definition and coordination of signal processing functions for telephone connections involving...

Definition and Coordination of Signal Processing Functions for telephone connections involving automotive speakerphones

Scott PennockSenior Hands-Free Standards SpecialistQNX Software Systems

2 All content copyright 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


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


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


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


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


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


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


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


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


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”


interface should be used ALC should be done after

other SPFs that will affect the speech signal level


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


interface should be used


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


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


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


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




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


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




Automatic Level Control (ALC)


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


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




Noise Compensation (NC)


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


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



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


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




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


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


Thank you

Scott Pennock

Senior Hands-Free Standards Specialist

[email protected]

definition and coordination of signal processing functions for telephone connections involving...

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