Voice Quality Enhancements

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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 echo
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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-wire transmission network
2-wire subscriber line
Hybrid
PSTN subscriber
Echo
Mismatching in the impedance
PSTN Public Switched Telephone Network
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Acoustic Echo
Microphone
Loudspeaker
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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
Echo Path Delay (EPD) G.168
Pure delay
time
Example of an impulse response of an echo path
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Different Echo type characteristics

• PSTN echo (Electrical echo)

time
64 ms
MS echo (Acoustic echo)
time
gt5-10 ms
120-320 ms
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Echo tolerance
Echo Return Loss (ERL)
dB
Tolerable
Echo Tolerance Curve
Intolerable
Delay (msec)
200
300
Analog Networks
Digital Networks
Satellite
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Mobile Cross-talk Control (MCC)Acoustic Echo
Control
Acoustic echo
Mobileservices Switching Center
Network echo
Digital cellular subscriber
PSTN subscriber
Digital Cellular System
ECP
Network Echo Canceller
Mobile Cross-talk Control
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Noise Reduction (NR)Reduce the background noise
on the mobile side
Mobile side
Network echo
Mobile Switching Center
Digital cellular subscriber
PSTN subscriber
Digital Cellular System
ECP
ECP Echo Canceller in Pool PSTN Public Switched
Telephone Network
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Noise Reduction (NR)Reduce the background noise
on the PSTN side
Network echo
Mobile Switching Center
Digital cellular subscriber
PSTN subscriber
Digital Cellular System
ECP
ECP Echo Canceller in Pool PSTN Public Switched
Telephone Network
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Noise Level Compensation (NLC)Improves
audibility in noisy environment
NLC
ECP