Complete Communications Engineering

When people think of acoustic echo cancellation, they think of its application as a requirement of a conferencing speakerphones or using a wireless handset in hands-free mode. In these situations, the loudspeaker and microphone are enclosed in the same device. Therefore, the physical characteristics of the device help shape the echo path. Acoustic echo cancellation can be applied to any voice communication system that desires to achieve a high quality full-duplex conversation.

For example, acoustic echo cancellation can be applied to a drive-thru order post, home intercom systems, baby monitors, patient-care intercom systems in hospitals and imaging centers, VoIP communications on laptops, videophones, and human/machine interfaces. All of these applications present their unique set of challenges.

In drive-thru order posts, reflections off of curbs and other various buildings and structures create an unique acoustic environment to every deployment. The impulse response (echo path) will be very dissimilar to that of a typical office environment and potentially could have a long echo tail. Besides the uniqueness of the echo path, a drive-thru application also has to be able to handle the non-stationary aspects of the background noise. This makes a noise reduction algorithm a requirement with AEC.

In home control and patient-care systems, the near-end speaker will most likely not be located near the loudspeaker and microphone. Therefore, the gain on the loudspeaker and microphones will both be high. This creates a particularly harsh echo environment. The main challenge is the near-end speaker to far-end speaker ratio (NFR) will be much less than 0dB. This means the echo signal will swamp the near-end signal during double-talk. This scenario makes double-talk detectors extremely unreliable, fortunately the divergence caused by the near-end speaker will not be as severe.

There are also applications in which the loudspeaker and microphone are not physically tied to the same device, as in distributed multimedia systems. This alleviates the acoustic coupling between loudspeaker and microphone. In this situation assuring synchronization between the sampling rates of the loudspeaker and microphone becomes an additional burden. A mechanism to either compensate for a clock drift must be in place in the AEC or the ability to timestamp signals coming from both directions is required for proper alignment.

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