Improved Full Duplex Echo Cancellation for Small Communication Devices
Original Publication Date: 2003-Apr-25
Included in the Prior Art Database: 2003-Apr-25
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It is well known that the acoustical / mechanical coupling between loudspeaker and microphone in communication devices results in annoying echo problems and / or howling. Up to now several possibilities exist to deal with this problem, for example: - Half Duplex Voice Operated Switch (VOX) - Adaptive echo cancellation with or without additional path attenuation - Time and/or Frequency implementations - Additional time-domain attenuation The last one is often required because of low or even negative Echo Return Loss (ERL) and non-linear distortion of the loudspeaker enclosure microphone (LEM) system, but it leads to semi-half-duplex solutions. The figure 1 shows a typical echo cancellation set-up where the undesired echo is subtracted from the microphone signal by applying an adaptive FIR (Finite Impulse Response) filter that approximates the transfer function of the echo path. With the steadily miniaturization of communication equipment seen e.g. in mobile phones variants like “pendant phones” and “watch phones” the extreme acoustical coupling has become an increasingly problem for a proper handsfree design. Error free discrimination between double talk and echo path changes is virtually impossible because of the very low or even negative ERL and non-linearities in the echo path. Additional voice operated attenuation of the echo is often required resulting in half- or semi-half-duplex solutions which are less attractive to the end user. The extreme acoustical coupling in such devices is often caused by a combination of the mechanical design, the very short distance between the transducers and the high gain necessary for handsfree operation. The resulting impulse response of the echo path is in these cases characterized by a fixed part with a very high amplitude and a time varying part with a much lower amplitude. The inventive step is to apply an extra filter with fixed transfer function, W1, to the typical echo cancellation solution (figure 2). Addressing the fixed part of the transfer function of the echo path, it is possible to reduce significantly the initial acoustical and mechanical coupling and thereby to improve the ERL making full duplex handsfree operation possible. The transfer function of W1 could either be preloaded or calibrated during production and could also include modeling of non-linearities.