the direction giving the largest SNR improvement. Subband decomposition scheme, optimization criteria and the method are Alango proprietary technologies. The optimization is very fast so that the microphone adapts itself to new conditions in less than 10 milliseconds. The purpose of Post Processing (PP) blocks is to compensate for changes in the frequency response, eliminate the proximity effect and reduce the wind noise. These features are also proprietary for Alango technology.

The functionality of the processing blocks above depends on the operational mode.

In the far-talk mode the delays are chosen to attenuate all signals but the one coming from the main direction. For the endfire configuration the theoretical polar pattern of ADM microphone is described as half of figure eight. Figure on the right shows ADM polar pattern as measured in an anechoic chamber using a pair of chip, not specially matched electret microphones. The blue line shows the figure eight pattern. About 20dB attenuation is achieved for back sounds in the whole frequency range.

In the close-talk mode ADM tries eliminate all sounds that create equal (with some specified tolerance) sound pressure level on its two sound pressure sensors. Thus all distant sounds coming from all directions are cancelled while a close sound that creates enough sound pressure difference is preserved without any distortion.

ADM technology white paper v3.2


Comparison of adm with a shotgun l.jpg

AT4073a

ADM

Comparison of ADM with a shotgun

Adaptive DualMicrophone technology was compared side by side with Audio Technica shotgun microphone (model AT4073a). Alango conference room was used for recording. Brüel & Kjær's mouth simulator was used as shown on the picture instead of a real speaker. High level of ambient babble noise was produced by a set of powerful loudspeakers.

Click on buttons to compare the recordings

AT4073a

ADM

ADM technology white paper v3.2


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ADM advantage in Bluetooth Headsets

Woman and lorry

Regular microphone

ADM microphone

Adaptive Dual Microphone

(ADM)

Man in a train

Regular microphone

ADM microphone

ADM technology white paper v3.2


Slide7 l.jpg

ADM in mobile phones

Close-talk (handset )“Bubble Of Sensitivity” (BOS)muting outside sounds

Far-talk (speakerphone )“Beam Of Sensitivity” (BOS)attenuating outside sounds

PossibleConfigurations

Beam Of Sensitivity

Bubble Of Sensitivity

ADM

ADM

Endfire

Broadside

ADM technology white paper v3.2


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Alango Voice Communication Package (VCP)

ADM (Adaptive Dual Microphone) : Attenuates all types of noises

Stationary Noise Suppressor : Suppresses stationary noises (traffic, engine, tiers, etc)

Acoustic Echo canceller : Eliminates acoustic echoes ensuring full-duplex communication

Speech Enhancer : Improves speech intelligibility without increasing the sound volume

Automatic Gain Control : Equalizes possible changes in signal levels

Noise Dependent Equalization : Equalizes loudspeaker signal according to the environmental noise.

ADM technology white paper v3.2


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Alango contact information

www.alango.com

Headquarters: Edgar 2, Tirat-HaCarmel, 39100, Israel

Telephone: +972 4 8580743

Fax: +972 4 8580621

e-mail: [email protected]

ADM technology white paper v3.2


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