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Definition and Coordination of Signal Processing Functions for telephone connections involving automotive speakerphones
Scott PennockSenior Hands-Free Standards SpecialistQNX Software Systems
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Outline
Introduction
List of Signal Processing Functions (SPFs)
Detail for each SPF
Problem trying to solve
SPF solution
Coordination between terminal and network Speech Processing Equipment (SPE)
Conclusions
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Introduction
Telephone connections involving automotive speakerphones are among the most challenging types of connections
Signal Processing Functions (SPFs), also known as speech enhancement functions, are essential for user acceptance
SPFs reduce or eliminate:
Fundamental problems caused by
Far-field transducers
Noisy environments
Terminal design problems
Network-related problems
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List of SPFs
SPFs include:Send or Ingress (entering network)
Acoustic Echo Cancellation (AEC)
Line Echo Cancellation (LEC)
Microphone Array Processing (MAP)
High-Frequency Encoding (HFE)
Noise Reduction (NR)
Equalization (EQ)
Automatic Level Control (ALC; also referred to as AGC)
Limiting (LM)
Receive or Egress (exiting network)Band-Width Extension (BWE)
EQ
ALC
Noise Compensation (NC; also referred to as ALE)
LM
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Acoustic Echo Cancellation (AEC)
Acoustic echo
Far end talker hears an echo of his/her own voice
Caused by sound coming out of the loudspeaker being picked-up by the microphone and transmitted back to the talker
NR BWE EQ ALC NC LM
LM ALC EQ NR HFE MAPNetwork A
EC
Far end
SPF solution Acoustic Echo Cancellation
(AEC) can eliminate acoustic echo heard by far end
Allows both ends to simultaneously talk without noticeable speech or noise level fluctuations
This is known as “Type 1” full duplex performance
AEC
Loudspkr
MicTonetwork
FromnetworkSPE coordination
Only SPE closest to source of acoustic echo should be used
Echo path delay likely known and stable
Echo path more linear Can be optimized to
acoustic interface
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Microphone Array Processing (MAP)
Variable speech levelsFar end hears speech level differences
Caused by different positions of users relative to microphone
High noise levelFar end hears high levels of background or electrical noise
Caused by far distance of microphone and high vehicle noise levels
NR BWE EQ ALC NC LM
LM ALC EQ NR HFE MixNetwork
Far end
SPF Solution Microphone Array
Processing (MAP) intelligently combines signals from multiple mics to optimize speech pick-up and reduce noise
Should be robust to mic type and spacing
AE
C
MAP
MAP Mic 2
Mic 1Out
SPE coordination No coordination required
because specific to acoustic interface
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High Frequency Encoding (HFE)
Poor intelligibility of narrowband calls
Far end listener has difficulty correctly recognizing speech sounds
Caused by the narrowband telephone network filtering out speech frequencies above around 3400Hz
NR BWE EQ ALC NC LM
LM ALC EQ NR HFE MAPNetwork
Far end
SPF solution High Frequency Encoding
(HFE) improves intelligibility of narrowband calls
High-frequency speech energy is encoded into lower frequencies that are transmitted by network
AE
C
HFE
MicOutHFE
Actual Before AfterCompare
SPE coordination Only SPE closest to
bandwidth restriction should be used
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Noise Reduction (NR)
High noise levelFar end hears high levels of noise
Caused by:Far distance of microphone
Acoustic noise sources
Airflow
NR BWE EQ ALC NC LM
LM ALC EQ NR HFE MAPNetwork
Far end
SPF solution Noise Reduction (NR)
reduces noise heard by the far end
Can be used to compensate for design problems
Important to track noise even during speech
AE
C
NR
Mechanical vibration
Electro-Magnetic Interference (EMI)
SPE coordination Only SPE closest to signal
source should be used
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Equalization (EQ)
Unnatural speech
Far end listener hears speech that does not have the same timbre as the talker
Caused by filtering of microphone and other components of the speech signal path
NR BWE EQ ALC NC LM
LM ALCALC WB NR HFE MAPNetwork
Far end
SPF Solution Equalization (EQ) makes
speech sound more like the person talking
Should use different EQ curves for low and high noise environments
AE
C
EQ
MicOutEQ
SPE coordination SPE closest to acoustic
interface should be used EQ should be done before
ALC
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Automatic Level Control (ALC)
Variable speech levels
Far end hears speech level differences
Caused by different speaking levels and distances of users relative to microphone
NR BWE EQ ALC NC LM
LMLM EQ NR HFE MAPNetwork
Far end
SPF solution ALC ensures far end
hears a consistent perceived loudness by adjusting level
Should be robust to different signal types and only train on speech
AE
C
ALC
Compare
ALC “Off”
ALC “On”
SPE coordination SPE closest to acoustic
interface should be used ALC should be done after
other SPFs that will affect the speech signal level
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Limiter (LM)
Speech distortion
Far end hears distorted speech (has “buzzy” quality)
Caused by amplitude clipping in signal path (e.g., exceeding dynamic range of telephone system)
NR BWE EQ ALC NC LM
ALC EQ NR HFE MAPNetwork
Far end
SPF solution Limiter (Lim) reduces
