iOS Stereo Playout/Recording

[ipavlidakis]

Stereo support is exposed via RTCAudioSessionConfiguration and the existing WebRTC audio device module; no extra glue code is required.

Enable stereo up front

import WebRTC

func configureForStereoIfAvailable() {
    let session = RTCAudioSession.sharedInstance()
    session.lockForConfiguration()
    defer { session.unlockForConfiguration() }

    let supportsStereoInput  = session.maximumInputNumberOfChannels > 1
    let supportsStereoOutput = session.maximumOutputNumberOfChannels > 1
    guard supportsStereoInput || supportsStereoOutput else {
        NSLog("Current audio route is mono-only; leaving configuration at 1 channel.")
        return
    }

    let config = RTCAudioSessionConfiguration.webRTC()
    config.inputNumberOfChannels  = supportsStereoInput  ? 2 : 1
    config.outputNumberOfChannels = supportsStereoOutput ? 2 : 1
    RTCAudioSessionConfiguration.setWebRTCConfiguration(config)
}

Call this once before the WebRTC audio session is activated (e.g. right after building your RTCPeerConnectionFactory). The native ADM will request multi-channel hardware, rebuild the voice-processing I/O unit with the right stream description, and automatically bypass the hardware voice-processing stage when stereo capture is enabled.

Toggle stereo at runtime

func setStereoEnabled(_ enabled: Bool,
                      factory: RTCPeerConnectionFactory) {
    let session = RTCAudioSession.sharedInstance()
    session.lockForConfiguration()

    let config = RTCAudioSessionConfiguration.webRTC()
    config.inputNumberOfChannels  = enabled ? 2 : 1
    config.outputNumberOfChannels = enabled ? 2 : 1
    RTCAudioSessionConfiguration.setWebRTCConfiguration(config)

    session.unlockForConfiguration()

    // Optional: restart playout/recording so the change takes effect immediately.
    let adm = factory.audioDeviceModule
    _ = adm.stopPlayout()
    _ = adm.stopRecording()
    _ = adm.initPlayout()
    _ = adm.initRecording()
    _ = adm.startPlayout()
    _ = adm.startRecording()
}

Route changes

When iOS reports a new audio route, WebRTC re-reads the hardware channel counts. If the route only exposes one channel (e.g. Bluetooth SCO), the ADM falls back to mono automatically, updates its buffers, and continues streaming. The “desired” stereo setting remains cached, so returning to a stereo-capable route re-enables multi-channel I/O without additional app logic.

Manual Validation (iOS device)

1. Built-in route (speaker + mic)

   • Run the app with configureForStereoIfAvailable() invoked before audio start.
   • Confirm StereoPlayout/StereoRecording report true and the call audio is healthy.

2. Plug in a stereo-capable headset (wired or AirPlay)

   • Verify audio continues uninterrupted and the ADM logs keep showing 2-channel playout/record.

3. Switch to a mono-only route (e.g., Bluetooth SCO headset)

   • Observe log warning about falling back to mono.
   • Confirm Stereo*IsAvailable returns false and the call still works.

4. Return to a stereo-capable route

   • Confirm the ADM automatically re-enables stereo (logs show 2 channels again) and audio remains stable.

[Copilot]

Pull Request Overview

This PR enables stereo audio playout and recording capabilities for iOS audio devices by implementing channel configuration management and automatic voice processing bypass when multi-channel audio is requested.

Key changes include:

• Addition of channel configuration methods and state management for stereo audio
• Automatic voice processing bypass when stereo is enabled (since iOS voice processing only supports mono)
• Dynamic audio session reconfiguration to apply channel changes during active streaming

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
voice_processing_audio_unit.mm	Adds channel configuration support and updates audio format handling for multi-channel streams
voice_processing_audio_unit.h	Adds new API methods for channel configuration and voice processing bypass control
audio_device_module_ios.mm	Updates stereo configuration methods to delegate to the underlying audio device implementation
audio_device_ios.mm	Implements comprehensive stereo support with session reconfiguration and voice processing management
audio_device_ios.h	Adds method declarations and member variables for stereo audio support
audio_device_module.h	Updates documentation to clarify voice processing bypass behavior with stereo capture

Comments suppressed due to low confidence (1)

sdk/objc/native/src/audio/voice_processing_audio_unit.mm:349

• Missing return statement after the closing brace. The function should return a value after the error handling block.

    }

---

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[Copilot]

Pull Request Overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Comments suppressed due to low confidence (1)

sdk/objc/native/src/audio/audio_device_ios.mm:1

• Missing return statement after the error case. The function continues execution even when SetStereoRecording fails, which may not be the intended behavior.

/*

---

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[Copilot]

Pull Request Overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

---

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[Copilot]

Pull Request Overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Comments suppressed due to low confidence (1)

sdk/objc/native/src/audio/audio_device_ios.mm:593

• Missing return statement before the closing brace. The function should return 0 to match the expected return type.

  }
}

---

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[Copilot]

Pull Request Overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

---

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[Copilot]

Pull Request Overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

sdk/objc/native/src/audio/voice_processing_audio_unit.mm:351

• Missing return statement. The function should return the result of the SetStereoRecording call, but it always returns 0 regardless of success or failure.

      }

---

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[Copilot]

Pull Request Overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Comments suppressed due to low confidence (1)

sdk/objc/native/src/audio/voice_processing_audio_unit.mm:1

• Missing return statement after line 346. The function will continue execution even after reporting an error, which could lead to unexpected behavior.

/*

---

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[Copilot]

Pull Request Overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 6 comments.

Comments suppressed due to low confidence (1)

sdk/objc/native/src/audio/voice_processing_audio_unit.mm:351

• Missing return statement. The function should return 0 on success after the closing brace on line 347.

      }

---

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