Transcript SHER Speech Harmonics Enhancement & Restoration

Adaptive Dual Microphone
ADM technology white paper v3.2
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 Executive Summary
Adaptive Dual Microphone (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. ADM automatically
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 configuration ADM has two modes of operation: “far-talk” and “close-talk”. In the far-talk
mode ADM works 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 mode ADM works 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
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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) .
d
Acoustic delay 

D(t)
d

Delay 
D(t)
+
Sound direction
Sound direction
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 farfield 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;
2.
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.
3.
The frequency response depends on the distance between the sound source and the microphone (proximity effect).
4.
They are difficult to build into mobile devices.
1.
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ADM approach
Endfire configuration

A/D
A/D
S(t)
Sound direction
Split
d
Split
DSP
Broadside configuration
Delay
R1
-
+
PP 1
Delay
F1
+
PP N
Combine from bands
Adaptive Dual Microphone (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.
D/A
D(t)
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.
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Comparison of ADM with a shotgun
AT4073a
ADM
Adaptive Dual Microphone 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 technology white paper v3.2
ADM
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ADM advantage in Bluetooth Headsets
Woman and lorry
Regular microphone
ADM microphone
Adaptive Dual Microphone
Man in a train
Regular microphone
ADM technology white paper v3.2
(ADM)
ADM microphone
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ADM in mobile phones
Close-talk (handset )
“Bubble Of Sensitivity” (BOS)
muting outside sounds
Far-talk (speakerphone )
“Beam Of Sensitivity” (BOS)
attenuating outside sounds
Possible
Configurations
Beam Of Sensitivity
Bubble Of
Sensitivity
ADM
Endfire
ADM technology white paper v3.2
ADM
Broadside
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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 :
Automatic Gain Control :
Improves speech intelligibility without increasing the sound volume
Noise Dependent Equalization :
Equalizes loudspeaker signal according to the environmental noise.
ADM technology white paper v3.2
Equalizes possible changes in signal levels
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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]
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