Download Glossary of Terms – Array Microphone and Smart Microphone

Glossary of Terms – Array Microphone and Smart Microphone
This document provides a quick guide to terms used in XMOS Array Microphone and Smart Microphone
product documentation. It is assumed that the reader is familiar with digital filtering and signal processing
terminology, such as FFT, FIR, etc.
Microphone Array Term
Definition
Beamformer
Signal processing technique used in sensor arrays for combining signals
coming from a preferred direction constructively.
A beamforming microphone is designed to be more sensitive to sound
coming from one or more specific directions than sound coming from other
directions.
Beamsteering
The direction of a beamformer can be manually or automatically steered
to specific areas of interest or follow a target source.
Broadside array
Broadside microphone arrays consist of multiple microphones arranged in
a line perpendicular to the direction of sound propagation.
DC offset
The mean value of the wave in the time domain. Typically it is removed
using a high pass filter.
May also be referred to as DC bias or DC coefficient.
Decimator
Reduces the sample rate of a signal. It is a specific example of sample
rate conversion when down sampling a signal.
ARRAY MICROPHONE AND SMART MICROPHONE GLOSSARY
Delay line
Allows a signal to be delayed by a variable amount of time.
Delay and sum
The simplest type of beamformer. Inputs are delayed to ensure the
maximum response for a given direction.
Diffuse noise
Sound that appears to have no single source i.e. the sound pressure level is
uniform.
Sensors in a diffuse noise field receive noise signals that are uncorrelated,
but have approximately the same energy.
Direction of Arrival (DOA)
The direction from which the voice (sound) arrives at a point (microphone).
Directivity index
A measure for rating the directionality of a sound source in a specific
direction relative to the level that would be radiated by a perfectly
symmetrical sound source of the same power, in decibels.
Directionality
Describes the microphone’s sensitivity to sound from various directions. The
main types of directionality are: omnidirectional, unidirectional,
bidirectional.
Dynamic range
The difference in dB between the loudest and quietest sounds detected by
a microphone.
End-fire array
End-fire microphone arrays consist of multiple microphones arranged in
line with the direction of sound propagation.
Far field
The sound source is far enough away from the microphone that the wave
appears as a plane wave orthogonal to the direction of travel and the
Sound Pressure Level obeys the Inverse Square Law. The distance is usually
greater than 1000mm.
Gain compensation
The gain is corrected so that the relative amplitudes of the channels are
matched.
Inter-Integrated Circuit (I2C)
Multi-master, multi-slave, single-ended, serial bus used for attaching lowerspeed peripheral integrated circuits to processors and microcontrollers.
© 2016 XMOS LTD
Third party trademarks are hereby acknowledged.
This is a preliminary glossary, contents are subject to change.
XM-010800-TM| 2016-11-07
Page 2 of 5
Inter-IC Sound (I2S)
Serial bus interface used for connecting digital audio devices; used to
transfer PCM audio data between integrated circuits.
Inverse square law
States that the sound pressure level decreases 6 dB with each doubling of
distance from the source.
Main lobe or beam
The lobe (or beam) in a beamformer with the maximum power and field
strength.
Micro-Electro-Mechanical
Systems (MEMS)
Miniaturized mechanical and electro-mechanical devices manufactured
using semiconductor fabrication processes that range in size from 0.02 to
1.0mm. The most common type of MEMS devices are (micro)sensors and
(micro)actuators which are often under the control of integrated
microelectronics.
MEMS microphone
Consists of a pressure sensitive diaphragm etched on a semiconductor
alongside an ASIC (Application Specific Integrated Circuit).
The
capacitance of the sensor changes in response to a sound wave, and ASIC
converts the capacitance in the sensor to a digital or analog output
depending on the type of microphone.
Microphone array
Any number of microphones, operating in tandem, whose outputs are
combined in some sort of array pattern. Topologies include: Linear, Circle,
Square, Star.
Near field
The sound source is relatively close (usually within about two wavelengths)
and the wavefront is spherical. There is no simple relationship between
sound level and distance. The sound pressure does not obey the Inverse
Square Law. The distance is usually less than 100mm.
Noise floor
The maximum noise power. The sum of all the noise sources within a
system, where noise is defined as any signal other than the one being
monitored.
Omnidirectional
A microphone that is equally sensitive to sound coming from all directions,
regardless of the orientation of the microphone.
