Date post: | 18-Dec-2015 |
Category: |
Documents |
View: | 217 times |
Download: | 4 times |
1
Voice Quality Enhancements
2
Outline
• Acoustic and network echo
• Noise Reduction (NR)
• Mobile Cross-talk Control (MCC)
• Noise Level Compensation (NLC)
3
Causes of echoReflection
Delay
4
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
5
Electrical (PSTN) Echo
4-wiretransmissionnetwork
Hybrid2-wiresubscriberline
PSTNsubscriber
Echo
PSTN: Public Switched Telephone Network
Mismatching in the impedance
6
Acoustic Echo
Loudspeaker
Microphone
7
Acoustic Echo Control - Terminals
• Terminals are different in terms of– Size
– Design
– Internal design of Acoustic Echo Cancellation
8
Pure delay and End Path Delay
time
Echo Path Delay (EPD) G.168
Pure delay
Example of an impulse response of an echo path
9
Different Echo type characteristics
PSTN echo (Electrical echo)
time
64 ms
time>5-10 ms
120-320 ms
MS echo(Acoustic echo)
10
Echo tolerance
EchoReturnLoss(ERL)
dB
Echo ToleranceCurve
Tolerable
Intolerable
Delay (msec)
AnalogNetworks
DigitalNetworks Satellite
200 300
11
Mobile Cross-talk Control (MCC)Acoustic Echo Control
Digitalcellularsubscriber
PSTNsubscriber
DigitalCellular System
Network echo
ECP
Mobileservices
SwitchingCenter
Acoustic echo
NetworkEcho Canceller
MobileCross-talk Control
12
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
13
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
14
Noise Level Compensation (NLC)Improves audibility in noisy environment
ECP
NLC