AEC Part 2: Echo, Echo, Echo…The Anatomy of Acoustic Echo Cancelling Technology

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Marco Hendel is an application engineer on Biamp Systems’ global application engineering support team, based out of Germany. He is interested in everything audio related and enjoys spending time working on his HiFi electronics and speakers.
Marco Hendel is an application engineer on Biamp Systems’ global application engineering support team, based out of Germany. He is interested in everything audio related and enjoys spending time working on his HiFi electronics and speakers.
If you have followed the previous article about Acoustic Echo Cancellation, you are already aware of what’s happening inside this mysterious ‘black box’ of an AEC processing circuit. This time we will move forward and focus on the implementation of AEC, aiming to achieve maximum performance of our distance conferencing system.

There are many different ways to design and setup AEC systems and the following guidelines are just recommendations for best results. The only constant design consideration that applies to ALL systems without exception is maintaining proper gain structure. AEC systems will perform best with unity gain (0dB) throughout your entire DSP chain, so always keep in mind there’s nothing more important than setting a proper gain structure.

biamp2_50Room Acoustics
Acoustic treatment matters! Whether you are using AEC or not, the rules of physics still apply. An AEC system is not a solution to improve intelligibility in a highly reverberant and noisy environment. Spaces that already yield poor intelligibility make for terrible conferencing rooms. Correcting an acoustic problem through the use of acoustic treatments and materials is always best first step.

Purpose-built acoustic treatments are the first line of defense. Surfaces such as carpets, acoustic tiles, and bookshelves can provide surface area to absorb, dampen, or at least attenuate acoustic echo. On the other hand, highly reflective surfaces (glass, bricks, etc.) will tend to reflect most of the acoustic energy and create reverberation that may exceed the AEC’s tail length.

Microphones and Speakers
Use appropriate placement of microphones to maximize signal to noise performance. Placing the mic as close as possible to the person talking will improve your system’s performance, while direct loudspeaker-to-microphone coupling should be avoided. Obviously this general design guideline is no different than a typical installation where the designer will try to optimize gain before feedback.

If possible position mics so the coverage pattern is directed away from air handling noise, machinery noise, or any other ambient noise.

Microphone choices include ceiling, boundary, gooseneck, DSP based microphone arrays, or beam forming. Try to find the best solution for the acoustic challenge at hand. While aesthetics are important, intelligibility and a great sounding system are more important to the successful functioning of a conferencing room.

Required Capabilities:
You will of course want to choose the best AEC tool for the application. When looking for an AEC solution, check for the following capabilities:

  • Full duplex capability
  • High audio quality
  • Fast convergence time
  • Short latency
  • Effectiveness at reducing echo

Reference
We must be careful of how to feed the reference: Whilst feeding the AEC reference with the “pure” far-end signal should provide the adaptive filter with the correct feed, that feed doesn’t necessarily take into consideration any processing or volume changes that may occur within the room. Any such changes will require re-convergence, potentially resulting in echo being sent to the far-end.

Thus, when we design a distance conference system, we must ensure that the reference is taken AFTER any processing and dynamic volume changes that may occur. The rule of thumb is: anything that may change the amplitude (volume) of the far end signal in the speakers must be applied to the AEC reference as well.

Never ever, under any circumstances, send a near-end microphone to its own AEC reference. The results will be catastrophic. In installations where local reinforcement is utilized, the routing can become a little challenging. This is where the routing flexibility of the system becomes even more important than ever, and when designing, we need to consider carefully what options to employ.

biamp3_50Multiple Microphones
Multiple microphones on a single-channel AEC processor? One other item to keep in mind when designing AEC systems is the amount of AEC circuits that will be needed. In a typical room, the echo picked up by each microphone may or may not be the same as that of others, as the echo paths and relative level are likely different. Costs savings can be achieved by combining the audio of multiple microphones and feed the resulting mix to a single AEC. Whether this will be sufficient to remove the entire echo and produce a system that allows for a true collaborative experience, really depends on whether a single adaptive filter is enough to handle all the various echo paths within the conferencing space. Will it work? Well, it depends.

Best practice is to have a dedicated AEC processing per microphone, ensuring that each microphone gets a dedicated filter.

Having said that, be weary of multiple AECs in the same audio path. If you are using a DSP processor with AEC, mixing and processing multiple microphones, and feeding the resulting signal to a Video Codec, make sure that any AEC in the Video Codec is turned off. The AEC on the Video Codec was not intended to deal with the resulting signal of a DSP processor managing multiple microphones, so the result will end up with two AEC in series.

Setting Up an AEC System
Each installation is unique, therefore each system will require unique EQing and adjustments. Nevertheless, the following steps provide good performance results for most typical AEC system installations:

1. Turn down your power amplifiers.
2. Adjust each microphone’s input gain so that the input meter displays approximately 0dB when someone is talking in front of the microphone.
3. Adjust the far end’s input gain so that the input meter displays approximately 0dB when the far end is talking.
4. Adjust your gain structure throughout the entire system to have nominal 0dB.
5. Slowly bring up the levels of your amplifiers until you have sufficient signal.
6. Make a test call and adjust inputs gains if required.
7. Once operational, make level changes in the room as required but do not change the volume of the amplifier, so as to not ruin the gain structure.

Professional AEC implementations are not always plug-and-play and there are important factors to be considered when implementing for the best possible results. In any AEC project, good preparation is key and it takes diligence to consider all of the aspects that are mentioned here. The end results are end users who are rewarded with an enjoyable, natural sounding, and productive distance conferencing experience. You may even achieve the highest form of appreciation in the A/V industry: no user complains at all!

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*Editor’s Note: This article first appeared in Systems Integration Asia Aug-Sep 2016 issue.