Pulse Code Modulation (PCM)
An encoding scheme in which an analog signal is sampled at regular
intervals, and the amplitude is then digitized.
PCM is the encoding method generally used for uncompressed audio.
Pulse Density Modulation
(PDM)
An encoding scheme in which an analog signal is represented as a stream
of single bits. The relative density of the pulses corresponds to the analog
signal's amplitude.
PDM is often used as the output for digital MEMS microphones as minimal
I/O connect is required for the 1bit data stream.
PDM is also the modulation scheme used in the DSD (Direct Stream Digital)
audio format.
© 2016 XMOS LTD
Third party trademarks are hereby acknowledged.
This is a preliminary glossary, contents are subject to change.
XM-010800-TM| 2016-11-07
Page 3 of 5
Signal to Noise Ratio (SNR)
SNR of a microphone specifies the ratio of a reference signal to the noise
level of the microphone. Measured in decibels dB. The larger the SNR the
better the quality of the output.
Synchronous Sample Rate
Conversion (SSRC)
Converts the sample rate of a signal between two sample rates where the
relative rates are a rational ratio.
Total Harmonic Distortion
(THD)
The measurement of the level of distortion on the output signal for a given
pure tone input signal
Total Harmonic Distortion plus
noise (THD+N)
The ratio of the noise plus harmonics of a signal to the signal for a given
signal frequency over a given bandwidth.
Voice Activity Detection (VAD)
Detects whether the audio is speech or silence/background noise.
Smart Microphone Term
Definition
Acoustic Echo Cancellation
(AEC)
A signal processing technique to remove acoustic echo.
Acoustic echo is produced by local audio loop back that occurs when a
near-end microphone picks up far-end signal from near-end
loudspeaker(s) integrated into a product or device.
Recently AEC has be used to describe the process of removing a known
signal, such as a keyword, from a pickup.
AEC can be implemented as:
 full duplex where there is a near-end signal and far-end signal
simultaneously and the far-end signal is removed from the pick-up to
leave just the near-end signal.
 half duplex where unlike in the full-duplex case only one signal should
be present at any one time. It prevents echoes in phones.
AEC tail length
Defines the maximum delay or “tail” in ms that the echo canceller (EC) can
handle. Simply put it is the length of time over which the EC can cancel
echo.
Note that the more reverberation a room has, the longer the tail length will
need to be.
Attenuation
A general term that refers to any reduction in the strength of a signal.
Attenuation of audio signals is quoted in dB.
Convergence rate
The rate at which an echo canceller converges. This is measured in dB per
second.
Compressor
Signal processing technique to reduce the dynamic range of audio signals.
De-reverb
A technique to remove reverberations. Commonly used to eliminate room
reflections in conference and far field voice applications.
© 2016 XMOS LTD
Third party trademarks are hereby acknowledged.
This is a preliminary glossary, contents are subject to change.
XM-010800-TM| 2016-11-07
Page 4 of 5
Echo Return Loss Enhancement
(ERLE)
ERLE is a measure of the performance of an echo canceller and is the
amount of additional signal loss (in dB) applied by the echo canceller.
Far-end
In AEC, the far-end signal refers to the signal that is not the near-end
signal. Typically the far-end signal is a music track or a measured noise
source played out of a speaker, or conference participants in a remote
location.
Limiters are compressors with fixed ratios of 10:1 or greater. Here, the
dynamic action prevents the audio signal from becoming larger than the
threshold setting and clipping.
Limiter
Near-end
In AEC, the near-end signal refers to the source of the signal produced in
the room that the microphone is in; the near-end microphone is the one in
front of you and it produces the near-end signal.
Noise cancellation
Removes unwanted noise from an environment. For example, noisecancelling headphones remove outside noise, and noise cancelling in a
truck may remove engine noise from the cab.
Noise suppression (NS)
Removes background noise from the captured voice signal, suppressing
unwanted signals.
Noise suppressors can be designed to remove stationary or non-stationary
noise. Noise suppression can be done in the cloud services as well as the
local front-end.
Reverberation
When a sound persists in an enclosed space as a result of repeated
reflection or scattering, after the sound source has stopped.
© 2016 XMOS LTD
Third party trademarks are hereby acknowledged.
This is a preliminary glossary, contents are subject to change.
XM-010800-TM| 2016-11-07
Page 5 of 5