distorted speech caused by amplitude clipping
Protects against speech distortion caused from large peaks in speech
AE
C
LM
SPE coordination SPE closest to acoustic
interface should be used
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Noise Reduction (NR) on Receive
Noise
Near end listener hears noise
Caused by some Network Access Device (NAD) chips
May only be audible in speakerphone mode because of increased gain
NR BWE EQ ALC NC LM
LM ALC EQ NR HFE MAPNetwork
Far end
SPF solution NR eliminates
objectionable noise heard by near end
AE
C
NR
SPE coordination Should only be run when
noise detected Needs to be run in tandem
with NR in SPE at far end acoustic interface
Need NR at far end of connection to help with speech encoding
Need NR at near end of connection because receive noise generated after far end NR
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BandWidth Extension (BWE)
Narrowband call quality
Near end listener complains of poor quality of narrowband calls
Caused by contrast effect with higher quality wideband calls and increasing user expectations due to exposure to wideband quality
NR BWE EQ ALC NC LM
LM ALC EQ NR HFE MAPNetwork
Far end
SPF solution BWE improves quality of
narrowband calls and helps minimize contrast effect and user dissatisfaction
BWE reconstructs low and high frequency information removed by the telephone network based on speech information that was transmitted
AE
C
BWE
To loudspkr
Fromnetwork
BWE
SPE coordination SPE closest to acoustic
interface should be used
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Equalization (EQ)
Unnatural speech
Near end listener hears speech that does not have the same timbre as the talker
Caused by filtering of loudspeaker and other components of the speech signal path
NR BWE EQ ALC NC LM
LM ALC EQ NR HFE MAPNetwork
Far end
SPF solution EQ makes far end talker
sound more like themself Compensates for
frequency response of loudspeaker in housing
Prevents loudspeaker distortion by attenuating frequencies which overdrive the loudspeaker
AE
C
EQ
SPE coordination SPE closest to acoustic
interface should be used
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Automatic Level Control (ALC)
Variable speech levels
Near end hears speech level differences across calls
There are several causes:Bandwidth differences between narrowband and wideband calls
Speaking level differences across talkers at far end
Other fixed gains that vary from call to call (e.g., far end terminal, network, Bluetooth, etc.)
NR BWE EQ AGC NC LM
LM ALC EQ NR HFE MAPNetwork
Far end
SPF solution ALC ensures near end
hears a consistent perceived loudness
Should account for effects of bandwidth (wideband vs. narrowband) and the dynamic nature of speech
AE
C
ALC
SPE coordination SPE closest to acoustic
interface should be used
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Noise Compensation (NC)
Variable speech levels
Near end hears speech level differences across noise conditions
Caused by a perceptual effect known as loudness masking in human auditory system
NR BWE EQ ALC DLC LM
LM ALC EQ NR HFE MAPNetwork
Far end
SPF solution Noise Compensation (NC)
causes near end to hear speech at the same loudness across noise conditions
Automatically adjusts receive volume control based on noise measurement—which is better than wheel-speed
Improves intelligibility in the presence of noise
AE
C
NC
NC “Off”
NC “On”
CompareSPE coordination SPE closest to acoustic
interface should be used
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Limiter (LM)
Speech distortionNear end hears distorted speech (has “buzzy” quality)
Caused by overdriving digital domain, electronic components, or loudspeaker
Acoustic echo or speech level fluctuationsFar end hears echo of his/her own voice and/or speech level fluctuations of near end talker
Caused by overdriving system components which leads to echo path distortion and ultimately poor AEC performance
NR BWE EQ ALC NC Lim
LM ALC EQ NR HFE MAPNetwork
Far end
SPF solution The Limiter reduces or
eliminates speech distortion, echo, and speech level fluctuations caused by overdriving system components
It does this by soft limiting the receive speech signal before it is hard-clipped
AE
C
LM
SPE coordination SPE closest to acoustic
interface should be used
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Line Echo Canceller (LEC)
G.161_F01
Hybrid
Networkelements
Networkelements
Sin
AECHO
Cancelledend
Rout Receive path
Echo estimator and othercontrol circuitry
Digital subtractor
ACANCSend path
Non-linearprocessor
ANLP
LRES
Sout
LRET
Non-cancelledend
LRin
Rin
ITU-T G.799.2/Figure 2 Block diagram of an echo canceller [ITU-T G.168]
Network echo
Near end talker hears an echo of his/her own voice
Caused by electrical reflection of speech energy at network hybrid
SPF solution Line Echo Cancellation (LEC)
can eliminate acoustic echo heard by Near end
Allows both ends to simultaneously talk without noticeable speech or noise level fluctuations
SPE coordination Only SPE closest to source
of network echo should be used
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Conclusions
SPFs are essential for user acceptance of telephone connections involving automotive speakerphones
Where SPFs are placed along the telephone connection is important to their effectiveness
Running SPFs in tandem can degrade performance
It is important to coordinate the operation of SPFs along the connection
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Thank you
Scott Pennock
Senior Hands-Free Standards Specialist