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  1. echo cancellation Ian Hung 2B Computer Engineering University of Waterloo August 17th, 2001

  2. Squelching Echo in a Digital Cellular Network A. Background and Significance B. Echo Basics C. Process of Echo Cancellation D. Echo Control E. Evaluating Audio Quality F. Future Trends Presentation Overview This presentation will explore the different methods of handling echo inherent within a digital cellular network.

  3. A. Background and Significance

  4. Speech quality is the benchmark at which the quality of the network is assessed. Wireless phones have gained enormous popularity and are becoming essential communication tools that impact our lives from day-to-day personal tasks to how business is performed. The key is to maintain customer loyalty by improving network quality thus generating additional revenue. For this reason, a considerable amount of research in the removal of inherent hybrid and acoustic echo with the aim of improving perceived voice quality has been conducted. A. Background andSignificance

  5. B. Echo Basics

  6. Mobile Switching Center (MSC) Cell Site 4 Wire 4 Wire 2 Wire Local Telephone Exchange 4 Wire Long Distance Toll Office Long Distance Toll Office Long Distance Toll Office 4 Wire 4 Wire Communications Network General Overview Due to the cost of cabling, two-wire circuits are used to connect the telephone to the local telephone exchange rather than four-wire trunks. • Hybrid Echo • Acoustic Echo • Other Echo Sources B. EchoBasics

  7. Hybrid transformers are used to interface this two-wire circuit with a four-wire station. 4 Wire Send Port This conversion does not work perfectly and results in electrical energy leaking back to the talker in the form of echo. Hybrid Device 2 Wire Port Echo 4 Wire Receive Port When there is little round-trip delay (28ms), it creates a positive sense that the call is live by adding sidetone. Hybrid Echo • Hybrid Echo • Acoustic Echo • Other Echo Sources B. EchoBasics

  8. Intrusive echo results when the total network delay exceeds 36ms. The amount of echo depends on how well the hybrid matches both circuits. This is measured as echo return loss (ERL). High ERL value = weak signal being reflected. • Hybrid Echo • Acoustic Echo • Other Echo Sources B. EchoBasics

  9. Audio reflected from window Audio reflected from dash Audio reflected from door Multipath echo is transmitted back to the distant end and heard as echo. This form of echo originates from handsets and is produced by poor isolation between the microphone and speaker. Acoustic Echo • Hybrid Echo • Acoustic Echo • Other Echo Sources B. EchoBasics

  10. Background noise is generated when the mobile is operated in hands-free mode. This noise is passed through the vocoder causing further distortion in speech. Delay in either speech compression or transmission equipment further induce echo and degrade voice quality. …especially true when the call is processed through mixed network infrastructures including copper wire, fiber optic lines, microwave connections, international gateways, and satellite transmission • Hybrid Echo • Acoustic Echo • Other Echo Sources B. EchoBasics

  11. C. Process of Echo Cancellation

  12. ERLE Send to Far End Echo Canceller S-in S-out Hybrid Drop-Side Line-Side Longhaul Receive from Far End R-out R-in Endpath Echo Canceller Implementation in the Digital Wireless Network C. Process of Echo Cancellation

  13. Hybrid's impulse response must be learned- known as adaptation Residual Echo S-in S-out Subtractor NLP Echo Estimate Error Signal Convolution Processor H-Register X-Register Hybrid Double-Talk Detector The endpath is mapped R-out R-in Contents within either registers are multiplied together, a process called convolution, producing the echo estimate that is fed to the Subtractor. Echo Canceller – Schematic Diagram C. Process of Echo Cancellation

  14. Subtractor eliminates the echo estimate from the convolution processor Residual Echo S-in S-out Subtractor NLP Echo Estimate Error Signal Convolution Processor H-Register X-Register Hybrid Double-Talk Detector H-register is adjusted accordingly, refining the estimate of the impulse response, in a process called convergence R-out R-in Echo Canceller – Schematic Diagram C. Process of Echo Cancellation

