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Unformatted text preview: ECE 421 - Sum 2010 Notes Set 13: Echo Cancellation 1 INTRODUCTION In this set of Notes we will study how the concepts of vectors and matrices provides one way to solve a very important problem in communications. The problem we will study is the generation of echos in long distance voice communications. However, similar phenom- ena arise in data communication networks, in which the term reflections, instead of echos, is used. Many comunications links (including voice, video, and data) are composed of varied trans- mission lines (like telephone lines, coaxial cables, and fiber optic cables) connected to- gether. Transmission line theory shows that an impedance mismatch between two trans- mission lines causes energy to be propagated back toward the sending location. If the signal energy is in audible frequency range, then this reflection may be interpreted as an audio echo at the senders location. The effect at a senders location is that an echo of the senders signal will be received at a time-delay corresponding to the round-trip propagation time on the link. If this signal is an audio signal it can be a very annoying echo which can disrupt the process of information transfer between the sender and receiver. By deriving a mathematical model for the echo generation process, we can create sig- nal processing devices called echo cancellers, which greatly improve the quality of long- distance telephony. The set of Notes will examine one method for creating these echo cancellation algorithms. The approach will use Least Squares techniques, which depend heavily on the vector and matrix techniques we studied earlier. ECE 421 - Sum 2010 Notes Set 13: Echo Cancellation 2 NETWORK ARCHITECTURE To appreciate the source of the echo generation problem it helps to consider the problem of simultaneous communication between Speakers A and B in a long-distance telephone network. Suppose both speakers are using wireline, residential telephones. A very basic block diagram of such as system is shown below: H H Local Loop Local Loop Long Distance Section Section Section Speaker A Speaker B s (t) A s (t) A s (t) A s (t) A r (t) A r (t) A r (t) A A detailed explanation of the system above will be given in class. We can understand the basic problem of echos by following the progress of Speaker As signal, represented by s A t , as it propagates to Speaker B. Begin with s A t , the rightward travelling signal originating from Speaker A. This signal travels over Speaker As Local Loop, and then is connected to the Long Distance link. The long distance connection from Speaker A terminates at Speaker Bs Hybrid, represented by the H on the right side of the figure. The function of the Hybrid for Speaker B is to couple the signal from Speaker A to the Local Loop for Speaker B. However, due to impedance mismatches, some of Speaker As signal leaks into the return long distance circuit connected from B back to A. This return, or echo, signal is denoted by r A t . ECE 421 - Sum 2010...
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This note was uploaded on 09/07/2010 for the course ECE 421 taught by Professor Hallen during the Summer '08 term at N.C. State.
- Summer '08
- Signal Processing