Acoustic echo cancellation for single-and dual-microphone devices: Application to mobile devices

With the increased flexibility and mobility which they provide, mobile terminals are arguably the most popular and widespread telecommunications devices of the present day. Mobile terminals are used in widely different and adverse conditions such as in handsfree mode or in noisy environments. Particularly in hands-free mode, part of the far-end voice signal from the loudspeaker is coupled to the microphone. Furthermore, in noisy environments the microphone also captures the ambient noise in addition to the useful near-end speech signal. In consequence, mobile terminals are generally equipped with speech signal processing algorithms in order to maintain acceptable speech quality. This thesis mainly focuses on echo cancellation. Acoustic echo cancellation is generally achieved through adaptive filtering followed by echo postfiltering. Adaptive echo cancellation aims to estimate the echo signal recorded by the microphone. The echo postfilter is used to attenuate the residual echo. Echo postfiltering often operates in the subband or frequency domain. Filtering in the frequency or subband domain is appealing on a computational complexity point of view. However, subband filtering suffers from significant signal delay whilst frequency domain filtering suffers from time domain aliasing due to circular convolution. Alternative filtering methods can be use to avoid these problems. The first contribution in this thesis aims to assess different filtering methods for combined noise reduction and echo postfiltering. Assessments show that all filtering methods approximately achieve the same amount of echo suppression. However, they are not equivalent in terms of perceived speech quality: time domain filtering methods introduce some crackling while other methods introduce musical noise. Although adaptive echo cancellation and echo postfilter both target the same problem, they are generally implemented separately. We propose a synchronized approach to adaptive echo cancellation and echo postfiltering. We also introduce a new stepsize computation methods. The synchronization approach and stepsize proposed method achieves a significant increase of the convergence rate and robustness of the adaptive filter. Recent approaches to improve the speech quality are based on multi-microphone devices. In line with this trend, dual-microphone mobile devices can be found on the market. The last part of this thesis deals with dual-microphone echo control. Recordings with dual-microphone devices show us that a significant level difference is observed between the microphone signals. Based on this observation, we propose a frequency domain based double-talk detector. The proposed DTD is assessed with a single microphone echo postfilter. Assessment show that the use of the double-talk detector leads to increased echo suppression and slightly reduced distortion. Our echo processing scheme is still composed of adaptive filtering followed by postfiltering. The novelty lies withtin the postfilter which uses two microphone signals instead of one. We introduced two novel approaches to echo postfiltering. The first is based on the level difference between the microphone and is suitable for a specific arrangement of the transducer on the device. The second postfilter is not restricted to a specific arrangements of the transducer. This solution is also advantageous as it can be used to tackle non-linear echo problem. It turns out that the proposed dual-microphone postfilter achieve good echo suppression whist keeping the distortion of the useful speech low.