  15. Double-talk occurs when both near and far-end callers are speaking at the same time. Residual Echo S-in S-out Subtractor NLP Near-end speech during double-talk can distort the error signal and confuse the adaptation process. Echo Estimate Error Signal Error signal from the Subtractor is ignored causing the contents within the H-register tofreeze Convolution Processor H-Register X-Register Hybrid Double-Talk Detector R-out R-in Echo Canceller – Schematic Diagram C. Process of Echo Cancellation

  16. Nonlinear echoes: clipped speech signals speech compression poor quality speakerphones pulse code modulation (PCM) …challenge convolution processor to develop an accurate echo estimate. Residual Echo S-in S-out Subtractor NLP Echo Estimate Error Signal NLP: Reduces the residual echo to inaudible levels based on the adaptive suppression threshold Convolution Processor H-Register X-Register Hybrid Double-Talk Detector R-out R-in Signals above the threshold are allowed to pass where as signals belowthethresholdareremoved Echo Canceller – Schematic Diagram C. Process of Echo Cancellation

  17. D. Echo Control

  18. Mobile Switching Center (MSC) Cell Site 4 Wire Echo Canceller 4 Wire Hybrid Public Switched Telephone Network (PSTN) 2 Wire 4 Wire When a call is made from the mobile user to the PSTN, the caller will experience echo-free speech, while the receiver may experience poor speech quality. Therefore, the echo canceller must work both ways to accommodate users from either ends. Echo Canceller Implementation in the Digital Wireless Network • Handling Hybrid Echo • Handling Acoustic Echo • Handling Other Complex Echo D. Echo Control D. Echo Control

  19. During conversation, the audio picture dynamicallychanges, which causes the canceller to adapt continually. From MSC Loudspeaker Echo Canceller Echo EC should be capable of receiving 270 ms of delay. Any less and the remaining echo would be beyond the ability for the canceller to remove them. To MSC (no Echo) Microphone (Speech w/Echo) Acoustic echo return loss enhancement (AERLE), the amount of attenuation applied to unfavorable echo, has a value of 65 dB as the minimum requirement with the non-linear processor enabled. Echo Canceller Implementation in the Mobile Handset • Handling Hybrid Echo • Handling Acoustic Echo • Handling Other Complex Echo D. Echo Control D. Echo Control

  20. Performance is dependent upon both network-side EC integration, and mobile-side handset design Due to strongpricepressures, most handsets do not control echo very well. In some cases, such handsets are known to produce a terminalcompilingloss of 24dB. Echo Cancelled Voice Sample Voice Sample Echo Cancelled Hybrid 2 Wire 4 Wire Back-to-Back Implementation in the Digital Wireless Network • Handling Hybrid Echo • Handling Acoustic Echo • Handling Other Complex Echo D. Echo Control D. Echo Control

  21. E. Evaluating Audio Quality

  22. A. Total Echo Removal 1. No echo at the beginning of the call 2. No Echo throughout the call - call is stable without bursts or distortion 3. Residual Echo (low-level) is absent throughout the call B. Double-Talk Clarity 1. No echo or scratching noises throughout the call 2. Speech is clearly audible during double-talk 3. No clipping at the beginning and end of speech *Occasionally, echo cancellers may be fooled due to high background noise. If double-talk detection is handled incorrectly, near-end speech, perceived to be echo, may be attenuated. C. Background Audio Transparency On contrary, background audio transparency is the ability for an echo canceller to allow background noise to be heard. Poor DSP = audio fading in and out Better DSP = consistent background audio. Noise injection to fill-in for background audio is occasionally used. Replacing these intelligible background sounds with random noise can be irritating. E. Evaluating Audio Quality D. Echo Control

  23. F. Future Trends

  24. In the pursuit for greater call clarity, echo cancellers have been the center of recent development, and have now become highly sophisticated transmission equipment at the heart of highly complex networks. Effective implementation of echo cancellation across the network holds the key to improving call quality and ultimately, strengthen customer satisfaction, boost revenues, and reduce subscriber churn. F. Future Trends D. Echo Control