2. Echo is Your Enemy Echo Cancellation is Your Solution

3. Agenda What is Echo? What Causes Echo? Echo Characteristics Echo and Speech Recognition: A Real-World Example Brooktrout Solutions

4. 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

5. Hybrid PSTN Network What Causes Echo? 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

6. Echo Characteristics …what ci t y will you be fly ing to..? Outgoing Prompt Incoming Echo 5-50 msec delay 6 – 11 dB attenuation

7. 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

8. 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…

9. Echo Cancellation Hybrid Echo Cancellation PSTN Network 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

10. 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

11. 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

12. 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

13. 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

14. 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 cancellation and highest channel density on the market today

15. http://www.brooktrout.com 250 First Avenue Needham, MA 02494 (781) 449-4100