EEL 6502
Project #2
Due December 5, 2006
Description: This is an adaptive echo cancellation problem, where the goal is to remove the echo from one signal
(the input to an adaptive filter) that corrupts the second signal (desired signal for the adaptive filter).
The data provided for this project represents two voice channels of a long-distance telephone conversation. The
signal labeled "music" is the voice channel from the far end, the signal labeled "corrupted_speech" is the speech
signal of the speaker on the near end, and "fs" is the sample frequency in Hertz. The speech on the near end, as the
name suggests, is corrupted as a result of an imperfect two-wire to four-wire hybrid, such as commonly used in
most switching offices.
The reason the hybrid is used is because the performance of long-distance communications using four wires is
significantly better than the performance using the two-wire system found in most homes. Therefore, a hybrid is
located at a switching office at each end of the conversation to facilitate conversion to and from four wires.
However, hybrids commonly allow leakage of the signal being received into the signal that is being sent. The
result is that a person's own speech is returned to them with some delay. When the delay is large enough, such as
occurs with long-distance calls, the effect is very annoying.
The assumptions for this project are that the far end hybrid is working well, so that the music signal is not
currupted, and the near end hybrid has some leakage. This causes the incoming signal and the outgoing signal to
be correlated with each other. The goal is to use an echo canceller to restore the outgoing signal and recover the
speech.
Scenario: Middle Earth is being overrun by Sauron and Saruman the White. Gandalf the Gray, while deep in the
Mines of Moria, has called the remainder of the fellowship from his cell phone to give them some important
information about the ring that Frodo wears around his neck. Unfortunately, the hybrid at the switching office is
far from perfect and has allowed a large amount of leakage. Oddly enough, the theme music from the Lord of the
Rings can be heard playing quite loudly in Lothlorien, where the fellowship has temporarily congregated. The
leakage has allowed the theme music to completely wipe out Gandalf's message. You must repair the hybrid so
that Gandalf's message can be recovered, so the quest can continue.
The data is in a Matlab format. After dowloading the file, "proj2.mat", simply type "load proj2" in Matlab.
Goals:
Compare the performance of two different structures, the tapped delay line and the Gamma filter.
(1) Plot an appropriate measure of performance of the Gamma filter as a function of the feedback parameter,
alpha, for a fixed number of taps.
(2) Plot the performance of the Gamma filter as a function of the number of taps, for a fixed feedback parameter.
(3) Plot the performance of the tapped delay line filter as a function of the number of taps.
(4) Recover the outgoing speech signal and compare the intelligibility of the recovered speech using each
structure.
(5) Show one set of weight tracks for each structure.
(6) For this application, can the input and desired signals be exchanged as was done for noise cancellation? Why
or why not?
