Definition and Coordination of Signal Processing Functions for
telephone connections involving automotive speakerphones
Scott Pennock
Senior Hands-Free Standards Specialist
QNX Software Systems
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
From
network
SPESPF
coordination
solution


Only
SPE Echo
closest
to source
Acoustic
Cancellation
of
acoustic
echo should
be
(AEC)
can eliminate
acoustic
used
echo heard by far end
 Echo
pathends
delaytolikely
Allows
both
Far
end
known and stable
simultaneously
talk without
noticeable
speech
noise
 Echo path
more or
linear
Network
level fluctuations
 Can be optimized to
Thisacoustic
is known
as “Type 1”
interface
full duplex performance
AEC
To
network
NR BWE EQ
ALC
NC
LM
NR
HFE MAP
Mic
LM
AEC

Loudspkr
ALC
5
EQ
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Microphone Array Processing (MAP)
Variable speech levels
Far end hears speech level differences
Caused by different positions of users relative to microphone
High noise level
Far end hears high levels of background or electrical noise
Caused by far distance of microphone and high vehicle noise levels
Solution
SPESPF
coordination
No
coordination
required
Microphone
Array
because
specific
Processing
(MAP)to acoustic
interface
intelligently combines
signals from multiple mics to
Far end
optimize speech pick-up and
reduce noise
Network

Should be robust to mic type
and spacing
Out
MAP
Mic 2
Mic 1
NR BWE EQ
ALC
NC
LM
LM
NR
HFE MAP
Mix
AEC

ALC
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EQ
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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
Actual Before After
Compare
SPESPF
coordination
solution

Only Frequency
SPE closest
to
High
Encoding
bandwidth
restriction
should
(HFE)
improves
intelligibility
of
be
used
narrowband
calls
High-frequency
speech energy
Far end
is encoded into lower
frequencies that areNetwork
transmitted by network
Out
HFE
NR BWE EQ
ALC
NC
LM
NR
HFE MAP
Mic
LM
AEC

ALC
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EQ
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Noise Reduction (NR)
High noise level
Far end hears high levels of noise
Caused by:
Far distance of microphone
Acoustic noise sources
Airflow
Mechanical vibration
Electro-Magnetic Interference (EMI)
SPESPF
coordination
solution


Noise
Reduction
Only SPE
closest to (NR)
signal
source
should
be heard
used by
reduces
noise
the far end

Far end
Can be
used to
compensate forNetwork
design
problems
NR BWE EQ
ALC
NC
LM
LM
NR
HFE MAP
AEC

ALC
EQ
Important to track noise
even during speech
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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
SPESPF
coordination
Solution


SPE
closest to acoustic
Equalization
(EQ) makes
interface should be used
speech sound more like
EQ
should
betalking
done before
the
person
ALC
Far end
Should use different EQ
curves for low and
high
Network
noise environments
Out
EQ
Mic
NR BWE EQ
ALC
NC
LM
LM
NR
HFE MAP
AEC


ALC WB
EQ
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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
ALC “Off”
Compare
SPESPF
coordination
solution


SPE
to acoustic
ALCclosest
ensures
far end
interface should be used
hears a consistent
ALC
should be
done after
perceived
loudness
by
other SPFs that will affect
Far endlevel
adjusting
the
speech
signal level
Should be robust
to
Network
different signal types
and only train on
speech
ALC “On”
NR BWE EQ
ALC
NC
LM
ALC
NR
HFE MAP
AEC


LM
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EQ
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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)
SPESPF
coordination
solution


SPE
closest
to acoustic
Limiter
(Lim)
reduces
interface should be used
distorted speech caused
by amplitude clipping
Far end
Protects against speech
distortion caused
from
Network
large peaks in speech
NR BWE EQ
ALC
NC
LM
ALC
NR
HFE MAP
AEC

LM
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EQ
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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
SPESPF
coordination
solution

Should
only be run when
NR eliminates
noise detected
objectionable noise
Needs
to
benear
run in
tandem
heard
by
end
with NR in SPE at far end
Far end
acoustic
interface


Need NR at far end
of
Network
connection to help with
speech encoding
Need NR at near end of
connection because
receive noise generated
after far end NR
NR BWE EQ
ALC
NC
LM
LM
NR
HFE MAP
AEC


ALC
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EQ
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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
SPESPF
coordination
solution
SPE
acoustic
BWE closest
improvesto
quality
of
narrowband
calls and
interface
should
be helps
used
minimize contrast effect and user
dissatisfaction

Far end
BWE reconstructs
low and high
frequency information removed
Network
by the telephone network
based
on speech information that was
transmitted
BWE
From
network
To
loudspkr
NR BWE EQ
ALC
NC
LM
LM
NR
HFE MAP
AEC

ALC
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EQ
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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
SPESPF
coordination
solution
EQ
talker
SPEmakes
closestfar
to end
acoustic
sound
more
likebe
themself
interface
should
used

Compensates for
frequency
Far endresponse of
loudspeaker in housing

Network
Prevents loudspeaker
distortion by attenuating
frequencies which
overdrive the loudspeaker
NR BWE EQ
ALC
NC
LM
NR
HFE MAP
LM
AEC


ALC
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EQ
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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.)
SPESPF
coordination
solution


SPE
tonear
acoustic
ALC closest
ensures
end
interface
should
be
hears a consistent used
perceived loudness
Faraccount
end
Should
for effects
of bandwidth (wideband vs.
narrowband) andNetwork
the
dynamic nature of speech
ALC NC
NR BWE EQ AGC
LM
AEC

LM
ALC
15
EQ
NR
HFE MAP
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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
NC “Off”
SPESPF
coordination
solution

Noise
Compensation
(NC)
SPE closest
to acoustic
causes
near
end to
interface
should
behear
used
Compare
NC “On”
speech at the same loudness
across noise conditions

Far end
Automatically adjusts receive
volume control based on noise
Network
measurement—which
is better
than wheel-speed
NR BWE EQ
NC
ALC DLC
LM
NR
LM
AEC

ALC
EQ
HFE MAP
Improves intelligibility in the
presence of noise
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Limiter (LM)
Speech distortion
Near end hears distorted speech (has “buzzy” quality)
Caused by overdriving digital domain, electronic components, or loudspeaker
Acoustic echo or speech level fluctuations
Far 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
SPESPF
coordination
solution
SPE
closest to
acoustic
The Limiter
reduces
or
interface
should
be
used
eliminates speech
distortion, echo, and
speech
Farlevel
end fluctuations
caused by overdriving
system components
Network

LM
Lim
NR BWE EQ
ALC
NC
LM
NR
HFE MAP
AEC


ALC
EQ
It does this by soft limiting
the receive speech signal
before it is hard-clipped
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Line Echo Canceller (LEC)
Network echo
Near end talker hears an echo of his/her own voice
Caused by electrical reflection of speech energy at network hybrid
Send path
ACANC
Network
elements
Sin
ANLP
Digital subtractor
Non-linear
processor
Sout
SPESPF
coordination
solution


ECHO
Only
SPE closest
to Asource
Line Echo
Cancellation
(LEC)
of
network
echo
shouldecho
be
can
eliminate
acoustic
Hybrid
used
heard by Near end Cancelled
end
Allows both ends to
simultaneously talk without
noticeable speech or noise
level fluctuations
Network
elements
LRET
LRES
Noncancelled
end
LRin
Echo estimator and other
control circuitry
Rout
Rin
Receive path
G.161_F01
ITU-T G.799.2/Figure 2  Block diagram of an echo canceller [ITU-T G.168]
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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
spennock@qnx.com
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