Echo is Your Enemy Echo Cancellation is Your Solution.

Echo is Your EnemyEcho Cancellation is Your Solution

Agenda

What is Echo?

What Causes Echo?

Echo Characteristics

Echo and Speech Recognition:A Real-World Example

Brooktrout Solutions

What is Echo?

Outgoing Signals are Reflected Back to the Input– Delayed: transmission latency– Attenuated: hybrid, network echo cancellation– Distorted: varying electrical characteristics of path

Types of Echo– Acoustic echo

– Speakerphones, hands-free phones– Multiple acoustic reflections from surrounding surfaces

– Echo in wireless networks– Very large, variable propagation delays

– Echo in VoIP networks– Codec, jitter buffers introduce latency

– Hybrid (line) echo– An inherent problem in PSTN networks

Hybrid HybridPSTNNetwork

What Causes Echo?

2 – wire 2 – wire

– 2–wire connections between home and switch

4 – wire connections within PSTN network

– 4–wire connections within the digital network

Impedance mismatch causes reflections

ASR System

Echo cancellation required for ASR systems

Echo Cancellation

Echo Cancellation

Echo Characteristics

…what ci t y will you be fly ing to..?

Outgoing Prompt

IncomingEcho

5-50 msec delay 6 – 11 dB attenuation

Echo and ASR:A Real-World Example

A Simple Speech-Enabled Travel Application…

Typical prompt and resulting echo

The user “barges in”

What the ASR system hears and recognizes– With 16 msec echo cancellation– With 32 msec echo cancellation

A Typical Prompt…

“Welcome to Brooktrout Air…”– An introductory prompt calibrates the echo

cancellation algorithm before barge-in is enabled

“What city would you like to leave from?”

The response “Boston” is correctly recognized

But echo can interfere with recognition…

Echo Cancellation

Hybrid Hybrid

Echo Cancellation

PSTNNetwork

ASR System

Barge-In With Echo

The ASR system plays a prompt…

…the user barges in…

…but the prompt echo mixes with the user’s voice, impacting voice detection and recognition accuracy

Echo

“Austin” mixed with the echo of the prompt “…be…”is misrecognized as “Boston” Boston

Echo and ASR

ASR accuracy depends on effective echo cancellation– Typical system requires 16-20 msec echo cancellation– Many instances where 32 msec echo cancellation is

recommended

Options for 32 msec echo cancellation– Install a standalone echo cancellation system (expensive)– Use a telephony board with onboard 32 msec echo

cancellation (inexpensive)

16 msec EC

A system with excessive echo

and/or delay

32 msec EC

Compare the echo

Brooktrout ASR Solutions:Carriers and Service Providers

TR1000™ Carrier-Grade Telephony Board– PCI, cPCI form factors– Digital interfaces (T1/E1, ISDN)– Broad OS support

– Windows, Solaris, Linux, Unixware

Full-Featured Media Resource Platform– High-density ASR support

– Up to 96 channels per board, 192 per system– The highest channel density on the market today

– Vendor-independent APIs for ASR/TTS

– IP with on-board packetization– Enhanced V.34 fax– High density conferencing

With onboard 16 or 32 msec echo cancellation

Brooktrout ASR Solutions:Enterprise Applications

TR1100™ High-Density Telephony Board– Up to 60 channels per board– 8/16 msec echo cancellation– Digital interface (T1/E1)– PCI– Windows NT and 2000

Vantage™ Series– Ideal for small and medium

business applications– Up to 4 analog channels per board– 8/16 msec echo cancellation– ISA, PCI – Windows NT and 2000

Summary

Echo is Your Enemy– It’s everywhere within the PSTN network– It can destroy barge-in recognition accuracy

Echo Cancellation is Your Solution– Telephony boards can provide effective

onboard echo cancellation– How much is enough?

– 16 msec typical

– 32 msec otherwise

The new TR1000 delivers the best echo cancellationand highest channel density on the market today

http://www.brooktrout.com250 First Avenue

Needham, MA 02494(781) 449-4100

Echo is Your Enemy Echo Cancellation is Your Solution.

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