1 Visit us at: & Your Way. Communicate.

Achieving Superior VoIP Sound Quality with Software Echo Cancellation

4 Which Echo Cancellation solution are you using? Who is affected by poor voice quality? You Your Customers Your Company’s Image Everyone!

5 Can you Talk the Talk? Definition of Echo Network Delays Network Scenarios Echo Cancellation Requirements & Features Echo Cancellation Solutions

6 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.

7 Hybrid-caused Electrical Echo Cause: mismatched impedances in Hybrid (H) Noticeable and distracting when delay > 20ms. – PSTN network: on long distance calls > 3000km (~1800miles) – On all VoP calls Person A Person B

8 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

Network & Delay

10 Voice over Packet Delay Calculations Cause of DelayTime (ms)Details Packetization10 – ms packets Processing1 – 2 Network Queuing2 – 45 Transmission time negligible VPN N.A. quoted by MCI PDV Buffering5 – 30 Processing2 – 20 Resource Queuing Total round trip delay 20 – 117 ms In Packet Networks, delay is always > 20ms, so echo is always noticeable and distracting.

Network Scenarios

12 Echo Canceller in Network Far end Local end Pointed this way Local end Far end Pointed this way

13 EC in Packet Network IP Network Trunking GW TDM Network DLC H Trunking GW TDM Network DLC H EC

14 EC in Enterprise Network IP Network IP PBX GW TDM Carrier Network DLC H EC (Acoustic) EC (Electrical)

15 EC in IAD / VoIP Home Gateway Access Platform IP Network IAD Inverse GW TDM Network DLC H TDM Network DLC H EC IAD MACVoIPSLICEC EthernetPOTS

16 EC in IAD / VoIP Home Gateway Access Platform IP Network WiFi HGW IP Phone TDM Network DLC H TDM Network DLC H EC IAD MACVoIPSLICEC EthernetPOTS

Echo Cancellation (EC) Requirements & Features

18 Inside an Echo Canceller Composed of 2 Parts 1)Adaptive Filter (AF) Removes linear echo 2)Non-Linear Processor (NLP) Removes residual, non-linear echo

19 Adaptive Filter’s (AF) Functionality 1)Samples R in and S in 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 (S in ) 5)Voice then enters NLP R in S in

20 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

21 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

22 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

23 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

24 NLP’s Design Characteristics Frequency-aware Able to handle different situations – Changes in frequency of BGN – Start and end of speech

Acoustic Echo Cancellation

26 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

27 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

28 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 Local Talk Echo

29 Acoustic Echo Saturation Normal SignalSaturated Signal No linear relationship

30 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 Local Talk Echo

31 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

Echo Cancellation Solutions

33 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 Delivers carrier-grade voice quality Eliminates support calls from your customers Plug & Play Cost-effective

34 Software Echo Cancellation Mobile & IP Telephony Hand-helds DECT Phones Speakerphones IP Phones Soft phones Acoustic Echo Cancellation (AEC) Line Echo Cancellation (LEC) EC Solutions for all Endpoints ® Asterisk is a trademark of Digium, Inc. IP PBX & Asterisk® Market Integrated Access Devices (IADs)

35 EC & Voice Quality Solutions Software Solution: SoftEcho 1-8 channels For consumer equipment –Asterisk® based IP PBX –Softphones –IP & speaker phones CPU usage: 5% per channel* 8 ch. high quality EC: 40% CPU usage Hardware Solution: OCT channels For enterprise & carrier equipment Available on interface cards from many vendors Max. power: 2W at 672 channels Deployed worldwide in wireless, wireline & VoP networks Digium For 8+ channels

Thank you!

37 Visit us at: & Your Way. Communicate.