[BUG] WebRtcApmModifier: AEC silently does nothing - far-end (reverse) stream is never fed to APM

[Keidran]

Requirements

• I have searched the existing issues to ensure this bug has not already been reported.
• I have provided a clear and concise description of the bug.
• I have provided detailed steps to reproduce the bug.
• I have included a minimal reproducible example (MRE) if applicable.
• I have provided error messages and stack traces if applicable.
• I have completed the environment information.

1. Describe the Bug

Summary

When using WebRtcApmModifier with AEC enabled on a FullDuplexDevice, the acoustic echo canceller never receives a far-end reference signal, so ProcessReverseStream is effectively never called and echo cancellation does nothing. Noise suppression (NS) and the high-pass filter still work on the near-end capture stream, which makes the audio sound processed and hides the fact that AEC is a no-op.

Root cause

WebRtcApmModifier wires its internal far-end handler (HandleAudioEngineProcessedForAec, which only acts on Capability.Playback frames) to the FullDuplexDevice/engine OnAudioProcessed event. However, that event is raised only for the capture device path — playback/render devices never raise OnAudioProcessed. As a result the handler is only ever called with capture (near-end) frames, which it ignores, and it is never called with the playback (far-end) frames it actually needs. The reverse stream therefore stays empty and AEC has no reference signal.

Suggested fix

The modifier needs to actually receive the render/playback post-mix as the far-end signal. Either:

• raise the processed-audio event on playback/render devices as well (so the existing Capability.Playback handler gets fed), or
• inside WebRtcApmModifier, explicitly drive ProcessReverseStream from the playback device's master mixer instead of relying on the capture-only OnAudioProcessed event.

2. Steps to Reproduce

1. Create a FullDuplexDevice and attach WebRtcApmModifier with aecEnabled: true (NS/HPF on or off — doesn't matter).
2. Play back a signal on the playback device while capturing on the mic in a shared acoustic path (speakers audible to the mic).
3. Observe that captured audio still contains the full echo of the playback signal — AEC has no effect. Instrumenting ProcessReverseStream shows it is never called with real render data.

3. Expected Behavior

With aecEnabled: true, the APM's reverse stream should be continuously fed with the render/playback signal (what is being sent to the speakers), so AEC3 can build its reference and cancel echo from the capture stream.

4. Current Behavior

The far-end handler is registered but never invoked with playback audio, so ProcessReverseStream is never driven. AEC produces no cancellation; the caller hears their own echo. Because NS/HPF still run on the near-end, there is no error and the output is not obviously broken — the failure is silent.

5. Minimal Reproducible Example (MRE)

6. Error Messages and Stack Trace (if applicable)

SoundFlow Version

1.4.0

.NET Version

.NET 10.0

Operating System

Windows 11

Architecture

x64

Audio Backend Used (if known)

Full-duplex WASAPI device

Specific Audio Hardware (if relevant)

No response

8. Affected Components/Modules (if known)

SoundFlow.Extensions.WebRtc.Apm 1.4.0

9. Impact

No response

10. Possible Workaround (if known)

I route the playback master-mix into the modifier's private far-end handler ourselves via a small SoundModifier attached to PlaybackDevice.MasterMixer, invoking the internal HandleAudioEngineProcessedForAec with Capability.Playback (currently via reflection, since the method is non-public). The buffer is passed through unmodified. With this in place AEC converges and cancels correctly.

11. Additional Context

No response