Communicate. Your Way.
Visit us at:
www.octasic.com & www.octware.net
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Achieving Superior VoIP Sound Quality
with Software Echo Cancellation
Who is affected by poor voice quality?
You
Which Echo
Everyone!
Cancellation
Your
Customers
solution are
you
using?
Your
Company’s
Image
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Can you Talk the Talk?
• Definition of Echo
• Network Delays
• Network Scenarios
• Echo Cancellation Requirements & Features
• Echo Cancellation Solutions
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Echo
• What is it?
– Echo is the repetition of a sound caused by the
reflection of sound waves.
• Types of Echo
– Electrical / Line
– Caused by impedance mismatch in hybrid in analog local
loop
– Acoustic
– Caused by sound emitted from a speaker and captured
by the microphone after a delay.
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Hybrid-caused Electrical Echo
Person A
Person B
• Cause: mismatched impedances in Hybrid (H)
• Noticeable and distracting when delay > 20ms.
– PSTN network: on long distance calls > 3000km (~1800miles)
– On all VoP calls
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Handset Cross-Talk
• Mechanical or electromagnetic problem
– Occurs with cell phones
• Many variables with wireless equipment
– Many makers & models
– Angle at which it’s held
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Network & Delay
Voice over Packet Delay Calculations
Cause of Delay
Packetization
Time (ms)
10 – 20
Processing
1–2
Network Queuing
2 – 45
PDV Buffering
5 – 30
Processing
2 – 20
Total round trip
delay
Details
10-20 ms packets
• Transmission time negligible
• VPN N.A. quoted by MCI
• Resource Queuing
20 – 117 ms
In Packet Networks, delay is always > 20ms,
so echo is always noticeable and distracting.
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Network Scenarios
Echo Canceller in Network
Pointed
this way
Pointed
this way
Far end
Local end
Local end
Far end
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EC in Packet Network
H
EC
TDM
Network
EC
IP
Network
H
DLC
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TDM
Network
Trunking
GW
Trunking
GW
DLC
EC in Enterprise Network
EC
H
(Electrical)
EC
(Acoustic)
IP
Network
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IP PBX
GW
TDM
Carrier
Network
DLC
EC in IAD / VoIP Home Gateway
H
EC
Ethernet
IAD
MAC
TDM
Network
POTS
VoIP
EC
SLIC
IP
Network
H
DLC
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DLC
Inverse
GW
EC
TDM
Network
Access
Platform
IAD
EC in IAD / VoIP Home Gateway
H
EC
Ethernet
IAD
MAC
TDM
Network
POTS
VoIP
EC
SLIC
DLC
EC
IP
Network
H
TDM
Network
QuickT i me™ and a
T IFF (Uncompressed) decompressor
are needed to see this picture.
IP Phone
DLC
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Access
Platform
WiFi
HGW
Echo Cancellation (EC)
Requirements & Features
Inside an Echo Canceller
•
Composed of 2 Parts
1) Adaptive Filter (AF)
•
Removes linear echo
2) Non-Linear Processor (NLP)
•
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Removes residual, non-linear echo
Adaptive Filter’s (AF)
Functionality
1) Samples Rin and Sin
2) Builds mathematical
model of echo path
3) Simulates echo and
feeds it to subtractor
4) Subtractor removes bulk
of echo (linear portion) on
return path (Sin)
5) Voice then enters NLP
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Rin
Sin
Echo Cancellation Algorithms
• Least Square (LS)
– Used in Block processing
– More robust than LMS
• Especially in presence of double-talk and background noise
• Least Mean Square (LMS)
– Recursive Algorithm
– Most common algorithm
– Requires a lot of adjustments
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Convergence
AF Algorithms for EC Compete in 2 Types of Races
1) Sprints for Fast Convergence
– Start of calls
– Re-converging after change in echo path
2) Marathons for Continual Improvements
– Despite noise in echo signal
– Through silence & double-talk
AF’s Challenge
To simultaneously achieve
2 conflicting characteristics:
Fast Convergence & High Stability
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Divergence
Adaptive Filter Problem
• Cause:
– No suitable solution for line model found with mathematical
algorithm
• Result:
– Can corrupt signal
– Can even add echo!
