Generation of Echoes in Communication
chapter ONE introduction 1.0 Background
A significant problem in communications is the generation of echoes. The echoes arise for a number of reasons, with the primary reason being an impedance mismatch. The impedance mismatch occurs when the two-wire network meets the four-wire network, this interface is known as the hybrid. This impedance mismatch causes some of the signal energy to be returned to the source as an echo [1]. This can be seen in Figure 1. (All figures appear at the end of the report.)
Figure 1: schematic layout of the Echo canceller [1]
The delays between primary and echo signals are directly related to the transmission distance. For example, if a signal was sent to a satellite that redirected the signal back to another location on earth, that signal would have a very large time delay compared to a signal sent to a local switching station and back. Short delays (less than 50 ms) will not affect the quality of the signal as much as longer delays. Delays of this length are not noticed by the receiver and therefore are not considered an annoyance. However, these echoes may have an effect on data being transmitted through transmission lines [2].
A sinusoid will be used as the input signal. The DSP board will create an echo of the sinusoid and add the echo to the original sinusoidal signal, thus creating a distorted version of the input signal. The DSP will then use LMS adaptive filtering to estimate the echo, and remove the echo from the distorted signal creating a reconstructed signal. The LMS algorithm seeks to minimize the excess mean-square error (MSE) between the echo signal and the estimated echo. The excess MSE refers to the LMS algorithm fluctuations about the adaptive filter coefficients after a large number of iterations [3].
This thesis consists of designing and implementing an echo canceling system. The DSP chip is used to simulate the echo creating system and to implement the adaptive filtering system to cancel
A significant problem in communications is the generation of echoes. The echoes arise for a number of reasons, with the primary reason being an impedance mismatch. The impedance mismatch occurs when the two-wire network meets the four-wire network, this interface is known as the hybrid. This impedance mismatch causes some of the signal energy to be returned to the source as an echo [1]. This can be seen in Figure 1. (All figures appear at the end of the report.)
Figure 1: schematic layout of the Echo canceller [1]
The delays between primary and echo signals are directly related to the transmission distance. For example, if a signal was sent to a satellite that redirected the signal back to another location on earth, that signal would have a very large time delay compared to a signal sent to a local switching station and back. Short delays (less than 50 ms) will not affect the quality of the signal as much as longer delays. Delays of this length are not noticed by the receiver and therefore are not considered an annoyance. However, these echoes may have an effect on data being transmitted through transmission lines [2].
A sinusoid will be used as the input signal. The DSP board will create an echo of the sinusoid and add the echo to the original sinusoidal signal, thus creating a distorted version of the input signal. The DSP will then use LMS adaptive filtering to estimate the echo, and remove the echo from the distorted signal creating a reconstructed signal. The LMS algorithm seeks to minimize the excess mean-square error (MSE) between the echo signal and the estimated echo. The excess MSE refers to the LMS algorithm fluctuations about the adaptive filter coefficients after a large number of iterations [3].
This thesis consists of designing and implementing an echo canceling system. The DSP chip is used to simulate the echo creating system and to implement the adaptive filtering system to cancel