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Abstract - AE5

AE5.1	Stereophonic Acoustic Echo Cancellation Using Nonlinear Transformations and Comb Filtering J. Benesty, D. Morgan, J. Hall, M. Sondhi (Bell Labs, USA) Stereophonic sound becomes more and more important in a growing number of applications (such as teleconferencing, multimedia workstations, televideo gaming, etc.) where spatial realism is demanded. Such hands-free systems need stereophonic acoustic echo cancelers (AECs) to reduce echos that result from coupling between loudspeakers and microphones in full-duplex communication. In this paper we propose a new stereo AEC based on two experimental observations:(a) the stereo effect is due mostly to sound energy below about 1 kHz and (b) comb filtering above 1 kHz does not degrade auditory localization. The principle of the proposed structure is to use one stereo AEC at low frequencies (e.g. below 1 kHz) with nonlinear transformations on the input signals and another stereo AEC at higher frequencies (e.g. above 1 kHz) with complementary comb filters on the input signals.
AE5.2	A Stereo Echo Canceler with Pre-Processing for Correct Echo-Path Identification Y. Joncour, A. Sugiyama (NEC Corp - C&C Media Research Labs, Japan) A new stereo echo canceler with pre-processing for correct echo-path identification is proposed. The pre-processing is accomplished by a two-tap time-varying filter which delays the input signal periodically by one sample in one of the two channels. Aliasing components and audible clicks by pre-processing are made inaudible by selecting appropriate parameters for the filter. Simulations with the NLMS algorithm and a white Gaussian signal confirm the correct echo-path identification. For speech signals, the convergence speed of the proposed echo canceler is more than three times faster than that of an echo canceler with nonlinear transformations. Results of a subjective listening test demonstrate that quality of the pre-processed signals is 4.38 using the CCIR five-grade impairment scale. This is acceptable for general teleconference applications.
AE5.3	Using Auditory Properties to Improve the Behavior of Stereophonic Acoustic Echo Cancellers A. Gilloire, V. Turbin (France Telecom / CNET, France) We focus on the problem of stereophonic acoustic echo cancellation for teleconference applications. To limit the well-known detrimental effect of the correlation between the loudspeaker input signals, we propose a new method which consists in adding to these signals random noises controlled by auditory properties. We describe this method in some details and we show that its complexity can be fairly low. We demonstrate experimentally that the improvement yielded by this method is higher than the one provided by a former method based on the use of a non-linearity.
AE5.4	New Configuration for a Stereo Echo Canceller with Nonlinear Pre-Processing S. Shimauchi, Y. Haneda, S. Makino, Y. Kaneda (NTT Human Interface Laboratories, Japan) A new configuration for a stereo echo canceller with nonlinear pre-processing is proposed. The pre-processor which adds uncorrelated components to the original received stereo signals improves the adaptive filter convergence even in the conventional configuration. However, because of the inaudibility restriction, the pre-processed signals still include a large amount of the original stereo signals which are often highly cross-correlated. Therefore, the improvement is limited. To overcome this, our new stereo echo canceller includes exclusive adaptive filters whose inputs are the uncorrelated signals generated in the pre-processor. These exclusive adaptive filters converge to true solutions without suffering from cross-correlation between the original stereo signals. This is demonstrated through computer simulation results.
AE5.5	Stereophonic Acoustic Echo Cancellation System Using Time-Varying All-Pass Filtering for Signal Decorrelation M. Ali (Texas Instruments, USA) This paper describes a novel technique for decorrelating the stereo signals in stereophonic acoustic echo cancellation (AEC) systems. At present, most teleconferencing systems use a single full-duplex audio channel for voice communications. However, in order to introduce spatial realism, future teleconferencing systems are expected to have more than one channel (at least stereo with two channels). However, in stereophonic AEC systems, the correlation between the stereo signals does not allow correct identification of the echo path responses. In this paper, we develop a signal decorrelation technique based on time-varying all-pass filtering of the individual stereo signals. Experiments show that this technique does not effect the perception of the stereo signals, but identifies the echo path responses correctly.

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