Stereophonic Acoustic Echo Cancellation: Theory and Implementation

The thesis treats theory and implementation aspects for stereophonic acoustic echo cancellation. In Paper I a complete implementation of a stereophonic acoustic echo canceler based on the two-channel fast recursive least-squares algorithm is presented. An analysis of the system calculation complexity is also given, in addition to simulation results on recorded real-life data. Paper II presents a comparison between adaptive filters for usage in stereophonic acoustic echo cancellation. The comparison includes, in addition to the standard normalized least mean square algorithm, the two-channel fast recursive least-squares algorithm and a two-channel frequency-domain adaptive algorithm. In the paper, the convergence rate, the calculation complexity, the signal transmission delay, and the memory usage for the evaluated systems are shown. Adaptive filters applied in subband structures may need to model a few non-causal taps even if the fullband impulse response is causal. This phenomenon is analyzed in Paper III. Formulas to calculate the number of non-causal taps needed are also presented in the paper. The fundamental problem in stereophonic acoustic echo cancellation is the signal correlation between the two channels. In Paper IV it is shown how a perceptual audio coder may decrease this correlation and thereby increase the performance of the stereophonic acoustic echo canceler. Paper V investigates the possibilities of using joint subband filterbanks or time to frequency-domain transforms for echo cancellation and perceptual audio coding. The usage of joint filterbanks/transforms not only decreases the computational complexity of the system, it also reduces the total signal transmission delay of the system if properly designed.