Good ECs are tuned to avoid divergence
in nearly all line conditions
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Non-Linear Processor’s Functionality
The NLP:
1) Evaluates residual echo
2) Removes all signals below a
certain threshold
3) Replaces those signals with
simulated background noise
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NLP’s Design Characteristics
• Frequency-aware
• Able to handle different situations
– Changes in frequency of BGN
– Start and end of speech
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Acoustic Echo Cancellation
Speakerphones and Double-talk
• Acoustic echo common with speakerphones
• Some speakerphone makers simplified EC’s task
– Walkie-talkie style design
• Only one person’s voice transmitted at a time.
– Result
• Unnatural & unpleasant conversation
• Solution: Allow & handle double-talk
– With High quality EC solution
• Adaptive filter key for Double-Talk
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Acoustic Echo Cancellation
• Adaptive Filter
– Limited amount of linear echo
– Changing signal
• Strong NLP required
– High quality background noise matching
– Long tail coverage
– Operate well despite varying environment & echo point
• With user’s movement
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Mic & Speaker Separation
• Most Acoustic Echo is inside the box
– Best isolation reduces echo
– The lower the echo level the better the sound quality
• A few things to watch
– Separate enclosure for Mic & Speaker
– Excellent Mic to chassis isolation
Device
Echo
Local Talk
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Acoustic Echo Saturation
Normal Signal
Saturated Signal
No linear
relationship
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Signal Saturation
• Speaker Output
– Amplifier is powerful enough (linear part of curve)
– DAC is linear
– Speaker is not overdriven
• Microphone Input
– Analog Circuit is linear
• No frequency cut-offs
– Microphone enclosure
• Designed to avoid membrane saturation
– A/D range is properly adjusted
Device
Echo
Local Talk
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Vibration on Casing
• Casing vibration is non-linear
– This cannot be removed by EC
– Can only be addressed through Noise Reduction
• Causes:
– The speaker is not attached properly
• Vibrates against the casing and creates noise
– The speaker is not isolated from the chassis
• Makes another part of the device vibrate
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Echo Cancellation Solutions
Software Echo Cancellation
The Need
• Long processing delays cause echo in all IP communications
• EC is essential to restoring voice quality
Essential Features
•
•
•
•
Transparent algorithm
High quality background noise handling
Excellent double-talk handling
Fast convergence
Benefits
•
•
•
•
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Delivers carrier-grade voice quality
Eliminates support calls from your customers
Plug & Play
Cost-effective
Software Echo Cancellation
EC Solutions for all Endpoints
Line Echo Cancellation (LEC)
Integrated
Access
Devices
Acoustic Echo Cancellation (AEC)
IP PBX &
Asterisk® Market
(IADs)
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® Asterisk is a trademark of Digium, Inc.
Mobile & IP Telephony
•
Hand-helds
•
DECT Phones
•
Speakerphones
•
IP Phones
•
Soft phones
EC & Voice Quality Solutions
Hardware Solution:
OCT6100
Software Solution:
SoftEcho
• 1-8 channels
• For consumer equipment
– Asterisk® based IP PBX
– Softphones
– IP & speaker phones
For 8+
channels
• 8-672 channels
• For enterprise & carrier
equipment
• Available on interface cards
from many vendors
• Max. power: 2W at 672
channels
• CPU usage: 5% per channel*
• 8 ch. high quality EC: 40%
CPU usage
Deployed worldwide in wireless, wireline & VoP networks
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Digium
Thank you!
Communicate. Your Way.
Visit us at:
www.octasic.com & www.octware.net
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