Acoustic Echo Cancellation

Requires Dissonance v6.0.2 or greater!

When playing audio from speakers and recording audio from a nearby microphone you will often encounter problems when the microphone picks up the audio from the speakers. In a voice session a single person doing this can cause annoying echoes to be transmitted and multiple people doing this simultaneously can cause painful feedback which persists until everyone stops transmitting! This can be particularly problematic when using Voice Activation Detection (VAD) because the VAD automatically transmits back all speech it detects, causing constant echoes of everything other people say. It can also be very bad on platforms where the mic and the speaker are very close together such as VR headsets and mobile phones. Acoustic Echo Cancellation (AEC) is a system to automatically remove these echoes from the transmitted voice signal.

How Does AEC Work?

Dissonance already runs an audio preprocessor on the microphone signal before it is transmitted, by default this is running Noise Suppression (NS) and Voice Detection (VAD). To enable AEC we introduce a postprocessor which has all game audio passed through it - this postprocessor informs the microphone preprocessor what audio is about to be played through the speakers. With this knowledge the preprocessor can remove the echo signal from the microphone signal when it appears a short time later.

The most complex part of the AEC system is working out the correct echo delay from postprocessing the speaker to preprocessing the capture signal. This is achieved automatically but is is important to understand that the AEC system can take several seconds to converge on the correct delay and will not cancel any echo until it has done so. If no audio is playing out of the speakers the AEC has no data to work with and will not converge! In most gaming scenarios the background music/sound effects will suffice to give the AEC data to work with. However, if you are encountering problems with the AEC not cancelling any echo you should consider adding some sound effects for the AEC to process such as a short (1-2 second) jingle when a new user joins the voice session.

AEC Setup

Before starting ensure you are using Dissonance 6.0.2 or greater!

Before Starting

There is a single known issue which affects AEC setup in Dissonance 6.0.2. A fix for this will be included in the next version of Dissonance (not yet available as of 2018-01-31).

The fix is very simple:

  1. Open Plugins/Dissonance/Editor/DissonanceCommsEditor.cs
  2. Find typeof(VoicePlayback) on line 89
  3. Replace it with typeof(GameObject)

1. Audio Postprocessor

The first thing required for the AEC to function is to attach the postprocessor mentioned above to an audio mixer. Attach the Dissonance Echo Cancellation audio filter to the very last audio mixer in the mixing system and disable the Auto Mixer Suspend option for this mixer. If you were not already using audio mixers simply create a new mixer in Window > Audio Mixer and attach the filter to that.

Audio Mixer With AEC Filter

2. Route Non-Voice Audio

The filter will only process audio which passes through the mixer it is attached to - how to achieve this depends on what kind of audio mixing system you already had setup before using AEC.

If you were already using audio mixers then ensure that all the mixers eventually pass through the filter with the Dissonance Echo Cancellation filter attached. If you were not already using mixers then simply modify all the AudioSource components you use to output to the new mixer you created in the previous step.

You can check that you have done this correctly by running the game and watching the audio mixer window. The dB meter on the mixer should move when non-voice audio is playing.

Audio Source With Output Highlighted

3. Route Voice Audio

Voice audio also needs to be re-routed to pass through the mixer with the filter through the filter. To change where voice audio is sent you need to create a custom playback prefab. Create a prefab with a VoicePlayback component and an AudioSource component. Set the output of the AudioSource to the correct mixer. Finally drop the prefab into the Playback Prefab field of the Dissonance Comms component.

Playback Prefab With Output Highlighted

If you were already using audio mixers then you may want to consider creating a mixer specifically for voice and outputting this mixer to the root mixer. This will allow you to attach sound effects specifically to voices.

If you were not using audio mixers then you should just send the voice data to the mixer you created in step 1.

4. AEC Configuration

Now that all the audio is routed to pass through the AEC filter AEC can run. Open the Dissonance quality settings menu Window > Dissonance > Quality Settings to set the amount of echo suppression applied. Desktop platforms and mobile platforms use completely different AEC systems internally and are configured separately. Dissonance will automatically switch to using the mobile AEC (AECM) when a mobile platform is detected.

AEC Settings Inspector

These settings can be set in the editor - they will be saved into an asset and used as the default values at runtime. They can be changed at runtime by accessing the VoiceSettings class:

//Change amount of AEC applied on Desktop
VoiceSettings.Instance.AecSuppressionAmount = AecSuppressionLevels.Moderate;

//Change amount of AEC applied on Mobile
VoiceSettings.Instance.AecmRoutingMode = AecmRoutingMode.Speakerphone;

Only the two settings shown above can be changed while Dissonance is running, doing so will trigger a reset of the audio input system (causing a small hitch in transmitted audio). Changes to any other AEC related settings will be ignored until the next time the audio input system is reset (e.g. by changing the settings above).

It is advisable to start with very low AEC settings and ask the user to increase them if echo becomes a problem - excessive levels of AEC can very badly distort voices.

5. Testing AEC

Once you have set all of this up you may want to test that AEC is working as intended. To do so simply add an AudioSource component to your scene playing some loud music - make sure it's routed through the correct mixer! Now run the scene in the editor and select the filter attached to the audio mixer, this will show a status screen for the AEC:

AEC Status Inspector

The second box shows the AEC status. In this image it is showing a warning - if everything is setup correctly it should show AEC filter is running.

When the filter first starts all of the numbers shown here will be zero or even negative. This indicates that the filter has not yet converged and will not yet be removing any echo. After a short period of time (5-10 seconds) it should converge and begin removing echoes, if Fraction Poor Delays is more than 0% then the AEC will likely perform very badly.

Once the AEC is running and has converged remote speakers in the session should not be able to hear the music you are playing. In our own tests we have had music playing loudly enough to drown out voices but even that was still cancelled!

Known Issue With iOS

On iOS some people have reported the following error:

Audio effect Dissonance Echo Cancellation could not be found. Check that the project contains the correct native audio plugin libraries and that the importer settings are set up correctly.

To fix this problem:

  1. Download AudioPluginInterface from the Unity native audio plugin SDK. Add it to your XCode project.
  2. add #import "AudioPluginInterface.h"; to in XCode.
  3. Find the preStartUnity method and add the line UnityRegisterAudioPlugin(&UnityGetAudioEffectDefinitions);

If this does not fix the issue, please add a comment to this issue.