Acoustic Echo Cancellation
- by Clearone Communications

Audio is an extremely important element in any type of teleconference. All too often, the audio component of a teleconferencing system is overlooked. Even in a videoconference, eighty percent of the information transferred is in the audio. If this statement is surprising, imagine for a moment the effect of losing audio during a videoconference compared to losing the video.

Teleconference Audio Quality

Several factors contribute to the overall quality of a teleconference. Proper microphone placement (close to the talker), microphone selection (type and pickup pattern), loudspeaker to microphone coupling, background noise, room reverberation and the use of echo cancellation technology.

Clearone Conferencing System

In a teleconferencing system, speech from the remote end of the conference is amplified by the conference system and exits from the local loudspeaker. Some of the sound goes directly into the microphones (direct echo), and some is reflected off the walls and other objects (indirect echo). To the microphone, these sound signals appear to be originating in the room and would normally be sent to the remote end, where they would be heard as unwanted echo. The delayed speech signal is particularly objectionable, making it difficult for the remote party to carry on a conversation.

The Use of Echo Cancellation

In the echo cancellation process, the incoming audio signal from the remote site is sent to the local loudspeaker. It is also converted into a digital signal and is stored in a computer memory. The signal from the local microphone is also converted into a digital signal, and a digital signal processor compares the two signals.

The signal processor has a stored image of the speech sent to the loudspeaker, and it compares this image to the received microphone signal and determines what similarities exist between them. These similarities are then electronically subtracted from the microphone input, leaving only local speech. The result, consists only of local speech, free of echo, is then sent to the remote site.

The processor builds and electronic model of the acoustic properties of the room. For effective echo cancellation, not only must this model by accurate, but it also must remain accurate over time, even though the acoustics of the room change.

Echo Cancellation Helpful Hints

1. Microphone/Speaker Placement; Always keep the microphone as close to the participant as possible and as far away from the speakers and ambient noise sources as possible.
   
2. Acoustic Treatment; Acoustic treatment is the only way to enhance total echo elimination without adversely effecting the transmit audio. In fact, in most cases, acoustic treatment enhances the transmit audio quality by reducing reverberation and ambient noise.
   
3. Automatic Mixing; If using an automatic mixer, make sure the speaker audio can't gate microphones on. Also leave one microphone open at all times, a "last mic on" mode is adequate.
   
4. Ambient Noise; Any noise picked up be the microphone will be transmitted to the remote location. Echo cancellers do nothing to eliminate noise. In addition, echo cancellers are only capable of canceling down to the noise floor. So, if there is a high noise floor, residual echo will be mixed with that noise and may be heard at the remote location, particularly in systems with delay.
   
5. Network Levels; Always calibrate four-wire systems to the network level standard, typically -10. This will help eliminate problems, particularly when multiply rooms and multiply manufactures equipment types are added to the system.
   
6. Muting Microphones and Speakers; Always mute microphones and speakers with the controls provided on the echo canceller. Following this rule will always assure an echo free transmission.

For more information on applying echo cancellation to your design, contact Solutions@genproj.com or fill out our Information Request form.