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Voice Quality Enhancements

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Outline

• Acoustic and network echo

• Noise Reduction (NR)

• Mobile Cross-talk Control (MCC)

• Noise Level Compensation (NLC)

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Causes of echoReflection

Delay

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Echo in telecommunications systems

• Network (or PSTN, or electrical) echo– Generated by electrical reflection of the signal

• Acoustic echo– Generated by acoustic reflection of the audio signal

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Electrical (PSTN) Echo

4-wiretransmissionnetwork

Hybrid2-wiresubscriberline

PSTNsubscriber

Echo

PSTN: Public Switched Telephone Network

Mismatching in the impedance

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Acoustic Echo

Loudspeaker

Microphone

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Acoustic Echo Control - Terminals

• Terminals are different in terms of– Size

– Design

– Internal design of Acoustic Echo Cancellation

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Pure delay and End Path Delay

time

Echo Path Delay (EPD) G.168

Pure delay

Example of an impulse response of an echo path

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Different Echo type characteristics

PSTN echo (Electrical echo)

time

64 ms

time>5-10 ms

120-320 ms

MS echo(Acoustic echo)

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Echo tolerance

EchoReturnLoss(ERL)

dB

Echo ToleranceCurve

Tolerable

Intolerable

Delay (msec)

AnalogNetworks

DigitalNetworks Satellite

200 300

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Mobile Cross-talk Control (MCC)Acoustic Echo Control

Digitalcellularsubscriber

PSTNsubscriber

DigitalCellular System

Network echo

ECP

Mobileservices

SwitchingCenter

Acoustic echo

NetworkEcho Canceller

MobileCross-talk Control

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Noise Reduction (NR)Reduce the background noise on the mobile side

ECP: Echo Canceller in PoolPSTN: Public Switched Telephone Network

PSTNsubscriber

DigitalCellular System

Network echo

MobileSwitching

CenterDigitalcellularsubscriber

Mobile side

ECP

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Noise Reduction (NR)Reduce the background noise on the PSTN side

ECP: Echo Canceller in PoolPSTN: Public Switched Telephone Network

PSTNsubscriber

DigitalCellular System

Network echo

MobileSwitching

CenterDigitalcellularsubscriber

ECP

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Noise Level Compensation (NLC)Improves audibility in noisy environment

ECP

NLC